| Cyanobacterial bloom leads to serious environmental problem. Microcystis spp. are the most common bloom-froming cyanobacteria. Microcystis can produce hepatotoxic microcystins. The toxins are great health risks for animals as well as human beings. Microcystis always occurs in a colony under natural conditions, but exists mainly as single cells and a few paired cells in axenic cultures in the laboratory. The colony can adapt to the severe environment better than the unicellular. The aggregation of cells is helpful to vertical migration and defense against predation pressure, and it is helpful for Microcystis to grow fast and dominate over other phytoplanktons. It is speculated that the difference of the exopolysaccharides between colonial and unicellular form might lead to their morphological difference. Slime polysaccharides embed the Microcystis cells in the colony. The exopolysaccharide could play a key role in the formation of Microcystis colony. Additionally, the exopolysaccharides play a significant role in the microbial food web, metal complexation, and formation of aggregates and transparent exopolymeric particles in theaquatic ecosystem.The water samples were collected from Lake Taihu during the bloom-forming period on August 22 and on October 15,2009. Microcystis were isolated and cultured in the laboratory. The representive unicellular and colonial strains were selected for further research. Effects of nitrogen, phosphorous and calcium conditions on the growth, colony size and exopolysaccharide production of Microcystis were studied. Nitrogen and phosphorus limitation inhibited the growth of both colony and unicell, and the colony grew slower than unicell at the same condition. Nitrogen limitation enhanced the production of exopolysaccharide of unicell. When the nitrate concentration was 187.48 mg/L, the polysaccharide production of unicellular increased by 1.39 fold. Under phosphorus limitation condition, the synthesis of polysaccharide was enhanced, and the enhancement was stronger than that of nitrogen limitation. The maximum polysaccharide production of unicell was increased by 1.75 fold and that of colony was increased by 1.71 fold. The polysaccharide production of colony was higher than that of unicellular under the same phosphrous concentration. Nitrogen and phosphorus limitation had no obvious influence on the conoly size of Microcystis. High calcium level inhibited the growth and polysaccharide production of unicell when the calcium concentration was higher than 19.62 mg/L. When the calcium concentration increased from 9.81 mg/L from 29.43 mg/L, the colony size increased. The change of calcium concentration had no effect on growth of colony. The colony produced more polysaccharide at high calcium levels. When the calcium concentration was 29.43 mg/L, the polysaccharide production of Microystis colony increased by 1.1 fold. The change of nitrogen, phosphorus and calcium concentration in the unicellular culture didn’t result in the colony formation of Microcystis. The previous reports normally studied the exopolysaccharide production of unicellular Microcystis. In this study, we investigated both unicellar and colonial Microcystis, and found that phosphorus limitation and incrase of calcium concentration was able to increase the exopolysaccharide production. Although phosphorus limitation and incrase of calcium concentration resulted in the increase of exopolysaccharide production in colony, only the increase of calcium concentration leaded to the increase of colony size. This indicated that the increase of calcium concentration might alter the physicochemical properties of the exopolysaccharide besides the increased polysaccharide production, and thus leaded to the increase of colony size. Our data would provide important information for the study on the exopolysaccharide release and colony formation of Microcystis in nature, and the data on colonial Microcystis would be more important. |
PDF Full Text Request