Photosynthesis regulates succession of toxic and nontoxic strains in blooms of Microcystis (Cyanobacteria)

Microcystis are common cyanobacteria of eutrophic lakes that can form harmful blooms in warm seasons. The genus includes toxic and nontoxic species/strains, and they show competitive succession in natural environments that is not well understood. Here we show the effects of microcystin production on species competitiveness when cultured under different conditions. When phosphate is rich, toxic Microcystis grew faster than nontoxic Microcystis. However, phosphate limitation significantly promoted photosynthesis of nontoxic Microcystis. Under phosphate limitation, toxic Microcystis was more sensitive than nontoxic Microcystis, and the maximum relative electron transport rate (rETR(max)) of nontoxic Microcystis was significantly higher than that of toxic Microcystis. Significantly higher light quantum efficiencies suggested that nontoxic Microcystis was more competitive than toxic Microcystis (P < 0.05) under light limitation. When in high light, the ratio of closed Photosystem II (PSII) to total PSII was higher and more sensitive to light in nontoxic than toxic Microcystis. Intense non-photochemical quenching (qN) was induced by high light for toxic Microcystis, while there was a serious limitation of qN formation leading to photoinhibition in nontoxic strains. Deconvolution of qN composition suggested that lots of qN(s) (slow component of qN) production might be caused by a reversible photoinhibition mechanism for toxic Microcystis. These observations suggest a photosynthetic explanation for bloom succession in which in early stages of the bloom, high phosphate concentration favours the rapid growth of toxic Microcystis. Subsequent bloom formation would lead to the decrease of water transparency and phosphate concentration that then promotes dominance of nontoxic Microcystis.