| Algae and bacteria are important members of the freshwater ecosystem in food chains. Algae, as a kind of important primary producers, can secret extracellular substances to their surrounding environment during their growth process, which result in a formation of special microenvironment, phycospere, where the bacteria are attracted. It has been proved the presence of a variety of bacteria within phycospere. Bacteria within phycospere can interact with algae and form complex interactions. On one hand, algae create a niche for the bacteria, where selectively attract some bacteria to form a special bacterial groups within phycospere; On the other hand, bacteria within phycospere can also produce some of metabolic active substances such as antibiotics, anti-tumor substances and toxins, which can adjust the algae population density by promoting or inhibiting the growth of algae.In view of the complex interaction between bacteria and algae and bacterial potential biological activity, we study the isolation, identification and ecological effects of strain Ma-B1, which is aimed to provide theoretical support for the control of Microcystis bloom and to provide a reference for strain Ma-B1’s development and utilization. The major results are as follows:1. By the traditional bacterial plate culture method, epiphytic bacteria Ma-B1 is isolated and identified from the cell surfaces of Microcystis aeruginosa from Taihu which is dominant in blooms. The strain Ma-Bl is identified to be Bacillus methylotrophicus based on morphology, physio-biochemical reaction and 16s rDNA sequence analysis.2. By the method of determination cell growth, the interaction relationship between strain Ma-B1 and M. aeruginosa was studied. M. aeruginosa can promote the growth of strain Ma-B1 by the secretion of metabolites, but the metabolites of strain Ma-B1 exhibit significant growth inhibition against M. aeruginosa. The results of GC/MS analysis indicate that the interaction of bacteria within phycospere and algae is related to the metabolites secreted by them.3. By mycelial growth inhibition assay, antifungal activities of strain Ma-B1 are detected. Resultes indicate that the culture filtrate (OD600=1.95) of strain Ma-B1 show good inhibitory activities against phytopathogen Alternaria alternate, Rhizoctonia solani, Glomerella glycines, Geotrichum candidum, Rhizopus sp., Verticillium sp. and Phytophthora capsiciwiih the inhibition rate 94.11%、91.03%、71.42%、65.29%、 60.59%、60.16% and 51.04%, respectively.4. Two compounds that have inhibitory effects on phytopathogens and no response on mass spectrometry are obtained from metabolites of strain Ma-B1 by macroporous resin, silicon gel and HPLC.5. By the single factor and orthogonal experiments, the medium components and culture conditions of strain Ma-B1 producing antimicrobial metabolites are optimized. The optimized culture condition for Strain Ma-B1 is as follows:Culture medium (1L):40 g mannose,40 g yeast extract,30 g NaCl.Inoculation:1%(v/v) of the overnight cultures of Strain Ma-B1 in a 50 mL flask containing 500 mL culture medium, and incubation in a shaking incubator at 120 rpm, 33 ℃ for 48 h. Under the optimum condition, the inhibition rate against G. candidum of Ma-B1 strain is 93.87% which is higher 41.84% than that before optimization. In addition, the anti-fungal ingredients of strain Ma-B1 have good stability to temperature (<100 ℃) and low pH(pH<8), but sensitive to high pH(pH>8); The culture medium of strain Ma-B1 (The concentration is 109cells/mL) towards silkworm has good bio-security.In summary, strain Ma-B 1 not only inhibit the growth of the dominant bloom-forming species, M. aeruginosa, but also may play an important role in the regulation of Microcystis population density, which are worthy of intensive study. In addition, strain Ma-B1 show good performance in inhibitory activities against phytopathogens with wider action spectrum, which has the potential of developing new biocontrol agent or new antifungal lead compounds and need further study. |
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