Abstract:
The method for treating microcystin-containing water which detoxifies microcystin in the microcystin-containing water, the method comprises the step of: bringing the microcystin-containing water into contact with a  Sphingomonas  bacterium to degrade biologically the microcystin in the microcystin-containing water, wherein: the  Sphingomonas  bacterium is a strain FERM P-19480 which is deposited as strain MDB1 with International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology.

Description:
This is a Divisional of application Ser. No. 11/270,745, filed Nov. 10, 2005, now U.S. Pat. No. 7,425,267, which claims the benefit of Japanese Patent Application No. 2004-327799, filed Nov. 11, 2004. The disclosures of the prior applications are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and an equipment for treating microcystin-containing water, more particularly to a method and an equipment for treating microcystin-containing water that can degrade and detoxify microcystin produced from algae such as water-bloom generated in closed-water areas such as lakes, dams, moats, and inland seas. 
     2. Description of the Related Art 
     When closed-water areas such as lakes, dams, moats, and inland seas are eutrophicated due to inflow of sewage or the like, a large amount of cyanobacteria, for example, cyanobacteria belonging to the genus  Microcystis  are proliferated in the closed-water areas to generate so-called water-bloom. Many types of the cyanobacteria including the generated water-bloom produce microcystin called water-bloom toxin, and thus the closed-water areas are polluted with microcystin. Microcystin is composed of a cyclic peptide consisting of seven amino acids, and has properties of toxic and carcinogenic to a liver of human or livestock. 
     Microcystin LR, microcystin RR, and microcystin YR detected as typical microcystins from closed-water areas make human or livestock poisoned when the microcystins are orally ingested. There are various reports on poisoning by the microcystins in Japan and other countries, irrespective of human or livestock (see Mariyo WATANABE, “Water-bloom”, University of Tokyo Press, 1999, and Kenichi HARADA, “Recent Advances of Toxic Cyanobacteria Researches”, Journal of Health Science, 45(3), 150-165, 1999). However, since the mechanism of liver diseases due to the microcystins has not been clear, there is no method for treating the poisoning actually. 
     Conventionally, various methods for treating microcystin have been studied. In order to prevent poisoning by microcystin, a method for suppressing the generation of water-bloom is generally used. 
     As the method for suppressing the generation of water-bloom, a method is adopted to add an algae proliferation inhibitor such as lysine to a closed-water area with generated water-bloom. In addition, a method is also adopted to spray an algaecidal copper ion compound over a closed-water area with generated water-bloom, as described in Japanese Patent Application Publication No. 9-239370. 
     In Japanese Patent Application Publication No. 8-33888, a method is also suggested to transmit ultrasonic waves to polluted water prepared by evacuating a part of a closed-water area with water-bloom generated, thereby crushing cells of water-bloom so as to destroy the algae. 
     However, the produced microcystin is accumulated in the cells of the cyanobacteria. Even if the water-bloom is removed by the above-described methods, a large amount of microcystin is released from the crushed cyanobacteria. Therefore, there is a problem that the closed-water area is increasingly polluted with microcystin. 
     In order to resolve this problem, Japanese Patent Application Publication No. 11-70395 discloses a method of oxidation degradation with ozone, and a method of oxidation with chlorine, as a method for treating microcystin produced from water-bloom. 
     However, in the method of adding an algae proliferation inhibitor, since the algae proliferation inhibitor itself is an organic substance such as lysine, then BOD concentration in a closed-water area is increased by the addition of the algae proliferation inhibitor, which causes organic substance pollution. 
     In the method described in Japanese Patent Application Publication No. 9-239370, since a large amount of copper ion compounds remains in water in a closed-water area, it is not preferable that the water is used to be drunk. 
     The method described in Japanese Patent Application Publication No. 8-33888 has drawbacks in that oscillation of ultrasonic waves requires a massive amount of energy, and the residual crushed cells cause secondary pollution such as eutrophication. 
     In the method described in Japanese Patent Application Publication No. 11-70395, and the method of oxidation with chlorine, it is possible to completely degrade microcystin itself in a closed-water area. However, in such methods, since ozone or chlorine is reacted with a large amount of contaminants in a closed-water area, there is problem in that microcystin cannot be degraded efficiently. In addition, there is also a problem that a toxic by-product such as trihalomethane is generated due to the strong oxidation ability of ozone or chlorine. 
     SUMMARY OF THE INVENTION 
     The present invention has been contrived in view of such circumstances, and an object thereof is to provide a method and an equipment for treating microcystin-containing water that can carry out biological treatment of extremely rapidly, and easily degradation and detoxification of microcystin which is contained in a closed-water area with generated water-bloom, without affecting the environment and ecosystem in the closed-water area. 
     In order to attain the aforementioned object, the present invention is directed to the method for treating microcystin-containing water which detoxifies microcystin in the microcystin-containing water, the method comprising the step of: bringing the microcystin-containing water into contact with  Sphingomonas  bacterium to degrade biologically the microcystin in the microcystin-containing water, wherein: the  Sphingomonas  bacterium is strain MDB1. 
     According to the present invention,  Sphingomonas  bacterium which makes contact with microcystin-containing water has a resolution of microcystin. Therefore, since microcystin in microcystin-containing water can be degraded biologically and detoxified, it is possible to prevent production of a by-product or presence of a harmful substance remaining in the treated water by the treatment. 
     Furthermore, since  Sphingomonas  bacterium can be naturally proliferated by degradation of microcystin, it is possible to reduce the running cost required for treatment considerably as compared with a conventional treatment method. 
       Sphingomonas  strain MDB1 was deposited under the Budapest Treaty as an international deposit on Nov. 9, 2005 under Accession No. FERM BP-10448 at the International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology (AIST) (AIST Tsukuba Central, 6, 1-1, Higashi 1-chome, Tsukuba-shi, Ibaraki-ken 305-8566, Japan), which is a transfer of a domestic deposit of  Sphingomonas  strain MDB1 made on Aug. 8, 2003 under Accession No. FERM P-19480 at the International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology (referred to as AIST, that address is AIST Tsukuba Central, 6, 1-1, Higashi 1-chome, Tsukuba-shi, Ibaraki-ken 305-8566, JAPAN) by Ho-Dong PARK (Assistant Professor, Ecosystem Analysis Laboratory, Department of Environmental Sciences, Faculty of Science, Shinshu University that address is 1-1, Asahi 3-chome, Matsumoto-shi, Nagano-ken 390-8621, JAPAN), and Hajime ISHIGURO (Representative, Hitachi Plant Engineering &amp; Construction Co., Ltd. that address is 1-14, Uchikanda 1-chome, Chiyoda-ku, Tokyo-to 101-0047, JAPAN). The strain MDB1 belonging to the genus  Sphingomonas  described above can degrade microcystin extremely faster than conventional bacterium for degradation microcystin while being able to be proliferated in an easy and stable manner at room temperature. Consequently, by use of this strain MDB1 as a member of  Sphingomonas  bacterium, microcystin-containing water can be detoxified extremely rapidly without requiring much labor for the treatment, and thus efficient treatment of microcystin-containing water can be provided in accordance with various circumstances. 
     Preferably, the  Sphingomonas  bacterium is sprayed over a water surface of a closed-water area so as to make contact with the microcystin-containing water, the closed-water area being a source of the microcystin-containing water. Herein, the term “closed-water area” includes a partially or wholly closed-water area such as a lake, dam, moat, and inland sea. 
     Although  Sphingomonas  bacterium can be efficiently proliferated in the presence of microcystin to be a dominant bacterium, the proliferation ability of  Sphingomonas  bacterium is lowered after microcystin is degraded, so that the dominant bacterium is one of the other microorganisms in microcystin-containing water. Therefore, when  Sphingomonas  bacterium is sprayed over the water surface of microcystin-containing water so as to make contact with the water, it is possible to effectively suppress uneven distribution of  Sphingomonas  bacterium in the microcystin-containing water, so that the time requiring for detoxifying microcystin can be further reduced. In addition, since it is also possible to minimize influence on the environment and ecosystem by spraying even after microcystin is degraded, the time and cost required after the treatment can be considerably reduced. 
     Preferably, the  Sphingomonas  bacterium is sprayed over the water surface of the closed-water area at a concentration of 10 8  cells/m 2  or higher. By spraying  Sphingomonas  bacterium at this concentration, it is possible to prevent the sprayed  Sphingomonas  bacterium from being predated by other organisms in a closed-water area, and to prevent degradation of microcystin by  Sphingomonas  bacterium from being inhibited. This rapidly allows  Sphingomonas  bacterium to be a dominant bacterium in a closed-water area, and thus the time required for detoxifying microcystin in a closed-water area can be minimized. 
     The present invention is also directed to the method for treating microcystin-containing water wherein: the  Sphingomonas  bacterium is immobilized so as to make contact with the microcystin-containing water. Preferably, the  Sphingomonas  bacterium is immobilized by being attached to an immobilizing material. Preferably, the  Sphingomonas  bacterium is immobilized by being entrapped in an immobilizing material. 
     Therefore, since  Sphingomonas  bacterium can evenly make contact with microcystin-containing water by making the bacterium immobilized to make contact with the water, it is possible to degrade and detoxify microcystin further efficiently. 
     In order to attain the aforementioned object, the present invention is directed to a equipment for microcystin-containing water which detoxifies microcystin containing in the microcystin-containing water, the equipment comprising: an introduction section which introduces the microcystin-containing water; a treatment section which degrades biologically the microcystin in the microcystin-containing water introduced from the introduction section by bringing the microcystin-containing water into contact with a plurality of immobilized members which immobilize  Sphingomonas  bacterium; an aeration device which aerates the treatment section; and a discharge section which discharges the water treated in the treatment section, wherein the  Sphingomonas  bacterium is strain MDB1. 
     According to the present invention, when microcystin-containing water is pumped up from a closed-water area polluted with microcystin by the introduction section, and makes contact with the pellets in the treatment section, microcystin contained in the water is degraded and detoxified by the  Sphingomonas  bacterium immobilized on or in the pellets as the immobilized members. Then, the water treated in the treatment section is fed back to the closed-water area as treated water via a feedback section. Since the strain MDB1 is used as the  Sphingomonas  bacterium, which is immobilized on or in the pellets provided in the treatment section, it is possible to simplify setting of treatment conditions in the treatment section. In addition, since the  Sphingomonas  bacterium can degrade microcystin at a degradation rate remarkably higher than that of conventional bacteria for degrading microcystin, microcystin can be degraded biologically and detoxified extremely rapidly. Therefore, since microcystin-containing water pumped up from the closed-water area makes contact with  Sphingomonas  bacterium extremely efficiently, it is possible to provide an equipment which minimizes a time required for purification of the closed-water area polluted with microcystin without secondary pollution. In addition, since microcystin-containing water in the treatment section is aerated by the aeration device, the water in the treatment section can be made more flowable so as to improve contact efficiency, and thus it is possible to improve ability of degrading microcystin by supplying air to  Sphingomonas  bacterium as aerobic microorganisms. 
     The present invention is also directed to the equipment for treating microcystin-containing water further comprising: a culture section which cultures the  Sphingomonas  bacterium and supplies a culture solution of the cultured  Sphingomonas  bacterium to the treatment section at a predetermined interval and in a predetermined amount of cell concentration of the  Sphingomonas  bacterium. 
     According to the present invention, the  Sphingomonas  bacterium is cultured in a culture section, and then the culture solution obtained by the culture section is supplied to the treatment section at a predetermined interval and in a predetermined amount so that a predetermined amount of cells of  Sphingomonas  bacterium are supplied to the treatment section and are attached to the pellets. Therefore, microcystin can be efficiently treated in a stable manner. 
     Preferably, the equipment is placed in a closed-water area as a source of the microcystin-containing water. Therefore, since it is not necessary to provide a space for treating microcystin-containing water, it is possible to reduce considerably the energy required for microcystin-containing water which is to be introduced into and discharged from a closed-water area. 
     As described above, according to the present invention, the strain MDB1 is particularly used as  Sphingomonas  bacterium to make contact with microcystin-containing water for degrading and detoxifying microcystin. Therefore, while the production of by-product due to the treatment can be prevented without adjusting the treatment environment or the like, it is possible to degrade microcystin at a degradation rate increased as compared with detoxification of microcystin by conventional biological treatment. In addition, it is also possible to detoxify microcystin in microcystin-containing water extremely rapidly and inexpensively, without causing secondary pollution due to the treatment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
         FIG. 1  is a diagram showing a family tree according to nucleotide sequences obtained by analyzing 16SrRNA of strain MDB1; 
         FIG. 2  is a graph showing a microcystin degradation rate of strain MDB1 according to the present invention in microcystin-containing water; 
         FIG. 3  is a graph showing a relationship between a microcystin degradation ratio and a sprayed cell concentration of strain MDB1 used in the present invention; 
         FIG. 4  is a graph showing a relationship between a microcystin degradation ratio and an immobilized cell concentration of strain MDB1 used in the present invention; 
         FIG. 5  is a general schematic drawing of a microcystin-containing water treatment equipment according to a first embodiment of the present invention; and 
         FIG. 6  is a general schematic drawing of an microcystin-containing water treatment equipment according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, there are no specific limitations to  Sphingomonas  bacterium used for treating microcystin-containing water. The present invention may also adopt the bacterium belonging to the genus  Sphingomonas  which can degrade and detoxify microcystin. In the present invention, strain MDB1 is used. The strain MDB1 is separated in a nutrient agar medium from the collected lake water with water-bloom proliferated in Lake Suwa, Nagano. 
     The deposited strain MDB1 is examined according to morphological characters, cultural characters, and physiological characters. The morphological characters of strain MDB1 are examined by inoculating the strain MDB1 into a medium and cultured at 30° C. for five days, using a light microscope and a transmission electron microscope. A nutrient agar medium (meat extract: 0.5 g/L, peptone: 1 g/L, sodium chloride: 0.5 g/L, agar: 1.0 g/L) or a nutrient broth medium (meat extract: 0.5 g/L, peptone: 1 g/L, sodium chloride: 0.5 g/L) is used as a basal medium. 
     Consequently, the strain MDB1 is rod shaped, 0.79±0.23 μm in length and 0.49±0.08 μm in width, and is gram-negative bacterium motile by flagella. Typically, the strain MDB1 grows well when cultured in a nutrient agar medium, and forms a round and convex yellow colony. The optimum growth conditions are 30° C. and pH 7.0 in aerobic culture. 
     In order to examine the chemical taxonomic characters of strain MDB1, the strain MDB1 are determined according to nucleotide sequences of 16SrRNA genes. Herein, the result is shown in a following Table 1. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 SEQUENCE LISTENING 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 &lt;110&gt; APPLICANT NAME: Tateo SUMINO, Takako OGASAWARA, and Ho-Dong PARK 
                   
               
               
                   
               
               
                 &lt;120&gt; TITLE OF INVENTION: METHOD AND EQUIPMENT FOR TREATING MICROCYSTIN- 
               
               
                 CONTAINING WATER 
               
               
                   
               
               
                 &lt;160&gt; NUMBER OF SEQ ID NOS: 1 
               
               
                   
               
               
                 &lt;210&gt; SEQ ID NO 1 
               
               
                   
               
               
                 &lt;211&gt; LENGTH: 1441 
               
               
                   
               
               
                 &lt;212&gt; TYPE: DNA 
               
               
                   
               
               
                 &lt;213&gt; ORGANISM:  Sphingomonas  sp. 
               
               
                   
               
               
                 &lt;400&gt; SEQUENCE: 1 
               
               
                    1 TGGAGAGTTT GATCCTGGCT CAGAACGAAC GCTGGCGGCA TGCCTAACAC ATGCAAGTCG 
               
               
                   
               
               
                   61 AACGAAGCCT TCGGGCTTAG TGGCGCACGG GTGCGTAACA CGTGGGAATC TGCCCTTAGG 
               
               
                   
               
               
                  121 TACGGAATAA CAGTTAGAAA TGACTGCTAA TACCGTATGA TGACTCCGGT CCAAAGATTT 
               
               
                   
               
               
                  181 ATCGCCTAAG GATGAGCCCG CGTCGGATTA GCTAGTTGGT GAGGTAAAGG CTCACCAAGG 
               
               
                   
               
               
                  241 CGACGATCCG TAGCTGGTCT GAGAGGATGA TCAGCCACAC TGGGACTGAG ACACGGCCCA 
               
               
                   
               
               
                  301 GACTCCTACG GGAGGCAGCA GTGGGGAATA TTGGACAATG GGCGAAAGCC TGATCCAGCA 
               
               
                   
               
               
                  361 ATGCCGCGTG AGTGAAGAAG GCCTTAGGGT TGTAAAGCTC TTTTACCCGG GATGATAATG 
               
               
                   
               
               
                  421 ACAGTACCGG GAGAATAAGC TCCGGCTAAC TCCGTGCCAG CAGCCGCGGT AATACGGAGG 
               
               
                   
               
               
                  481 GAGCTAGCGT TGTTCGGAAT CACTGGGCGT AAAGCGCACG TAGGCGGCTT TGTAAGTTAG 
               
               
                   
               
               
                  541 AGGTGAAAGC CCGGGGCTCA ACTCCGGAAC TGCCTTTAAG ACTGCATCGC TTGAATCTGG 
               
               
                   
               
               
                  601 GAGAGGTAAG TGGAATTCCG AGTGTAGAGG TGAAATTCGT AGATATTCGG AAGAACACCA 
               
               
                   
               
               
                  661 GTGGCGAAGG CGGCTTACTG GACCAGAATT GACGCTGAGG TGCGAAAGCG TGGGGAGCAA 
               
               
                   
               
               
                  721 ACAGGATTAG ATACCCTGGT AGTCCACGCC GTAAACGATG AGAACTAGCT GTCAGGGCCT 
               
               
                   
               
               
                  781 TTTAGGCTTT GGTGGCGCAG CTAACGCATT AAGTTCTCCG CCTGGGGAGT ACGGTCGCAA 
               
               
                   
               
               
                  841 GATTAAAACT CAAAGGAATT GACGGGGGCC TGCACAAGCG GTGGAGCATG TGGTTTAATT 
               
               
                   
               
               
                  901 CGAAGCAACG CGCAGAACCT TACCAGCGTT TGACATCCTG ATCGCGGTTA CCAGAGATGG 
               
               
                   
               
               
                  961 TTTCCTTCAG TTCGGCTGGA TCAGTGACAG GTGCTGCATG GCTGTCGTCA GCTCGTGTCG 
               
               
                   
               
               
                 1021 TGAGATGTTG GGTTAAGTCC CGCAACGAGC GCAACCCTCG TCCTTAGTTG CCATCATTCA 
               
               
                   
               
               
                 1081 GTTGGGCACT CTAAGGAAAC CGCCGGTGAT AAGCCGGAGG AAGGTGGGGA TGACGTCAAG 
               
               
                   
               
               
                 1141 TCCTCATGGC CCTTACGCGC TGGGCTACAC ACGTGCTACA ATGGCGGTGA CAGTGGGCAG 
               
               
                   
               
               
                 1201 CAAACCCGCG AGGGTGAGCT AATCTCCAAA AGCCGTCTCA GTTCGGATTG TTCTCTGCAA 
               
               
                   
               
               
                 1261 CTCGAGAGCA TGAAGGCGGA ATCGCTAGTA ATCGCGGATC AGCATGCCGC GGTGAATACG 
               
               
                   
               
               
                 1321 TTCCCAGGCC TTGTACACAC CGCCCGTCAC ACCATGGGAG TTGGTTTCAC CCGAAGGCTG 
               
               
                   
               
               
                 1381 TGCGCTAACC GCAAGGAGGC AGCAGACCAC GGTGGGATCA GCGACTGGGG TGAAGTCGTA 
               
               
                   
               
               
                 1441 ACAAGGTAGC CGTAGGGGAA CCTGCGGCTG GATCACCTCC TT 
               
               
                   
               
             
          
         
       
     
     The above-described nucleotide sequences of strain MDB1 are searched in a homology search system FASTA of National Institute of Genetics. Consequently, it is found that strain MDB1 is a microorganism taxonomically close to the genus  Sphingomonas , as shown in a family tree of  FIG. 1 . 
     Next, the method for culturing the strain MDB1 used in the present invention will be described. As the culture method in the present invention, a conventional method for culturing aerobic bacterium is generally used. Any of synthetic medium and natural medium may be used as a culture medium if the medium appropriately contains a carbon source, a nitrogen source, an inorganic substance, and a necessary proliferation promoting substance which are available for culturing. As the carbon source, saccharides such as glucose, starch, dextrin, mannose, fructose, sucrose, lactose, xylose, arabinose, mannitol, and molasses; organic acids such as acetic acid; alcohols such as glycerol; and the like, may be used alone or in combination. As the nitrogen source, ammonium chloride, ammonium sulfate, sodium nitrate, urea, peptone, meat extract, yeast extract, dry yeast, corn steep liquor, soybean meal, casamino acid, and the like, may be used alone or in combination. In addition, inorganic salts such as sodium chloride, potassium chloride, potassium phosphate, ferrous sulfate, calcium chloride, manganese sulfate, zinc sulfate, and copper sulfate are added as required. Furthermore, a trace component for promoting growth of the strain may be appropriately added. Liquid culture is most suitable for the culture method. The culture temperature is suitably 30° C. The strain can be suitably cultured by adjusting the medium to pH 5 to 10, and preferably pH 7. By inoculating one platinum loop into 150 ml of a nutrient broth medium and carrying out shaking culture at 30° C. and 120 rpm for four days, the medium can be adjusted to a cell concentration of 10 9  cells/ml. 
       FIG. 2  is a graph showing microcystin degradation rate of strain MDB1 in microcystin-containing water, showing the results of carrying out batch treatment in a state in which the strain MDB1 with a cell concentration of 4×10 8  cells/ml makes contact with microcystin-containing water. In  FIG. 2 , a dotted line shows degradation rates in the case of carrying out conventional biological treatment under the same conditions using sludge in a closed-water area with generated water-bloom. 
     In the graph of  FIG. 2 , almost no change in the microcystin concentration occurs in relation to time in the conventional treatment, as shown by the dotted line. When the treatment continued (not shown in the graph), the microcystin concentration is finally reduced after two days from the start of treatment, and the water is treated to finally attain a microcystin concentration of 1.0 μg/L or lower after 28 days. On the other hand, in the treatment using strain MDB1 according to the present invention, the microcystin concentration is drastically reduced concurrent with the start of the experiment, the water is degraded at a microcystin concentration of 1.0 μg/L or lower after 30 hours. Consequently, while the conventional biological treatment using the sludge mixed various microorganisms is carried out only in the order of days, microcystin is treated in the order of hours by use of strain MDB1, and therefore, it is found that the time required for treating microcystin can be reduced considerably. In addition, it is confirmed that the strain MDB1 can degrade all three types of microcystin (microcystin LR, microcystin RR, and microcystin YR) which are often detected in lakes or the like. Therefore, the strain MDB1 is a most suitable microorganism for biologically treating microcystin. 
     In the present invention, a method is also suggested in which the cultured strain MDB1 is sprayed at a predetermined concentration over a closed-water area which contains microcystin due to generation of water-bloom, as a method for bringing strain MDB1 into contact with microcystin-containing water. 
       FIG. 3  is a graph showing a relationship between a microcystin degradation ratio and a sprayed cell concentration of strain MDB1 used in the present invention. 
     As shown in the graph of  FIG. 3 , when strain MDB1 is sprayed at a cell concentration of 10 8  cells/m 2  or higher, preferably 10 10  cells/m 2 , then the sprayed strain MDB1 is proliferated in a closed-water area so that microcystin contained in the water can be efficiently degraded. On the other hand, when the strain MDB1 is sprayed at a cell concentration of 10 8  cells/m 2  or lower, the microcystin degradation ratio tends to be extremely reduced. As this reason, it is considered that an organism in a closed-water area over which the strain MDB1 has been sprayed predates the strain MDB1 or inhibits degradation of microcystin. 
     In the present invention, a method is also suggested that the cultured strain MDB1 is immobilized at a predetermined cell concentration to bring the strain into contact with microcystin-containing water, as another contacting method. As a result of studies on the method for immobilizing strain MDB1, it is found that an attachment immobilization method or an entrapping immobilization method is most suitable treatment for degradation of microcystin. 
     In a treatment using the attachment immobilization method, a culture solution of strain MDB1 is immobilized on an immobilizing material with many irregularities such as globular pellets, cylindrical pellets, gel pellets, or non-woven cloth pellets. Therefore, since the strain MDB1 can be stably attached to the immobilizing material, it is possible to degrade microcystin at an improved efficiency. 
     On the other hand, in a treatment using the entrapping immobilization method, first, a culture solution of strain MDB1 is mixed with an immobilizing material such as a monomer or a prepolymer. Next, the mixture is polymerized so that strain MDB1 is entrapped and immobilized in the gel to form pellets. Then, microcystin contained in microcystin-containing water is degraded by bringing the water into contact with the pellets. As the monomer material, it is possible to use acrylamide, methylenebisacrylamide, triacrylformal, or the like. As the prepolymer material, it is preferable to use polyethylene glycol diacrylate, or polyethylene glycol methacrylate, and a derivative thereof may also be used. Entrapping immobilization pellets are preferably shaped into globes, cubes, strings, or non-woven clothes, or the like having many irregularities. Therefore, since the contact area of the strain MDB1 increases, it is possible to improve the microcystin degradation ratio. 
       FIG. 4  is a graph showing microcystin degradation ratios of strain MDB1 immobilized at each cell concentration. 
     As shown in the graph of  FIG. 4 , it is found that microcystin can be treated at a high degradation ratio of 80% or higher when strain MDB1 is immobilized on or in pellets at a cell concentration of 10 6  cells/ml or higher, and preferably 10 7  cells/ml or higher. 
     Table 2 shows microcystin degradation rates in immobilization pellets as results of bringing the pellets on or in which strain MDB1 is immobilized at an initial concentration of 8×10 8  cells/ml into contact with microcystin-containing water with a microcystin concentration of 100 μg/L. 
     
       
         
               
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Microcystin degradation rate 
               
               
                 Type of pellets 
                 (μg/L-pellets/h) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Polyethylene globular attachment pellets 
                 30 
               
               
                 Carbon fiber string-shaped attachment 
                 80 
               
               
                 pellets 
               
               
                 Plate-shaped non-woven cloth attachment 
                 60 
               
               
                 pellets 
               
               
                 Acrylamide entrapping pellets 
                 120 
               
               
                 Polyethylene glycol diacrylate entrapping 
                 180 
               
               
                 pellets 
               
               
                   
               
             
          
         
       
     
     As shown in Table 2, it is found that, though microcystin can be degraded rapidly by both of the attachment immobilization pellets and the entrapping immobilization pellets, and microcystin degradation rate of the two types of entrapping immobilization pellets is particularly faster than that of the three types of attachment immobilization pellets. As the reason, it is considered that an optimum cell concentration can be maintained constantly in the entrapping immobilization pellets. 
     However, entrapping immobilization pellets have drawbacks in that such pellets require much cost and labor in production thereof, as compared with attachment immobilization pellets. Accordingly, by selecting attachment immobilization or entrapping immobilization according to the application, microcystin-containing water can be efficiently treated. 
     EXAMPLE 1 
     Treatment by Spraying Strain MDB1 
     In an Example 1, the above-described strain MDB1 is cultured in the nutrient broth medium of 2 liter at 30° C. for one week, and a culture solution obtained by culturing the strain MDB1 is centrifuged so as to separate an exclusively bacterial suspension from the medium components. Then, the separated bacterial suspension is adjusted so as to be a cell concentration of 10 8  cells/m 2 , and is sprayed over 4 m 2 -wide water surface of an experimental aquarium which is packed with 4 m 3  of water having a microcystin concentration of 20 μg/L, so as to carry out an experiment. According to the Example 1, as time passed since the start of the experiment, the microcystin concentration in the microcystin-containing water is reduced. After two weeks, the water can be treated so that a microcystin concentration is equal to or more than 1 μg/L which is defined as a standard by WHO. 
     As a modification of the Example 1, the above-described bacterial suspension is adjusted so as to be a cell concentration of 10 9  cells/m 2 , and is sprayed over the 4 m 2 -wide water surface of an experimental aquarium which is packed with 4 m 3  of water containing a microcystin concentration of 18 μg/L once a day, so as to carry out an experiment. In the modification, as time passed since the start of the experiment, the microcystin concentration in the microcystin-containing water is reduced. After five days, the water can be treated so that a microcystin concentration is equal to or more than 1 μg/L which is defined as a standard by WHO. 
     As described above, since the strain MDB1 aerobically degrades microcystin, then the strain MDB1 can be rapidly allowed to be a dominant bacterium in a closed-water area over having sprayed the strain MDB1 by adjusting the concentration of cells to be sprayed with reference to the water surface area in the above-described manner. Therefore, microcystin contained in the closed-water area can be rapidly degraded. 
     EXAMPLE 2 
     Treatment Using Entrapping Immobilization Method of Strain MDB1 
     In an Example 2, in order to carry out a following experiment, a closed-water area polluted with microcystin is treated using a microcystin-containing water treatment equipment  10  according to a first embodiment of the present invention shown in  FIG. 5 . 
     As shown in  FIG. 5 , in the treatment equipment  10 , microcystin-containing water is pumped up from a closed-water area by an introduction pump  12   a  which is attached to an introduction tube  12  as an introduction section, and the water is supplied to a reaction vessel  14  through the introduction tube  12 . A large number of pellets  16  are formed by entrapping and immobilizing the strain MDB1, and are incorporated in the reaction vessel  14  as a treatment section. A diffuser tube  18  as a diffusing device is disposed on the bottom of the reaction vessel  14  so as to diffuse air by driving a blower  18   a  attached to the diffuser tube  18 . Therefore, while the air is supplied to the reaction vessel  14  to promote aerobic treatment of strain MDB1 in the pellets  16 , the packed pellets  16  are stirred to increase the contact ratio. The reaction vessel  14  is provided with a discharge tube  20  as a discharge section, and the microcystin-containing water treated in the reaction vessel  14  as treated water is derived from the bottom of the reaction vessel  14  via the discharge tube  20 . 
     As a method for making the pellets  16  used in the water treatment equipment  10 , first, the above-described strain MDB1 is aerobically cultured in the nutrient broth medium at 30° C. for four days. Next, cells of the strain MDB1 is separated from the culture solution by centrifugation, and separated cells are adjusted so that the cell concentration of strain MDB1 is 10 9  cells/cm 3 . Next, the separated and adjusted bacterial suspension of strain MDB1 is mixed with 10% polyethylene glycol diacrylate, and then 0.25% potassium persulfate is added to the mixture so that the cells of strain MDB1 are entrapped and immobilized by polymerization of the polymer. Then, the pellets  16  are formed in globes with a diameter of 3 mm. 
     Microcystin-containing water containing microcystin of 10 μg/L is treated in the treatment equipment  10 , setting the flow rate so that the microcystin-containing water is retained in the reaction vessel  14  for 30 minutes. Consequently, in the treatment equipment  10 , the microcystin-containing water can be continuously treated in a stable manner that a microcystin concentration is 1 μg/L or lower. 
     EXAMPLE 3 
     Treatment Using Attachment Immobilization Method of Strain MDB1 
     In an Example 3, in order to carry out a following experiment, an agricultural pool  100  as a closed-water area containing microcystin is treated using a microcystin-containing water treatment equipment  30  according to a second embodiment of the present invention as shown in  FIG. 6 . 
     In the treatment equipment  30 , a culture tank  32  is disposed as a culture section and a supply section on one shore of the agricultural pool  100 , which cultures the strain MDB1 in the nutrient broth medium stored in the culture tank  32 . The cultured bacterial suspension of the strain MDB1 is supplied via a supply tube  36  to a treatment section  38  which is placed in the agricultural pool  100 , by driving a supply pump  34  attached to the supply tube  36 . The treatment section  38  is provided so that a plurality of net rings  40  are linked and fixed to a linking member  42 , and a plurality of floating members  44  are floated on the water surface. Preferably, the net rings  40  are composed of a material that the strain MDB1 supplied from the supply tube  36  can be attached to and immobilize on. A diffuser tube  46  is disposed in the lower part of the treatment section  38 , and aerates the attached and immobilized strain MDB1 on the net rings  40  by driving an air pump  48  placed on the one shore of the agricultural pool  100 . While an introduction section  50  is formed in the lower part of the treatment section  38 , a discharge section  52  is placed in the upper part of the treatment section  38 . Accordingly, in the treatment section  38 , while water of the agricultural pool  100  is flowed into the introduction section  50 , the water in which microcystin has been degraded is discharged as a treated water from the discharge section  52  to the agricultural pool  100 . 
     In order to carry out a following experiment, microcystin-containing water containing microcystin of 40 μg/L in the 1,000 m 3 -volume agricultural pool  100  is treated using the treatment equipment  30  described above. The culture tank  32  having a volume of 100 liter is used. A bacterial suspension is prepared by culturing the strain MDB1 for one week in the nutrient broth medium filled in the culture tank  32 , and is supplied to the treatment section  38  from the supply tube  36  at a flow rate of 10 L/day for 10 days. As a result, at 20 days after the start of the operation, microcystin contained in the agricultural pool  100  can be reduced to a concentration of 1 μg/L or lower. 
     It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.