Microcystis

Microcystis is a genus of freshwater cyanobacteria, including 22 main European and tropical morphospecies such as Microcystis aeruginosa, flos-aquae, and wesenbergii (Komárek and Komárková, 2002). Microcystis is a unicellular colony-forming genus. Because Microcystis cells contain gas vesicles, they are buoyant, which allows for the formation of surface scums. These high density surface scums are associated with a plethora of negative environmental consequences, including production of the hepatotoxin microcystin.
Microcystins are harmful to organisms big and small, from microalgae to mammals. Humans are exposed to microcystins primarily through drinking contaminated water, but may also be exposed through recreational contact, food, or hemodialysis.
Microcystins are heptapeptides which are synthesized nonribosomally. Microcystins are the most structurally diverse group of cyanobacterial toxins, containing approximately 90 isomers, which differ in degree of methylation, hydroxylation, epimerization, peptide sequence, and toxicity (Welker and von Döhren, 2006). The mcy gene cluster codes for microcystin production in Microcystis. The mcy genomic locus covers 55 kilobases and includes 10 genes which are organized into 2 bidirectionally transcribed operons, mcyA-C and mcyD-J (Tillett et al., 2000). The two operons are separated by a 750 base pair promoter region (Kaebernick and Neilan,
However, many of these correlations are believed to reflect the effect of environmental parameters on Microcystis cell growth, as opposed to directly affecting microcystin transcriptional regulation, since microcystin production is believed to be proportional to cell growth rate (Kaplan et al., 2012; Orr and Jones, 1998). Nevertheless, a few key environmental stimuli are believed to have a direct effect on mcy gene transcription, chief among them light and iron-limiting conditions. The effects are manifested in small adjustments in mcy transcription as opposed to on-off regulation. For example, the abundance of mcy transcripts has been shown to be upregulated above a critical light threshold (Kaebernick et al., 2000) and under iron deplete conditions (Sevilla et al., 2008). Additionally, both mcy operons exhibit alternate start sites of transcription under differing light levels (Kaebernick et al., 2002). It’s also interesting to note that individual genes in the mcy cluster exhibit differing levels of upregulation under oxidatively stressful conditions, indicating that genes are individually regulated within the mcy gene cluster (Straub et al., 2011). The cause of this differential regulation amongst mcy genes is