| Microcystis aeruginosa, a dominant bloom-forming species in freshwaters, can result in a serious threat to freshwater ecosystems and human health because of its microcystin (MC) production. In the past 100 years, many studies have been carried out on the structure, toxicity, biosynthetic pathway and ecological roles of MC, however, the study on the influence of species competition on MC production of M. aeruginosa has not been reported. In this study, we studied the Microcystins-LR (MC-LR) production of M. aeruginosa that was co-cultured with Anabaena flos-aquae (cyanobacteria) and Scenedesmus obliquus (green algae), respectively, under different environmental conditions (temperature, light, nutrients and N/P). The major results are as follows:(1) Under co-cultured condition, M. aeruginosa severely inhibited the growth of A. flos-aquae, however A. flos-aquae had no effects on the growth of M. aeruginosa. The abundance of A. flos-aquae decreased by 68.2% under co-cultured condition compared with that under uni-cultured condition at 20 days after inoculation. Moreover, M. aeruginosa and S. obliquus exhibited a competitive relationship under co-cultured condition. The abundance of M. aeruginosa and S. obliquus under co-cultured condition decreased by 83.9% and 53.6%, respectively, compared with that under uni-cultured condition at 20 days after inoculation.(2) The production of MC-LR by M. aeruginosa was independent on its initial cell density based on HPLC analysis of the toxic compound.(3) The MC-LR production of M. aeruginosa was significantly reduced by 68.3%,67.5% and 49.8% respectively when it was co-cultured with Quadrigula chodatii, S. obliquus and A. flos-aquae compared to that under the uni-cultured condition.(4) The reduction of MC-LR production of M. aeruginosa under co-cultured condition with S. obliquus and A. flos-aquae was enhanced by the increase of temperature. For instance, the amount of MC-LR reduced by 38.3%,54.9% and 61.9% when M. aeruginosa was co-cultured with A. flos-aquae, and 34.3%,87.5% and 99.8% with S. obliquus at 15℃,25 oC and 35 oC, respectively.(5) MC-LR production of M. aeruginosa under co-cultured condition with A. flos-aquae was also affected by light intensity. MC-LR production of M. aeruginosa under co-cultured condition reduced by approximately 54% and 97% under low light intensity (10000 to 150001x) and high light intensity (20000 to 250001x), respectively, compared with that under the uni-cultured condition. However, light intensity did show significant effect on MC-LR production of M. aeruginosa when it was co-cultured with S. obliquus.(6) MC-LR production of M. aeruginosa when it was co-cultured with A. flos-aquae was influenced under different N/P ratio. Except for a reduction of 21.5% under N/P 1:1 The MC-LR production of M. aeruginosa increased by 80.7%,78.2%, 111.4% and 37.5%, respectively under the N/P ratio of 4:1,16:1,64:1 and 100:1 at 6 days after inoculation.(7) When the initial cell ratio of A. flos-aquae to M. aeruginosa were 96:4,93:7, 90:10,70:30,50:50 and 30:70, MC-LR production of M. aeruginosa decreased by 8.0%,18.7%,22.1%,32.6%,47.8% and 62.1%, respectively when the two species were co-cultured.(8) According to qPCR analysis, the expression of microcystin synthetase gene mcyB and mcyD were inhibited by about 1/5 when M. aeruginosa was co-cultured with A. flos-aquae.Conclusion:The MC-LR production of M. aeruginosa was significantly inhibited when it was co-cultured with other algal species, and affected by a variety of environmental factors (N/P, temperature and light intensity) and the ratio of species density. In addition, the influence of population competition on MC-LR production of M. aeruginosa was mainly achieved by inhibiting the expression of microcystin synthetase gene mcyB and mcyD. These results provided us a new view of the roles of species competition in the production of microcystin during water bloom outbreak in the lake. |
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