A HIGH PRODUCTIVITY, CONTINUOUS FERMENTATION PROCESS FOR CAROTENOID-PRODUCING MICROORGANISMS

Abstract: A high productivity method for efficiently fermenting heterotrophic algae. The method includes diluting the fermentation broth. Additionally, a carbon source for the growth of the microorganisms is limited in a manner designed to restrict the growth rate to a level less than the maximum rate. The method is especially useful in the fermentation of Neospongiococcum algae. In preferred embodiments, the temperature of the fermentation broth is maintained from 30 DEG C to about 38 DEG C and the pH of the fermentation broth is maintained from about pH 5.5 to about pH 6.5. Using this method, volumetric productivity has been increased to at least about 20 mg total xanthophylls per liter per hour.

Description: A HIGH PRODUCTIVITY, CONTINUOUS FERMENTATION PROCESS FOR CAROTENOID-PRODUCING MICROORGANISMS This is a continuation-in-part of U.S. Patent Application Serial No. 07/058,512, filed June 5, 1987, and entitled "Carotenoid Producing Strains of Microorganisms and Methods for Selecting", which is incorporated herein by reference in its entirety. Field of the Invention The present invention is directed to a method for the efficient fermentation of heterotrophic algae which produce carotenoids and, in particular, a continuous process for fermenting Neospon iococcum algae in a manner designed to maximize the level of xanthophyll in the algae. Background of the Invention Batch and continuous fermentation methods are known. In certain batch fermentation processes, the microorganism populations increase to a certain level and then the growth rate decreases or ceases altogether. In particular, this problem has been discovered in connection with the unicellular green alga Neosponαiococcum excentricum. This alga is useful due to its production of carotenoids. Carotenoids are accessory pigments in photosynthetic microorganisms which function by absorbing light and transferring energy to chlorophyll for photosynthesis. Carotenoids also act to protect cells from photooxidative damage from oxygen radicals produced during photosynthesis. See generally Spurgeon, et al., The Biochemistry of Plants. Chap. 14 (1980) . Although the occurrence of carotenoids among photosynthetic organisms is ubiquitous, the occurrence among dark green heterotrophic algae is somewhat less common. Carotenoids, and xanthophylls in particular, have been observed to be formed under heterotrophic conditions by certain bacteria, yeasts and unicellular algae, with the highest levels observed in the latter. Xanthophylls are oxygen-containing carotenoids and are useful as natural coloring agents. Xanthophylls and, in particular, lutein and zeaxanthin, are used as feed supplements for poultry to produce a yellowish color in skin, shanks and egg yolks. Production of carotenoids by fermentation of microorganisms is known. In Farrow, U.S. Patent No. 3,280,502 (1966), a process for the preparation of lutein is disclosed in which a strain of Chlorella pyrenoidosa is fermented and up to 235 g of lutein per liter is produced in ninety-six hours. Kathrien, U.S. Patent No. 2,949,700 (1960) discloses a process for the production of carotenoids by the cultivation of algae of the phyla Chlorophvta, including genera of Chlorococcum, Chlorella, and Chlamvdomonas. Microbiological production of carotenoids by fungi is also known. For example, see Farrow, U.S. Patent No. 2,974,044 (1961). However, a problem with prior art processes is that the amount of carotenoid produced is typically too low to be economically feasible. The unicellular green alga Neosponαiococcum excentricum produces xanthophylls and has been used as a feed additive to provide color. See, for example, Marusich, et al., Oxycarotenoids in Poultry Pigmentation, Poultry Sci. 49 (6) pp. 1555-1566 (1970) . While Neosponαiococcum excentricum are known for producing carotenoids, when batch fermentation processes are employed to produce the algae, a barrier to growth and productivity has been discovered. In particular, the growth rate of the population is maintained for approximately 90 hours, after which time productivity declines and growth eventually ceases. Accordingly, it would be advantageous to overcome this barrier to growth and productivity encountered during the fermentation of Neosponαiococcum excentricum. Additionally, it would be advantageous to develop fermentation methods in which algae is produced having high levels of carotenoids. Summary of the Invention In accordance with the present invention, a continuous fermentation process has been developed which results in a high level of growth and productivity that is sustainable over an extended period of time. As used herein, the term "continuous fermentation process" means a fermentation process in which fresh fermentation medium is added and spent fermentation broth is removed, either without interruption or on a periodic basis. Preferably, the fresh fermentation medium is continuously added to a fermentation vessel and spent fermentation broth is continuously removed from the vessel in order to keep the volume of fermentation broth in the vessel substantially constant over a period of time. As used herein, the term "fresh fermentation broth" refers to that fermentation medium which is added to the fermentation vessel, the term "fermentation broth" refers to the fermentation culture (e.g. medium, cells and fermentation products) contained within the fermentation vessel and the term "spent fermentation broth" refers to the fermentation culture removed from the fermentation vessel. In accordance with an embodiment of the present invention, a continuous fermentation process is provided in which a Neoε onαiococcum algae culture is efficiently grown and a high level of carotenoids are produced. The high level of productivity is sustained over a long period of time by dilution of the fermentation broth with fresh fermentation medium. Additionally, the amount of the carbon source added to the fermentation broth is controlled. The carbon source, for example glucose, is fed to the microorganisms at a rate designed to maximize the production of carotenoids, and in particular xanthophyll, without unduly limiting the length of time during which successful fermentation can be accomplished. Temperature and pH are also controlled. In accordance with another embodiment of the present invention, the growth of Neosponαiococcum algae is maximized and the production of xanthophyll is maximized by continuously diluting the fermentation broth with fresh fermentation medium, by limiting the carbon source which is available to the algae, by maintaining the temperature of the fermentation broth between 30 C and 38 C and by controlling the pH of the fermentation broth between about pH 5.5 and about pH 6.5. Preferably, the productivity of xanthophyll is at least 10 mg xanthophyll per liter of fermentation broth per hour of fermentation time, more preferably 15 mg/l/hr and most preferably 20 mg/l/hr. Brief Description of the Drawings Figure 1 is a graph illustrating the effect of temperature on growth rate. Figure 2 is a graph illustrating the effect of temperature on the percent carotenoid content of the cells. Figure 3 is a graph illustrating the effect temperature changes have on the percent carotenoid content. Figure 4 is a graph illustrating the effect temperature changes have on the dry cell weight and the percent carotenoid content. Figure 5 is a graph illustrating the effect variations in pH have on the growth rate. Figure 6 is a graph illustrating the effect variations in pH have on the percent carotenoid content. Figure 7 is a graph illustrating the changes in dry cell weight, percent carotenoid content and productivity for a fermentation period of 90 hours, when cell growth ceases. Detailed Description of the Invention The present invention involves a method for maintaining continuous fermentation of microorganisms in a fermentation broth, and, in particular, for continuously fermenting heterotrophic microorganisms which produce carotenoids. The method includes diluting the fermentation broth with an effective amount of fresh fermentation medium which contains a growth rate-limiting amount of carbon source. By practicing the present invention, the productive phase of a fermentation can be effectively extended. As a result, depending upon the nature of the fermentation, high cell densities can be achieved and/or the production of materials, such as pigmentation compounds, can be increased. The method further includes controlling various fermentation parameters, including temperature and pH, to improve cell growth and pigmentation content in heterotrophic algae. The present method is particularly advantageous for heterotrophic algae such as algae of the genera Amphora, Ankistrodesmus, Astasia, Bracteacoccus, Botrydiopsis, Bumilleriopsis , Chlamydomonas r Chiore11idium, Chloroαlorea, Cyanidium, Euαlena. Gyrodinium, Haematococcus, Neochloris, Nitzschia, Prototheca, Scenedes us and Tribonema and, in particular, Neosponαiococcum and Chlorella. More particularly, this method is particularly advantageous for a strain of Neosponαiococcum excentricum identified by ATCC No. 40335, this strain having been deposited with the ATCC under the terms of the Budapest Treaty. The present invention involves conducting fermentation while diluting the fermentation broth by the addition of fresh fermentation medium. The fresh fermentation medium is generally an aqueous solution added to a fermentation vessel which includes a growth rate limiting amount of a carbon source. In particular, the fresh fermentation medium can be water, or water with nutrients, such as glucose, and other materials added to it. The fermentation broth can be diluted either by increasing the overall volume with the addition of fresh fermentation medium or, preferably, by maintaining a substantially constant volume of fermentation broth by the addition of fresh fermentation medium and the removal of spent fermentation broth. In the latter case, the dilution rate (D) is equal to the volumetric flow rate of fresh fermentation medium into the fermentation vessel and spent fermentation broth out of the vessel divided by volume of the fermentation broth. The units for D are reciprocal time, and for present purposes will be hr "1 unless indicated otherwise. Dilution is equal to the number of vessel liquid volumes which pass through the vessel per hour, and is the reciprocal of mean residence time. The addition of fresh fermentation medium and the removal of spent fermentation broth can be either constant or periodic, but is preferably constant. The addition of fresh fermentation medium and the removal of spent fermentation broth can either be simultaneous or not, but is preferably simultaneous. The present method requires a dilution rate which is less than the maximum growth rate (u ma ) . As used herein, the term "growth rate" (which will be indicated by the symbol "u") is the mass of cells formed per mass of cells in the fermentation broth per unit time. Unless indicated otherwise, the units for growth rate will be hr "1 , and the symbol will be "u hr"1 ". In the case of maintaining a relatively constant volume of fermentation broth for the growth of a typical culture, dilution is typically effective at between about 0.03 hr "1 to about 0.13 hr "1 , more preferably between about 0.06 hr "1 to about 0.12 h "1 and most preferably between about 0.09 hr "1 and about 0.11 hr "1 . At steady state, D = u in the continuous fermentation method of the present invention. Dilution rate (D) is dictated by the maximum growth rate (u" 13 *) of a particular organism. This can be determined by measuring u max in a batch culture using a nutrient-rich medium (e.g one having an excess of required nutrients) under optimal conditions. One may set the dilution rate in continuous culture at any level less than u max . For optimal productivity, D is preferably from about 25% to about 95% of u max , and more preferably is about 90% of u ax . The system will tend to ba more unstable as mx is approached. Washout occurs if D is greater than u. It should be recognized that with the introduction of fresh fermentation medium, the nutrient levels in the fermentation broth must be maintained at concentrations sufficient to support growth and/or production of extracellular material. Such nutrients can be added either in the fresh fermentation medium or independently of it. Specifically, such nutrients include sources of carbon; nitrogen; phosphates; sulfates; and magnesium, iron, and other trace metals. Another means of increasing productivity is by increasing the concentration of the carbon source in the feed which will increase the biomass concentration. However, an embodiment of the present invention includes conducting a fermentation while restricting the carbon source in the fermentation to a growth rate-limiting level. In this manner, it has been discovered that the time of fermentation can be extended indefinitely. In addition, by selectively limiting the carbon source in the fermentation, the yield factor (Y χ/S ) can be increased. As used herein, the term "yield factor" (Y χ/S ) refers to the amount of cell mass (X) formed for every mass unit of substrate (S) consumed. In the present instance, the substrate is the carbon source. The foregoing aspect of the present invention involves restricting the rate of addition of the carbon source to a rate-limiting level. Preferably, all other nutrients necessary for growth are present in excess compared with the carbon source. In other words, if the rate of addition of the carbon source were to be increased, the growth rate would increase. However, if the rate of addition of the other nutrients were to be increased, the growth rate would remain substantially unchanged. Without intending to be bound by theory, it is believed that by restricting the availability of the carbon source, the production of toxins is prevented or substantially reduced because microorganisms will be forced to preferentially utilize available carbon for growth. The rate of addition of the carbon source can be monitored and the carbon source can be fed at a rate to achieve the desired restricted growth rate. The carbon source is fed on a continuous basis to maintain the desired restricted level. Specifically, it has been determined that it is useful to maintain the feed of the carbon source at a rate designed to restrict the growth rate to a range from about 25% of u-" 3 " to about 95% of u 11 - 3 *, more preferably to about 90% of u max . The embodiment of the present invention relating to carbon source restriction is effective for use with any carbon source used in a fermentation. Specifically, the invention is useful for any type of carbohydrate and, in particular, any monosaccharide, disaccharide or trisaccharide. In particular, for the fermentation of Neosponαiococcu , the carbon source is preferably glucose. In the practice of the present method of restricting the feed rate of a carbon source to obtain the benefits of extended fermentation and improved product yield, it is recognized that by restricting the carbon source addition rate too greatly, the ability to grow and/or produce materials may be negatively affected to an undesirable extent. Therefore, it is desirable to maintain the carbon source feed rate at the desired restricted level but to also maintain a sufficient carbon source feed rate to avoid having a negative impact on growth and/or production of extracellular materials. In a preferred embodiment of the present invention the fermentation broth is diluted with fresh fermentation medium and the carbon source feed rate is restricted to a rate-limiting level. Preferably, this method includes three phases: (1) an inoculation phase; (2) a fed-batch growth phase; and (3) a carbon source limited, continuous culture growth phase. The inoculation phase includes providing an inoculum of actively growing microorganisms to a fermentation vessel. The fed-batch growth phase includes propagating microorganisms in the fermentation vessel with continuous medium additions while maintaining the carbon source concentration at less than 5 grams per liter. Typically, the carbon source concentration is between about 1 g/1 and about 5 g/1 during the fed-batch period. During the transition from the fed-batch phase to the steady state continuous phase, the rate of continuous feed of fresh fermentation medium containing the carbon source is gradually increased. The continuous fermentation phase commences when a desired steady state cell concentration is attained. Preferably, the carbon source is a carbohydrate, such as glucose. Prior to switching to the glucose limited, continuous culture growth phase, glucose is depleted to a minimal concentration. This concentration is as low as practical, typically about 1 g/1. The fermentation is then switched to the glucose limited, continuous culture growth phase. A continuous feed stream, including glucose and otherwise nutritionally balanced for growth (i.e. including nutrients such as potassium, phosphate, iron, sulfate, citric acid, magnesium and other trace metals) is introduced to the fermentation medium at an initially low dilution rate. The dilution rate is then gradually increased until the growth rate of the microorganisms is almost as great as the maximum growth rate achieved during the batch growth phase and a desired steady-state dilution rate (D=u) is attained. Preferably D is about 90% u max . Fresh fermentation medium is added at a rate equal to the removal rate for the spent fermentation broth. In conjunction with the process parameters of the present invention, as described above, the present invention includes the control of additional process parameters. In the fermentation of heterotrophic algae, temperature, pH, dissolved oxygen content and carbon dioxide level are controllable process parameters. The present invention further includes control of these variables to obtain high growth rates and high yields of carotenoids. Preferably, the volumetric productivity of xanthophyll is at least 10 mg xanthophyll per liter of fermentation broth per hour, more preferably 15 mg/l/hr and most preferably 20 mg/l/hr. The following discussion regarding control of the temperature, pH, dissolved oxygen and carbon dioxide process parameters is generally applicable to all heterotrophic algae, including Neosponαiococcum and Chlorella. More particularly, the following discussion is especially applicable to a strain of Neosponαiococcum excentricum identified by ATCC No. 40335. With regard to temperature, it has been found that maximum growth of heterotrophic algae can be achieved over a specific range of temperatures. In particular, high growth rates have been found between about 30 C and about 38 C and more particularly between about 33 C and about 38 C. The growth rate (u) is equal to the slope of a best- fit line drawn through a plot of the points indicating dry cell weight at various time intervals drawn on a semi- logarithmic scale, in other words, the slope of a line representing the plot of the log of the dry cell weight versus time. It has also been determined that carotenoid levels are maximized by conducting the fermentation at 36 C. It will be recognized that the present invention can be conducted within a range of temperatures around 36 C, for example, the fermentation can be conducted from about 33 C to about 38 C. It has also been found that control of pH in fermentations of heterotrophic algae is important to maximize growth and carotenoid production. In particular, it has been found that heterotrophic algae have a maximum growth rate between about pH 5.5 and about pH 6.5. Below about pH 5.5 and above about pH 6.5, growth rate drops rapidly. It should be recognized, however, that reference to pH between "about" pH 5.5 and "about" pH 6.5 includes pH values just less than pH 5.5 and just greater than pH 6.5 to the extent such values provide for a growth rate substantially equivalent to growth rates obtained by operating between pH 5.5 and pH 6.5. It has further been determined that carotenoid production is maximized by conducting fermentation at about pH 6.0. Reference to "about" pH 6.0 includes values which are less than and greater than pH 6.0, but which are close enough to pH 6.0 to obtain high values of carotenoid production. In view of the foregoing discussion relating to the effect of pH on growth rates of heterotrophic algae and on carotenoid content of heterotrophic algae, the present invention includes conducting fermentations and controlling growth rate and/or carotenoid content by controlling pH in accordance with the above-described parameters. Specifically, growth rate is maximized at between about pH 5.5 and about pH 6.5. Alternatively or in addition, carotenoid content is maximized by controlling pH at about pH 6.0. The effect of pH on carotenoid formation has further been found to be reversible. More particularly, the effect of pH on carotenoid formation was found to be reversible within about three generations. Accordingly, the present invention includes conducting fermentations and adjusting pH to an appropriate level in accordance with the above parameters at a point in the fermentation adequate to obtain desired carotenoid levels at a necessary time. For example, it may be desirable to conduct a fermentation at a pH at which carotenoid formation is not maximized for a portion of the fermentation and subsequently adjusting pH several generations prior to termination of the fermentation to a level at which carotenoid formation is maximized. An analogous situation exists with respect to other parameters, e.g. temperature, dissolved oxygen level and carbon dioxide level, as will be explained in more detail in connection with the Examples set forth hereinafter. The level of dissolved oxygen in fermentation medium of heterotrophic algae has also been found to be of importance. It has been found that growth and carotenoid formation is relatively constant at dissolved oxygen levels equal to or greater than about 10%. However, it has also been found that at dissolved oxygen levels below about 10%, carotenoid formation can be drastically reduced. Accordingly, another aspect of the present invention includes maintaining the level of dissolved oxygen in fermentations of heterotrophic algae above at least about 10%. Further, it should be noted that appropriate levels of dissolved oxygen depend in large part upon fermentation vessel configuration and mixing patterns. Specifically, at relatively low levels of dissolved oxygen close to a critical level, it is necessary to maintain sufficient mixing to avoid localized areas of fermentation medium having dissolved oxygen content below the minimum necessary to avoid a decrease in growth and carotenoid formation. It has also been found that carbon dioxide concentration in the fermentation medium is important to carotenoid formation. Specifically, levels of carbon dioxide greater than a partial pressure of about 0.04 are detrimental to carotenoid formation. It has further been found that the effect of carbon dioxide on carotenoid levels is reversible in less than one generation. Accordingly, the present invention includes conducting fermentation and maintaining the concentration of carbon dioxide below a partial pressure of about 0.04 for at least some portion of the fermentation. More specifically, carbon dioxide levels are maintained below a partial pressure of about 0.04 prior to harvesting of the algal biomass for a sufficient time to increase carotenoid levels to the desired level. The following experimental results are provided for purposes of illustration and are not intended to limit the scope of the invention. Preparation of Inoculum and Medium The following procedures were followed for Examples 1-6 and 8, as appropriate. An inoculum was prepared using a strain of Neosponαiococcum excentricum identified as ATCC No. 40335. Initially, a Yeast-extract/Peptone (2X YEP) inoculum medium was prepared comprising 6.0 g/1 of Difco " yeast extract, and 10.0 g/1 of Bacto"peptone in 960 ml deionized water. The mixture was sterilized by autoclaving at 121 C for 25 minutes. Subsequent to autoclaving, 40 ml of a 50% glucose solution was added. A single colony of Neosponαiococcum was inoculated into a 1 liter shaker flask containing 200 ml of the above-described inoculum medium. The flask was incubated in a gyrotory shaker at 400 rpm and at 35 C plus or minus 1 C. The flask was incubated until an optical density of 7 to 10 (absorbance at 620 nm) was attained, indicating mid-exponential growth phase. A fermentation medium was prepared which includes 3.88 g/1 of KH 2 P0 A , 1.50 g/1 of Amberex " yeast extract, 2.75 g/1 of (NH) 2 S0 4 , and 0.20 g/1 of citric acid in 9 1 of deionized water. This solution was autoclaved for 60 minutes at 121 C. Subsequently, the autoclaved solution was brought to 5 g/1 of glucose, 5.0 ml/1 of trace metal 8 solution, 4.5 ml/1 of MgS0 4 "7H 2 0, and 0.1 g/1 of FeS0 '7H 2 0. Trace metal 8 solution includes, in 6.0 liters of deionized water, 200 ml of concentrated hydrochloric acid (97%), 114 grams of CaCl-2H 2 0, 12.3 grams of manganese sulfate, 0.314 grams of copper sulfate-5H 2 0, 1.6 grams of cobalt chloride-6H 2 θ, 9.12 grams of boric acid, 17.65 grams of zinc sulfate-7H 2 0, 0.48 grams of sodium molybdenate-2H 2 0, 0.8 grams of vanadyl sulfate-2H 2 0, 0.4 grams of nickel nitrate'6H 2 0, 0.4 grams of sodium selenite, and 2.5 grams of sodium citrate. After all solids were added and fully dissolved, the final volume was brought to 10 liters with deionized water. The pH of the fermentation medium was approximately pH 4.8. Ammonium hydroxide solution (14%) was used to adjust the pH to 6.0 prior to inoculation. Pre- inoculation conditions were as follows: temperature = 36 C + 1 C; pH = 6.0 + 0.2; agitation = 400 rpm; aeration = 1 wm; and dissolved oxygen = 100% saturation. The inoculum was aseptically transferred to the fermentation vessel. The final concentration of inoculum in the vessel was 5% by volume. The initial optical density was about 0.5. For the fed-batch phase, the glucose fresh fermentation medium feed includes 500 g/1 of glucose, which was autoclaved for 40 minutes at 121 C, with the subsequent addition of 28.57 ml/1 of MgS0 4 .7H 2 0 and 5.72 ml/1 of H 3 P0 4 . The continuous phase fresh fermentation medium feed stream includes sterilized glucose and 0.048 grams of KH 2 P0 4 per gram of glucose, 0.0025 grams of FeS0 4 .7H 2 0 per gram of glucose, 0.003 grams of anhydrous citric acid per gram of glucose, 0.11 ml of one molar MgS0 4 .7H 2 0 per gram of glucose, and 0.125 ml of trace metal 8 solution per gram of glucose. Example 1 A batch fermentation was conducted to evaluate the effects of temperature on heterotrophic algal growth and carotenoid formation. The fermentation was conducted using the strain of Neosponαiococcum excentricum identified by ATCC No. 40335 in a Braun Biostat E fermenter. The fermentation was run for 35 hours. The pH of the medium was maintained at about pH 6.0. The dissolved oxygen content of the fermentation medium was maintained at greater than 20%. The inoculation was conducted and the fermentation medium was prepared as described above. An inoculum of actively growing microorganisms was introduced into the fermentation medium. During the course of the fermentations, results were evaluated over the temperature range of 30 C to 40 C. The growth rate maxima (u max ) were determined by obtaining the slope of a best-fit line, on a semi-logarithmic plot, drawn through dry weight points taken over time for each temperature. The results of these fermentations given as average growth rates at the selected temperatures, are shown in Figure 1. The percent of carotenoids per cell on a dry weight basis was also determined by averaging data from the fermentations taken at intermediate temperatures. These results are shown in Figure 2. It can be seen from Figure 1 that high rates of growth are obtained at temperatures between about 33 C and about 38 C. Incubation at 40 C resulted in cell death. It should be noted, however, that these fermentations only study temperatures above 30 C and, therefore, effective fermentations may be conducted at temperatures lower than 30 C. From Figure 2, it can be seen that carotenoid formation is highly dependent on temperature and is maximized between about 34 C and about 38 C, and is particularly maximized at about 36 C. Example 2 The effect of temperature on carotenoid formation was also studied using data from chemostat fermentations. The strain of Neosponαiococcum excentricum studied was identified by ATCC No. 40335. The pH of the fermentation medium was maintained at about pH 6.0. The dissolved oxygen content of the fermentation medium was maintained at greater than 20%. The medium was prepared as described above. The tests were designed to illustrate the effect of shifting the temperature from 34 C to 30 C to 36 C. The temperatures are indicated at the top of Figs. 3 and 4. The results of the chemostat fermentation are shown in Figure 3. The effects of temperature on dry cell weight were also studied. The results, superimposed on Fig. 3, are shown in Figure 4. As can be seen from Figure 4, a temperature of about 36 C provides for optimal carotenoid formation, when multiplied times the dry cell weight. Following a shift in temperature during steady state, maximum impact on carotenoid formation occurred three generations removed from the point of perturbation. These temperature-induced effects, however, are reversible within three generations upon return to the pre-perturbation steady state. Example 3 The influence of pH on growth of Neosponαiococcum excentricum strain ATCC No. 40335 was determined, once again, in a series of batch fermentations. Temperature was maintained at 36 C and pH was the variable. The pH was controlled by addition of NH 4 0H (14%) . Growth rate maxima were determined at various pH levels. From these fermentations, a curve of pH vs. u m x was plotted, as illustrated in Fig. 5. Based on these data, the effects of pH on growth indicates that a growth rate maximum at 36 C of 0.130 was achieved between pH 5.5 and 6.5, and that growth rate declined abruptly either below pH 5.5 or above pH 6.5, but more drastically above pH 6.5. The effect of pH on carotenoid formation was determined by 2 methods: (1) averaging carotenoid values taken during a series of batch fermentations, and (2) shifting pH in glucose-limited steady state chemostats. Again, the latter method was employed to minimize any ambiguity caused by cell growth dynamics, and to arrive at a more predictable model of pH perturbations in the process. From the series of batch fermentations, a curve was obtained by plotting % carotenoids vs. pH. From this plot, shown in Fig. 6, it is clear that, maximum carotenoid formation at 36 C occurs at about pH 6.0, and that carotenoid formation is quite sensitive to pH, indicating the need for precise control of this parameter. These results were also verified in a series of chemostat studies, whereby the pH set point was periodically adjusted either upward or downward. In these studies, as in the temperature studies, the maximum impact of pH perturbations required about 3 generations to appear. Also, as in the temperature studies, the effect on carotenoid formation was reversible within 3 generations once conditions were returned to normal. Based on the above collective data, a pH set point optimum of about pH 6.0 is preferred, with a slight bias towards the lower end. Although the cells grow at near maximal growth rates between about pH 5.5 and about pH 6.5, it is the apparent sensitivity of the carotenogenic machinery in the cells which mandates precise control of pH. Example 4 The effects of dissolved oxygen on the productivity of Neosponαiococcum excentricum were assessed in steady- state chemostats (glucose-limited) . Dissolved oxygen levels were maintained and adjusted by varying the agitation rate (rpm) while keeping the airflow constant at 1 volume of air per chemostat volume per minute. In an initial study, a 10% Cerelose " (trademark for refined glucose) feed was used with the dilution rate (D) at 0.03 to 0.04 per hour. The experimental dissolved oxygen levels tested included 70%, 20% and 10% of saturation. Cells were maintained at each level of dissolved oxygen for at least 4 generations. These data are summarized in Table 4.1. Based on these data, volumetric productivity as well as % carotenoids remained stable when dissolved oxygen was decreased from 70% saturation to 20% and finally 10% saturation. Productivity is equal to (gX/1) (mg cartenoids/g cells) (D, hr "1 ) (0.72), where 0.72 is the factor relating carotenoid assay values to external Association of Official Analytical Chemistry (AOAC) xanthophyll values. The volumetric productivity figures are the steady state figures obtained during the continuous phase of the chemostat process. Table 4.1 10% Glucose Feed Xanthophyll % Avg. Volumetric Avg. Yield Avg. 0 2 Avg. Dissolved % Productivity Coefficient Dilution Trans. Rate Respiratory 0 2 Carot. (mg/l/hr) £ x/S .) Rate (millimols/1/hr) uotient 70 0.86 11.5 0.38 3.4% 45.5 20 0.91 11.2 0.35 3.4% 42.9 10 0.93 11.1 0.34 3.4% 44.5 A second study was performed to further evaluate the response of cells to low-end dissolved oxygen levels (i.e. 10-20%) with a 13% Cerelose " feed and near-maximal dilution rate (D=0.095 hr '1 ) . The test results are shown in Table 4.2. Under these conditions, there was no significant difference in productivity when the process was performed at 20% dissolved oxygen vs. 10% dissolved oxygen. Table 4.2 Effects of Dissolved Oxygen on Productivity Xanthophyll Dilution Dissolved Carot. Productivity Rate (hr "1 ) Oxygen Content (mg/l/hr) 0.095 20% 0.71% 30 0.095 10% 0.75% 31 In spite of the demonstrated ability to successfully perform the process at low (10%) dissolved oxygen levels, it has been observed that oxygen limitation not only results in a loss of steady-state with glucose accumulation, but also has a dramatic negative effect on carotenoid formation. In these cases, a drastic decline in carotenoids occurs in less than a single generation, and is reversible within one generation, suggesting a physiological response. Based on the above results, it appears that the continuous portion of the process may be performed at 10% dissolved oxygen. This parameter, of course, would be highly dependent on vessel configuration and mixing patterns. As the detrimental effects of oxygen limitation are well documented, care should be taken to maintain dissolved oxygen above this critical level. The oxygen uptake rate (OUR) and respiratory quotients (RQ) were determined by mass-spectral analysis of the off-gas streams from steady-state che ostats run at various cell concentrations and flow rates. Typical OUR and RQ values obtained during one study in which the temperature was 36 C, the dissolved oxygen level was 20%, and a 13 weight/volume % Cerelose " feed was added, are cited below on Table 4.3. Dilution Dry cell Rate weight (g/1) 0.10 hr "1 43.2 A series of chemostat experiments were performed in order to assess the potentially toxic effect of carbon dioxide (C0 2 ) on cell growth and/or carotenoid formation. These experiments were performed by maintaining dissolved oxygen at about 20% and keeping airflow at a constant 1 volume air per chemostat volume per minute (vvm) . Carbon dioxide gas was blended with incoming air to obtain the desired levels in the fermenter off-gas. An initial chemostat study demonstrated the effects of gradually increasing the C0 2 in the off-gas from 2% to 10%. The results indicated that C0 2 levels exceeding 4% are detrimental to carotenoid formation. Increasing the C0 2 in the airstream from 4% to 10% resulted in a decline in carotenoids from 0.89% to 0.72%. On average, carotenoids declined in 0.03% increments for each 1% C0 2 in the off-gas. A significant increase in cell yield factor on glucose (Y χ/s/ g cells/g glucose) was observed (0.32 vs. 0.35) at 10% C0 2 . These results also demonstrated that, while elevated C0 2 levels inhibit carotenogenesis, there was no apparent adverse effect on cell growth. A second chemostat study was performed in order to determine whether detrimental effects of C0 2 could be reversed once incurred. For this experiment, steady state cells at ambient (i.e. not C0 2 enriched) airflow were subjected to a 10% C0 2 environment for several generations, and then returned to ambient conditions. Following C0 2 enrichment, carotenoid levels declined from 0.93% to 0.71% within one generation. When C0 2 levels were returned to ambient, carotenoid levels returned to near normal (about 0.90%) in less than one generation, indicating that the deleterious effects of C0 2 , incurred during fermentation or holding of biomass, may be reversed by increasing ventilation (airflow) to the system. An extrapolation of these results may also be applied to determine a parameter for airflow. In tests, using an airflow of 1 wm in the continuous phase with 13% (w/v) Cerelose " and D=0.10 hr '1 , off-gas C0 2 averages about 4.5%. Therefore, while this airflow seems to pose minimal losses in productivity, additional ventilation should be considered if cell densities (i.e Cerelose " feed concentration) is further increased. Example 6 The following experimental results illustrate the productivity decline and cessation of growth of a microorganism fermentation in the absence of a continuous dilution of fermentation medium and restriction of a carbon source feed, as is disclosed in the present invention. A fermentation was conducted using Neospongiococcum excentricum identified by ATCC No. 40335. The fermentation was conducted at a pH of about 6.0 and with a dissolved oxygen content of about greater than 2%. The results of this fermentation are shown in Figure 7. As can be seen from Figure 7, a cell density of about 113-114 g/1 dry cell weight was obtained after 90 hours with 0.81% of the cell dry weight as carotenoids. The carotenoid productivity and cell growth ceased after 90 hours. Example 7 The following example illustrates the use of the process parameters described above to achieve maximum productivity in a pilot plant fermentation. A. Preparation of inoculum: A 14 liter fermenter was charged with 7 liters water containing 31 g KH 2 P0 4 , 30 g Amberex 695" yeast extract, 22 g (NH 4 ) 2 S0 4 , and 1.0 ml of MAZU 204" antifoam. The entire contents of the vessel were steam sterilized at 121 C for 60 min. A 500 ml solution of the remaining nutrients were then aseptically filter-sterilized into the fermenter. This nutrient solution contained 160 g glucose, 40 ml of trace metal 8 solution, 9 g MgS0 4 -7H 2 0, 0.8 g FeS0 4 -7H 2 0, and 1.6 g citric acid. This fermenter is then inoculated with 500 ml of a 48 hr culture grown in YEP (9 g/1 yeast extract, 15 g/1 peptone, and 10 g/1 glucose) . Aeration is kept at 1.0-1.2 vvm with the agitation at 400-600 rpm throughout the run. The pH was controlled at 6.0 with anhydrous ammonia addition. When the residual glucose decreases to approximately 5 g/1, a nutrient feed was initiated consisting of the following components: This feed solution was maintained in such a fashion that the residual glucose in the fermenter ranges from 2-5 g/1. After approximately 24 hr of feed, the biomass accumulated to 40-50 g/1, and was used to inoculate the pilot fermenter. B. Medium preparation and sterilization: A 450 liter fermenter was filled to 190 liter with water containing Sigma " corn steep liquor (600 g — 3.0 g/1) and MAZU 204" antifoam (10 ml — 0.05 ml/1). this solution was sterilized for 60 minutes at 121 C, and then cooled to 36 C. The following components were then added to 5 liters (final volume) of water, and the total contents aseptically added to the fermenter through a Minidisk sterile filter. Medium Component Potassium Hydroxide (87.5%) Phosphoric Acid (85%) (NH 4 ) 2 S0 4 MgS0 4 -7H 2 0 Trace metal 8 solution The volume of the fermenter was adjusted to 200 liters with sterile water, and the pH adjusted to pH 6.0 with anhydrous ammonia. C. Fed-batch fermentation phase: A this point, the entire contents of the 14 liter inoculum vessel was aseptically transferred to the 450 liter fermenter. When the residual glucose level in the fermenter decreased to 5 g/1 (in approximately 12-15 hr) , nutrient addition to the fermenter commenced. Two sterile nutrient feed streams were aseptically added to the fermenter in a 1:1 ratio such that the residual glucose was maintained at 3.5 g/1 during this "fed-batch" portion of the fermentation. The two nutrient feed streams are listed below. Feed Solution #1 Feed Solution #2 Glucose (600 g/1) Phosphoric acid (55.05 g/1) FeS0^'7H 2 0 (0.6 g/1) Potassium hydroxide (29.8 g/1) Citric acid (1.2 g/1) Trace metal 8 solution (85.8 ml/1) NH 4 N0 3 (37.5 g/1) During this period of nutrient feeding, the pH was controlled at pH 6.0 + 0.2 by the addition of anhydrous ammonia, and the dissolved oxygen was maintained at greater than 40% of saturation. After approximately 15 hr of nutrient feed, the biomass accumulated to 25-30 g/1, and the continuous phase of carotenoid production was initiated. D. Continuous fermentation phase: After the biomass accumulated to approximately 25 g/1 / the continuous phase of the fermentation was initiated. The continuous phase is characterized by allowing the glucose addition rate to become growth limiting, and by main¬ taining a fixed dilution rate once the desired dilution rate is achieved. In the present example, the target dilution rate was 0.08 hr "1 . As in the fed-batch portion of the fermentation, the nutrient feed consists of two different sterile feed streams, and the composition of these streams is shown below. In this case, however, the ratio of the glucose feed to the salts feed is maintained at 1:3. Feed Solution #1 Feed Solution #2 Cerelose " (600 g/1) Phosphoric acid (6.1 g/1) FeS0,-7H 2 0 (0.6 g/1) KOH (34 g/1) Citric acid (12 g/1) MgS0 4 "7H 2 0 (47 g/1) Trace metal 8 solution (4.8 ml/1) NH 4 N0 3 (4.2 g/1) In the present example, the combined nutrient feed was initiated to give an initial dilution rate of 0.06 hr "1 , and this dilution rate was increased to 0.07 hr "1 and finally 0.08 hr "1 in 10 hr increments. The dilution rate was then maintained at this level for approximately 80 hr of continuous operation. During this continuous fermenta¬ tion phase, the carotenoid level of the biomass averaged 0.85% (this calculates to 0.61% AOAC xanthophylls using the 0.72 correlation factor). The cumulative volumetric productivity for the run described was 24.7 mg AOAC xanthophyll/1-hr. Example 8 The following results illustrate the increase in cell yield per unit of glucose by limiting glucose in a fermentation medium. In a continuous growth phase of a fermentation using a strain of Neospongiococcum excentricum identified by ATCC No. 40335, as described above, the dilution rate was increased from 0.073 to 0.139 hr "1 and the corresponding volumetric productivity was measured. The results of this investigation are shown in Table 8.1. Table 8 . 1 Effect of Dilution Rate on Steady State Growth and Carotenoid Yields in a Glucose-Limited Chemostat Xanthophyll Volumetric Dilution Biomass Cell Yield Carotenoids Productivit Example 9 In order to demonstrate the improved productivities obtained with the present invention, calculations are made in order to determine the volumetric productivity of prior art processes and the present processes. For Example 9 only, the volumetric productivity was calculated on a cumulative basis by multiplying the amount of biomass (in grams per liter) times the carotenoid content (in milli¬ grams carotenoids per gram of cells) times 0.72 (where 0.72 is the factor relating carotenoid assay values to external xanthophyll values according to the Association of Official Analytical Chemists) and dividing by the total reaction time (in hours) . This was necessary to accurate¬ ly compare the total productivity of prior art batch reactions to the present continuous method. In other words: volumetric productivity = (g biomass/liter broth) (mg carotenoids/g cells) (0.72)/(hr reaction time) The values obtained are shown in Table 9.1. Table 9.1 COMPARISON OF XANTHOPHYLL PRODUCTIVITY VALUES Productivity Reference Organism (mg/l/hr) US Patent #2,949,700 Chlorella sp 0.30 (Corn Processing Corp) US Patent #3,280,502 Chlorella sp 2.5 (Hoffmann-La Roche) Theriault, 1965 Chlorella sp 4.0 A. Ciegler (1956 Adv. Neospongiococcum 1.36 Appl. Microbiol. 7:1-34) Pat. Appl. #07/058,512 Neospongiococcum 7.0 (Coors BioTech, Inc.) Present Process Neospongiococcum greater (Coors BioTech, Inc.) than 10 Table 9.1 indicates the productivity gains resulting form the method of the present invention. While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adapta¬ tions are within the scope of the present invention, as set forth in the following claims.

Claims: What is claimed is: 1. A continuous fermentation method for carotenoid- producing heterotrophic microorganisms comprising: (a) diluting a fermentation broth comprising said microorganisms by continuously adding fresh fermentation medium comprising a nutrient to said fermentation broth, wherein the rate of addition of said nutrient is less than that necessary to provide for a maximum growth rate of said microorganisms; (b) maintaining the pH of said fermentation broth between about pH 5.5 and about pH 6.5; and (c) maintaining the temperature of said fermentation broth between about 30 C and about 38 C. 2. A method, as claimed in Claim 1, wherein said microorganisms produce xanthophyll at a rate of at least about 10 mg/l/hr. 3. A method, as claimed in Claim 1, wherein said microorganisms produce xanthophyll at a rate of at least about 15 mg/l/hr. 4. A method, as claimed in Claim 1, wherein said microorganisms produce xanthophyll at a rate of at least about 20 mg/l/hr. 5. A method, as claimed in Claim 1, wherein said microorganisms are heterotrophic algae. 6. A method, as claimed in Claim 1, wherein said microorganisms are algae of the genus Neospongiococcu . 7. A method, as claimed in Claim 1, wherein said microorganisms are algae of the species Neospongiococcum excentricum. 8. A method, as claimed in Claim 1, wherein the microorganisms are Neospon iococcum excentricum identified by ATCC No. 40335, or mutants thereof. 9. A method, as claimed in Claim 1, wherein said microorganisms are algae of the genus Chlorella. 10. A method, as claimed in Claim 1, wherein said nutrient is a carbon source. 11. A method, as claimed in Claim 1, wherein said nutrient is glucose or sucrose. 12. A method, as claimed in Claim 1, wherein the rate of dilution is from about 0.03 hr '1 to about 0.13 hr " 1 13. A method, as claimed in Claim 1, further comprising removing spent fermentation broth, wherein the volume of said fermentation broth is maintained substantially constant by maintaining substantially equal in-flow of fresh fermentation medium and out-flow of spent fermentation broth. 14. A method, as claimed in Claim 1, wherein the pH of said fermentation broth is about pH 6.0. 15. A method, as claimed in Claim 1, wherein the temperature of said fermentation broth is between about 33 C and about 38 C. 16. A method, as claimed in Claim 1, wherein the temperature is maintained at about 36 C. 17. A method, as claimed in Claim 1, wherein the growth rate of said microorganisms is from about 25% to about 95% of the maximum growth rate. 18. A method, as claimed in Claim 1, wherein said fermentation broth contains greater than 10% dissolved oxygen. 19. A method, as claimed in Claim 1, wherein the partial pressure of carbon dioxide contained in said fermentation broth is less than about 0.04. 20. A method for maintaining continuous heterotro¬ phic fermentation of Neosponαiococcum in a fermentation broth, said method comprising: a) diluting said fermentation broth with fresh fermentation medium at a rate from about 0.03 hr "1 to about 0.13 hr "1 ; b) limiting a carbon source addition rate to a level below the level necessary for a maximum rate of growth; c) maintaining a fermentation temperature from about 30 C and to about 38 C; d) maintaining a pH from about pH 5.5 to about pH 6.5, and e) producing at least about 10 mg xanthophyll per liter of fermentation broth per hour. 21. A method, as claimed in Claim 20, wherein the dilution rate is from about 0.09 hr "1 to about 0.11 hr "1 . 22. A method, as claimed in Claim 20, wherein the temperature is from about 33 C to about 38 C. 23. A method, as claimed in Claim 20, wherein the temperature is about 36 C. 24. A method, as claimed in Claim 20, wherein the pH is about pH 6.0. 25. A method, as claimed in Claim 20, wherein xanthophyll is produced at a rate of at least 15 mg/l/hr. 26. A method, as claimed in Claim 20, wherein xanthophyll is produced at a rate of at least 20 mg/l/hr. 27. A method, as claimed in Claim 20, wherein the rate of growth of Neosponαiococcum is from about 25% to about 95% of maximum. 28. A method, as claimed in Claim 20, wherein the rate of growth of Neosponαiococcum is about 90% of maximum. 29. A method for the production of carotenoids, comprising fermenting heterotrophic algae, wherein said fermentation is conducted at a temperature from about 33 C to about 38 C, a pH from about pH 5.5 to about pH 6.5 and at least 10 mg carotenoids are produced per liter of fermentation broth per hour. 30. A method, as claimed in Claim 29, wherein at least 15 mg of carotenoids are produced per liter of fermentation broth per hour. 31. A method, as claimed in Claim 29, wherein at least 20 mg of carotenoids are produced per liter of fermentation broth per hour. 32. A method, as claimed in Claim 29, wherein said heterotrophic algae are Neosponαiococcum excentricum. 33. A method, as claimed in Claim 29, wherein said algae are Neosponαiococcum excentricum identified by ATCC No. 40335, and mutants thereof. 34. A method, as claimed in Claim 29, wherein the heterotrophic algae produced include at least about 0.6% carotenoids. 35. A method, as claimed in Claim 29, wherein said temperature is about 36 C. 36. A method, as claimed in Claim 29, wherein the pH is about pH 6.0. 37. A method, as claimed in Claim 29, further including the step of diluting a fermentation broth with an amount of fresh fermentation medium. 38. A method, as claimed in Claim 29, further comprising the step of limiting a carbon source addition rate to a fermentation broth.

Inventors: BAILEY RICHARD B, MEDWID RICHARD D, BAILEY, RICHARD, B., MEDWID, RICHARD, D.
Applicants: COORS BIOTECH INC, COORS BIOTECH, INC.
Application year: 1991
Main classification: C12N1
IPC codes: C12N1, C12P23, C12R1