Nitrogen uptake by heterotrophic bacteria and phytoplankton in the nitrate-rich Thames estuary

The uptake of ammonium, nitrate, amino acids and urea was examined in the nitrate- rich Thames estuary and adjacent area in the North Sea during February 1999. The majority of uptake was by heterotrophic bacteria, as demonstrated by addition of a prokaryotic inhibitor that lowered uptake rates by 82, 66, 49 and 86% for ammonium, nitrate, amino acids and urea, respec- tively. Amino acids were preferred over ammonium and urea, which in turn were preferred over nitrate. Urea was not important as nitrogen substrate. Amino acids were the main nitrogen substrate offshore and at the mouth of the estuary, but in the inner estuary nitrogen was mainly taken up as ammonium and, even more, as nitrate. Nitrate appeared to be the main substrate in the inner estuary (60 to 90%) despite its low affinity for nitrate. The high nitrate uptake by heterotrophic bacteria is probably due to high ambient concentrations of nitrate (up to 650 µM).

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title: Nitrogen-specific uptake rates of marine phytoplankton isolated from natural populations of particles by flow cytometry
abstract: Phytoplankton were isolated from natural, heterogeneous populations of part~cles in Boothbay Harbor, M a ~ n e , USA using flow cytornetry with cell sorting to measure true rates of nitrogenspecific uptake of 15N without the bias due to detritus, bactena or other heterotrophs that affects tradltional filtration techniques. After incubation under light or dark conditions with I5NH4+ or "NO3-, particles were either directly filtered or were sorted to isolate phytoplankton and then filtered for isotopic analysis. Nitrogen-specific (pN, t i m e ' ) and absolute uptake rates (mass volume-' time-l) of ammonium were greater for small (3-10 pm) phytoplankton than for larger (10-53 pm) phytoplankton, whereas the opposite was observed for uptake of nitrate. Accounting for pronounced diel variations in uptake from both nitrogen sources, algal growth rates were calculated to be 0.38 d' on ammonium and 0.05 d ' on nitrate. The absolute rate of NH,+ uptake by all particles as assayed by filtration was approximately balanced by the summed rates of the 2 sorted fractions and the small (<3 ).]m) particles that were removed prior to sorting: phytoplankton represented only 50% of the total rate. In contrast, phytoplankton uptake of nitrate represented only about one-third of the total uptake, and the remaining fractions contributed only 10% leaving about 60% unaccounted for. Future applicat~ons of flow cytornetry with stable isotopes could be used to investigate diel variations in p~ and pc (carbon-specific uptake rates) and provide an estimate of phytoplankton growth rates.

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title: Organic matter mineralization in intertidal sediments along an estuarine gradient
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title: Nitrogenous nutrition of the plankton in the Chesapeake Bay. 1. Nutrient availability and phytoplankton preferences
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When the sum of all available N nutrients was less than that required to satiate the phytoplankton demand, NIL’, urea N, NO,-, and NO,were all utilized at rates proportional to their availability. For the midbay region in October 1973, NO, was the dominant N nutrient present both in the water and in the diet of the phytoplankton. The Chesapeake Bay and its tributaries receive nitrogen in discharges of treated sewage from metropolitan areas of the District of Columbia and Baltimore at rates excceding 10 and 7 tonnes d-l. Carpenter et al. (1969) observed high levels of NOS(30100 pg-atoms N liter-l) in the open waters of the north half of the bay during winter and spring, levels undoubtedly sufficient to meet the nutritional needs of the indigenous phytoplankton. The persistence of such levels for several weeks in spring suggests that the rates of supply and loss were approximately balanced during this period. In summer and early autumn the NOs-concentration fell to <l lug-atom N liter-l; often it was analytically undetectable. A pronounced north-south gradient led Carpenter et al. (1969) to conclude that the Susquehanna River, which enters the bay l This research was supported by National Science Foundation grants GA 33445 and DES 7502846 and by U.S. Energy Research and Dcvelopment Administration contract E ( ll-l)-3279, document No. COO-3279-21. Chesapeake Bay Institutc Contribution 240. above the major metropolitan arcas, was the major source of NOsfor the open waters of the Chesapeake Bay, but since the distribution of NI14+ showed little spatial or temporal variability they postulated that the primary origin of NIL+ in the open water of the bay was linked to local biological activity. Studies with unialgal laboratory cultures and artificially enriched coastal seawater have shown that marinc phytoplankton prefcr NH4+ .to NOs( cf. Grant et al. 1967; Eppley et al. 1969; Eppley and Rogers 1970; McCarthy and Epplcy 1972). The preferential assimilation of NH‘*+ in the presence of NO:%has been observed or inferred from data for the Peru Current ( MacIsaac and Dugdale 1969)) the subtropical Pacific Ocean (Goering et al. 1970), the Saronikos Gulf ( MacIsaac and Dugdale 1972), and the California Current (McCarthy 1972a). The importance of urea as a source of N for natural populations of phytoplankton (e.g. Carpenter et al. 1972; Eppley et al. 1973; Webb and Haas in press) enlarges LIMNOLOGY AND OCEANOGRAPHY 996 NOVEMBER 1977, V. 22( 6) N nutrition of phytoplankton conceptualIy the importance of “regenerated nitrogen” advanced by Dugdalc and Goering (1967). Whereas NH4+ at elevated concentrations will be used in preference to urea, urea N and NOsare utilized simultaneously over a wide range of concentrations ( McCarthy and Eppley 1972). At low ambient concentrations of all forms of nitrogenous nutrient, compensation for any difference between the concentration required for optimal growth and the concentration of the most highly preferred form, i.e. NH4+, might presumably be by use of an alternate form. For NOs-, this requires an energy expenditure for both induction of the enzyme system and reducing potential (Eppley et al. 1969). In eucaryotic algae, either an ATP : urea amidolyase enzyme system or a urease system is considered essential for the assimilation of urea N (Leftley and Syrett 1973). Some marine phytoplankters do not use urea (Guillard 1963; Thomas 1968; McCarthy 1972h) and there is no evidence that either of the identified enzyme systems is other than constitutive. Low ambient lcvcls of nitrogenous nutrients in most offshore marine waters and low densities of algal biomass preclude careful scrutiny of the nutritional preferences of unperturbed phytoplankton asscmblages. The relatively high standing crop of phytoplankton and the great temporal and spatial variability of nitrogenous nutrient concentrations in the Chesapeake Bay allow us to observe in situ the rate of utilization of a single N nutrient when others are present over a wide range of concentrations. In particular, we can examine the significance of the rapidly recycled NIL+ and urea during periods of both high and low NOsconcentration. We acknowledge M. E. Loftus for technical assistance and R. W. Eppley and K. L. Webb for helpful comments on an earlier draft of this manuscript.

title: Temporal and spatial variability in phytoplankton ammonium and nitrate uptake in the Delaware Estuary
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title: Carbon and nitrogen primary productivity in three North Carolina estuaries.
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title: Nitrogen assimilation by phytoplankton and other microorganisms in the surface waters of the central North Pacific Ocean
abstract: Assimilation rates of 15N-labelled ammonium, urea, and nitrate by plankton in the upper euphotic zone were measured in 2 summer, 2 winter, and 1 spring cruise in the central North Pacific Ocean. Average rates of ammonium plus urea assimilation could not be determined precisely, but were estimated to be 7 to 25 μg-at. N m-3 day-1. Indirect evidence suggested that non-photosynthetic microorganisms contributed to these rates. Nitrate assimilation was negligible in the upper waters considered in this report (above the chlorophyll maximum and the nutricline). Potential, nitrate-saturated rates were in the range 1 to 8 μg-at. N m-3 day-1. Seasonal comparison showed lowest rates of both carbon and nitrogen assimilation rates per chlorophyll a in winter.

title: Use of “Specific” Inhibitors in Biogeochemistry and Microbial Ecology
abstract: The above statement, although meant to be tongue in cheek, contains an essential truism: all work with inhibitors is inherently suspect. This fact has been known by biochemists for some time. However, use of chemical inhibitors of enzymic systems and membranes continues to be a common approach taken toward unraveling the biochemistry and biophysics of plants, animals, and microorganisms. Various types of “broad-spectrum” biochemical inhibitors (e.g., poisons, respiratory inhibitors, and uncouplers) have been employed by ecologists for many years in order to demonstrate the active participation of microbes in chemical reactions occurring in natural samples (e.g., soils, sediments, and water). In recent years, considerable advances have been made in our understanding of the biochemistry of microorganisms of biogeochemical interest. Concurrent with these advances have been the discoveries of novel types of compounds that will block the metabolism of one particular group of microbes, but have little disruptive effect on other physiological types. Thus, the term “specific inhibitor” has been applied to these types of compounds when they are used to probe the functions of mixed populations of microorganisms. These substances provide powerful experimental tools for investigating the activity and function of certain types of microorganisms in natural samples.

title: Effectiveness of various treatments in retarding microbial activity in sediment trap material and their effects on the collection of swimmers
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title: Amino acid utilization by marine phytoplankton: A novel mechanism
abstract: An enzyme on the cell surface of some marine phytoplankton, particularly the genus Pleurochrysis (ffyrnenomonas), oxidizes many L-amino acids to produce H,O,, NH,, and an ol-keto acid extracellularly. The NH,+ is subsequently taken up and used for growth. The enzyme synthesized by Pleurochrysis carterue (clone CoccoII) has a half-saturation constant of 0.25 PM assayed with L-W aminobutyric acid as a model substrate. The enzyme activity is absent in NH,+-grown (logarithmic phase) cells, is present in NO,--grown (logarithmic phase) cells, and is enhanced 3.S-fold in -N-limited cells. Some Heurochrysis species show different patterns of regulation, The existence of .this novel, high-affinity mechanism for amino acid utilization-one that would not be detected by typical field methods-suggests that amino acids may be a more important source of N for phytoplankton than is currently believed.

title: Relations between bacterial nitrogen metabolism and growth efficiency in an estuarine and an open-water ecosystem
abstract: Bacterial uptake or release of dissolved nitrogen compounds (amino nitrogen, urea, ammonium and nitrate) were examined in 0.8 pm filtered water from an estuary (Santa Rosa Sound [SRS], northwestern Florida) and an open-water location in the Gulf of Mexico (GM). The bacterial nutrient dynamics were related to oxygen consumption and activity of enzymes involved in nitrogen assimilation (glutamate dehydrogenase [GDH], glutamine synthetase [GS] and aminopeptidase activity [leu-MCA]). Dissolved free amino acids (DFAA) were the dominant N source to the bacteria, followed by dissolved combined amino acids (DCAA), ammonium and nitrate. Nitrogen budgets of the bacteria (assimilation of N compounds relative to accumulated N biomass) demonstrated that, except for the initial 24 h period in the GM cultures, the assimilated N compounds sustained all of the bacterial N demand. Urea was released in both sets of cultures, but in the SRS cultures the produced urea was reassimilated. Major differences in the bacterial N metabolism between the open-water GM and the estuarine SRS stations were observed. Relative to the GM station, bacteria in the SRS cultures had (1) a 2.4 to 18x higher cell-specific DFAA assimilation, (2) a 2 . 5 ~ higher cell-specific leu-MCA activity, (3) a 3 to 10x higher GDH:GS activity ratio and (4) a 1.9 to 4 . 1 ~ lower cell-specific respiration. The larger nutrient availability at the estuarine station probably caused these differences in uptake and metabolism of nitrogen as well as the lower respiratory rate, relative to the open-water station. Therefore, we hypothesize that availability of nitrogen, rather than carbon, controlled the bacterial activity in the cultures. Our results suggest that bacteria in eutrophic conditions can be more important mediators of nitrogen than those in oligotrophic conditions, in which bacteria temporarily may immobilize labile nitrogen.

title: Dependent coupling of inorganic and organic nitrogen uptake and regeneration in the plume of the Chesapeake Bay estuary and its regulation by large heterotrophs
abstract: Nitrogen uptake and regeneration in the plume of the Chesapeake Bay estuary was studied during a series of four cruises (1985-1986). During each season we followed the short-term patterns in inorganic (NH,‘, NO3 -, NO,-), and organic (urea and dissolved free amino acids-DFAA) nitrogen uptake and regeneration as plume water aged and became incorporated into ambient coastal water over l-3 d. There was little influence of seasonally changing temperature on inorganic nitrogen uptake rates: the highest specific and absolute hourly rates of uptake occurred in April, when the availability of total N was al a seasonal high; rates were somewhat lower in early and late summer and were lowest in February. NH,+ regeneration rates, on the other hand, were highly correlated with temperature, with the highest rates occurring in late summer. Rates of DFAA uptake were highest in June. Urea contributed up to 70-80% of the total N utilized during winter and summer; in spring most nitrogen uptake was in the forms of NO, and NH,‘. The seasonal differences in the patterns of release of dissolved organic N, and the role of zooplankton in mediating this flux, paralleled the patterns previously reported for the release of dissolved organic C in the Chesapeake Bay plume. That is, in winter most organic N release was not grazer mediated, but instead was more likely due to nutrient-deficient phytoplankton. In contrast, in late summer, there seemed to be more dissolved organic N release by grazers, either directly or from “sloppy feeding.” At all seasons, in the transition from the bay environment ‘to coastal waters, autotrophic populations appeared to become nutrient deficient, and a system dominated by heterotrophic processes rapidly became established.

title: Size-fractionated uptake of ammonium, nitrate and urea and phytoplankton growth in the North Sea during Spring 1994
abstract: The growth of 2 different algal size classes was studied in March/April 1994 during the establishment of the spring bloom along a transect from the Dogger Bank to the Shetland Islands in the North Sea. Size-differential growth rates were estimated on the basis of independent measurements of carbon and nitrogen uptake. At the shallower stations near the Dogger Bank (DB) area, chlorophyll a (chl a) levels were up to 5.8 mu g l(-1). In the bloom 89 % of the chi a was in the >5 mu m fraction. In the central North Sea (cNS) and near the Shetland Islands (SI) total chi a was 0.52 and 0.38 mu g l(-1), respectively; 60 % was in the >5 mu m fraction. Depth-integrated primary production at the DB, cNS and SI was 46, 145 and 149 mg C m(-2) d(-1), respectively, for the 5 mu m size fraction. Since the calculated specific growth rates, based on either nitrogen uptake or inorganic carbon uptake, were in good agreement with each other for both size fractions, it was concluded that smaller algae apparently grow faster than larger ones in this typically light-controlled environment. The >5 mu m fraction, however, dominated the bloom at the Dogger Bank. Our findings consolidate the concept of size differential control of phytoplankton communities under typical spring bloom conditions which originally was demonstrated in a coastal area (Riegman et al. 1993; ).  phytoplankton growth; size fractionation; nitrogen uptake; carbon uptake; North Sea Marine diatoms; cell-size; nitrogen assimilation; carbon fixation; populations; plankton; station; system; waters; light]

title: High rates of nitrification and nitrate turnover in undisturbed coniferous forests
abstract: THE importance of nitrate (NO–3) in the internal nitrogen cycle of undisturbed coniferous ecosystems has not been widely recognized1,2. Nitrate concentrations in soils from these forests tend to be low, and assays measuring net nitrification usually show exceedingly slow rates3,4. It may be, however, that microbial assimilation of NO–3 is substantial in these soils, and that net nitrification rates greatly underestimate gross rates5. Here we use a 15N isotope-dilution technique in intact soil cores to measure gross rates of nitrification and microbial assimilation of NO–3 in eleven undisturbed forest ecosystems of New Mexico and Oregon. We found that gross nitrification rates were surprisingly high in all of the forests examined. Net nitrification rates poorly predicted gross rates because the soil microbial communities had the capacity to assimilate almost all of the NO–3 produced. To our knowledge, this is the first report of gross nitrification and NO–3 assimilation rates in intact soil samples from a large number of contrasting forest ecosystems. Our results contradict previous assumptions that nitrification rates are low in mature coniferous forests and suggest that current models greatly underestimate the role of the microbial community in preventing NO–3 loss.

title: Biomass and nitrogen uptake by heterotrophic bacteria during the spring phytoplankton bloom in the North Atlantic Ocean
abstract: N biomass and N uptake by heterotrophic bacteria and phytoplankton were examined during the North Atlantic spring bloom (May 1989). Phytoplankton N had reached its zenith by the time the authors arrived at the station and increased only slightly during the first week of the study, while particulate nitrogen (PN) nearly doubled (increase of 450 mg N m−2), apparently due to the growth of heterotrophic organisms other than bacteria. During the second week, bacterial N doubled, increasing by 280 mg N m−2. These increases in biomass N and total PN were large compared to NO3− uptake and to N export from the upper 100 m. To examine the role of bacteria in greater detail, we estimated total N uptake and the uptake of free amino acids (DFAA), NH4+, and NO3−. The ratio of total N uptake by bacteria to total N uptake by phytoplankton varied greatly and was often high (0.2–0.9). When uptake was corrected for phytoplankton activity, heterotrophic bacteria accounted for 22–39% and 4–14% of NH4+ and NO3− uptake. DFAA, NH4+ and NO3− supplied 12–34%, 19–29% and 2–8% respectively, of the nitrogen needed for bacterial production. Preference indices further indicated that NO3− was the N source least preferred by heterotrophic bacteria. Nitrogen uptake fuelled a buildup of bacterial biomass and other suspended material that needs to be considered in reconciling new production and N export from the upper layers of the ocean during the spring bloom.