Abstract
Toxic cyanobacteria such as Microcystis aeruginosa are a worldwide concern in freshwater reservoirs. Problems associated with their mass occurrence are predicted to increase in the future due to global warming. The hepatotoxic secondary metabolite microcystin is of particular concern in this context. This study aimed to determine whether co‐occurring microorganisms influence the expression of microcystin biosynthesis genes. To this end, we performed cocultivation experiments and measured mcyB and mcyD transcripts in M. aeruginosa using RT‐qPCR. We utilized representatives from three different plankton groups: the picocyanobacterium Synechococcus elongatus, the unicellular flagellate grazer Ochromonas danica, and virioplankton from two different lakes. The presence of S. elongatus significantly increased mcyB and mcyD transcription in M. aeruginosa. Cocultivation with the mixotrophic chrysophyte O. danica did not increase the transcription of mcyB and mcyD; in fact, mcyD transcripts decreased significantly. The virioplankton size fraction of environmental water samples induced a significant increase in mcyB and mcyD transcription when obtained from lakes with cyanobacterial blooms. Our results show that co‐occurring microorganisms influence the expression of microcystin biosynthesis genes in M. aeruginosa.
Highlights
Many strains of Microcystis aeruginosa and other cyanobacteria produce the toxic secondary metabolite microcystin, which inhibits protein phosphatases in eukaryotes (MacKintosh, Beattie, Klumpp, Cohen, & Codd, 1990) and is toxic to humans and animals.microcystin is a growing concern in freshwater management worldwide (Moreira, Spillane, Fathalli, Vasconcelos, & Antunes, 2014; Sivonen & Jones, 1999; Van Gremberghe et al, 2011).The adverse effects of microcystin on aquatic organisms have been studied intensely
Cocultivation of M. aeruginosa with the picocyanobacterium S. elongatus or spent medium from S. elongatus cultures resulted in increased transcription of microcystin biosynthesis genes in M. aeruginosa (Figure 1)
For mcyD transcripts, a significant increase was found for cultures exposed to S. elongatus cells (p = .025) but not cultures exposed to mere spent medium of S. elongatus cells (p = .064)
Summary
Many strains of Microcystis aeruginosa and other cyanobacteria produce the toxic secondary metabolite microcystin, which inhibits protein phosphatases in eukaryotes (MacKintosh, Beattie, Klumpp, Cohen, & Codd, 1990) and is toxic to humans and animals. The adverse effects of microcystin on aquatic organisms have been studied intensely. Much less is known about the inverse relationship, that is, the effect that co-occurring organisms might have on microcystin producers such as Microcystis sp. One of the few studies to address this question found an increase in McyB protein in Microcystis sp. Colony formation as a response to grazing danger has been observed but is not restricted to toxic strains (Fulton & Paerl, 1987a; Yang, Kong, Shi, & Cao, 2006)
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