Environmental factors controlling colony formation in blooms of the cyanobacteria Microcystis spp. in Lake Taihu, China

摘要

Nitrogen (N) and phosphorus (P) over-enrichment has accelerated eutrophication and promoted cyanobacterial blooms worldwide. The colonial bloom-forming cyanobacterial genus Microcystis is covered by sheaths which can protect cells from zooplankton grazing, viral or bacterial attack and other potential negative environmental factors. This provides a competitive advantage over other phytoplankton species. However, the mechanism of Microcystis colony formation is not clear. Here we report the influence of N, P and pH on Microcystis growth and colony formation in field simulation experiments in Lake Taihu (China). N addition to lake water maintained Microcystis colony size, promoted growth of total phytoplankton, and increased Microcystis proportion as part of total phytoplankton biomass. Increases in P did not promote growth but led to smaller colonies, and had no significant impact on the proportion of Microcystis in the community. N and P addition together promoted phytoplankton growth much more than only adding N. TN and TP concentrations lower than about TN 7.75-13.95 mg L~(-1) and TP 0.41-0.74 mg L~(-1) mainly promoted the growth of large Microcystis colonies, but higher concentrations than this promoted the formation of single cells. There was a strong inverse relationship between pH and colony size in the N&P treatments suggesting CO_2 limitation may have induced colonies to become smaller. It appears that Microcystis colony formation is an adaptation to provide the organisms adverse conditions such as nutrient deficiencies or CO_2 limitation induced by increased pH level associated with rapidly proliferating blooms.