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.
Microcystin exposure causes both acute (WHO, 1998) and chronic (Zhou et al, 2002) effects in humans through inhibition of protein phosphatases 1 and 2. For example, long-term consumption of low levels of microcystins has been linked to increased risk of liver cancer (Ueno et al., 1996) and colorectal cancer (Zhou et al., 2002).
Microcystin biosynthesis
Cyanobacteria produce a variety of bioactive secondary metabolites, including hepatotoxic microcystins.
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,
2001).

Regulation of microcystin production Microcystin production has been correlated with a multitude of environmental parameters (nutrients, trace metals, light, pH, temperature, etc.), indicating that a wide variety of factors may play a role in mcy transcriptional regulation.
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
unknown.
Further evidence for the role of iron as well as N in the transcriptional regulation of microcystin biosynthesis is found in the presence of ferric uptake regulator (Fur) and three global N regulator (NtcA) transcription factor binding sites in the mcy promoter region (Kaebernick et al., 2002). Fur is involved in iron availability and redox status (which is affected by both iron and light conditions). NtcA, on the other hand, is involved in N availability and redox status. Since predicted changes in mcy transcript abundance under differing N conditions were not seen experimentally, the importance of NtcA on microcystin regulation likely lies in its connection to redox status, as opposed to N availability (Sevilla et al., 2010).
Non-toxic Microcystis
Some Microcystis strains do not have the ability to produce microcystin. At the biochemical level, non-toxic strains are unable to produce microcystin because they lack some or all of the microcystin synthetase mcy genes (Meißner, et al., 1996). Lack of even a single mcy gene results in the inability to produce microcystin (Dittmann et al., 1997). Phylogenetic evidence indicates that microcystin biosynthesis evolved early on in cyanobacterial evolutionary history. The last common ancestor of a great number of cyanobacteria had the microcystin synthetase genes, which means that modern day non-toxic Microcystis have lost the ability over time. Despite their inability to synthesize microcystin, non-toxic strains do have the genes to produce other nonribosomal peptides, which may fulfill a similar role as the toxin (Rantala et al., 2004).
1.3 Oxidative stress in cyanobacteria
Reactive oxygen species
Prior to the evolution of cyanobacteria, the Earth had a reducing atmosphere (Dietrich et al., 2006). Around 3 billion years ago, cyanobacteria evolved and began oxygenating the atmosphere through photosynthesis (Brocks et al., 1999). As the first O2-producers, cyanobacteria were also the first organisms to encounter the damaging effects of the reactive oxygen species (ROS) unavoidably produced as a byproduct of aerobic metabolism. ROS include the superoxide anion (O2-), the hydroxyl radical, and hydrogen peroxide (HOOH), among others (Latifi et al., 2009). As a stable diradical and a weak univalent electron acceptor, O2 does not efficiently oxidize amino acids or nucleic acids. O2-, HOOH, and the hydroxyl radical are stronger univalent oxidants and are highly reactive with most biomolecules. The hydroxyl radical in particular is extremely reactive, with reaction rates limited only by diffusion rates (Imlay, 2003).