Light affects colony size and buoyancy in two buoyant types of Cyanobacterium Microcystis

摘要

During the cyanobacterial Microcystis bloom, vertical movement is directed either upward or downward. This movement, known as heterogeneity in buoyancy, occurs even at the same light intensity. In this study, four buoyant(floating on surfac) and two non-buoyant (sinking on bottom) colonial Microcystis were used to examine the effects of light intensity on their colony size and buoyancy. With the increase of light intensity (80-200 μmol m-2s-1), the colony sizes of all six Microcystis were dramatically enlarged. The enlargement was accompanied by the increase of extracellular polysaccharide (EPS) production in buoyant strains, and the increase of growth rather than EPS in non-buoyant strains. Based on the phenomena that buoyant strains sank downwards while non-buoyant strains floated upwards at high light intensity, gvp expression and photosynthetic activity were determined in order to understand the regulation of buoyancy in two buoyant types of Microcystis. It was found that the transcription level of gas vesicle and Fv/Fm decreased rapidly in four buoyant Microcystis, but increased in the two non-buoyant species at high light intensity, indicating that variation of buoyancy is related to the difference in adaptation to light intensity between the strains of two buoyant types. In addition, the velocity of vertical migration was increased significantly at high light intensity for all tested strains. In conclusion, the vertical migration and buoyancy regulation in cyanobacterial Microcystis by light intensity were correlated with colony enlargement, EPS production and gvp transcription. Our results provide one of solid evidences how light mediate and play a role in the dominancy of cyanobacterial Microcystis bloom in ever-changing environments.