SLOT | Introduction
Cyanobacteria are found in many different aquatic or terrestrial environments such as lakes, oceans and rivers, soil and cliffs or more extreme habitats such as geysers or biologicals crusts formed in arctic or aride climate. The ability to form blooms in lakes and rivers under certain circumstances (such as water stratification and eutrophication) and the rich metabolic pathways to produce toxins (microcystins, nodularin, and others) and bioactive metabolites (anabaenopeptides, aeruginosins, cyanopeptolins, among others) make them an interesting subject of study.
In general the changing climate is leading us to conditions where cyanobacterial blooms are going to be more prolific , affecting the usage of water sources and recreation. Certain cyanobacteria such as Microcystis and Planktothrix form algal blooms associated with potentially high concentrations of the above mentioned toxins causing human diseases and death of animals either through drinking water exposure or the food chain. Thus those blooms are nowadays called cyanobacterial harmful algal blooms (cHAB) because containing cyanotoxins.
Landsat 8 real color image of cHAB formed by Nodularia spumigena in the Baltic Sea (23-July-2018)
Lake Erie Bloom in Michigan, Ohio (24-Sept-2017) formed by Microcystis aeruginosa and Planktothrix agardhii. NASA Earth observatory https://earthobservatory.nasa.gov/
The most common toxins produced by cyanobacteria are the microcystins, with more than 200 described structural variants. Microcystins are hepatotoxic heptapeptides produced by non-ribosomal peptide synthetases (NRPS), using (non-)proteinogenic amino acids. The microcystin general structure is composed of (1)D-Ala-(2)X-(3)D-MeAsp-(4)Z-(5)Adda-(6)D-Glu-(7)Mdha, where X and Z denote variable L-amino acids. Microcystins and nodularin are toxic to eukaryotic organisms because of the inhibition of protein phosphatases 1 and 2A. Various (not necessarily) exclusive functions have been proposed, reaching from protection against predators and chemical defense to the protection of cellular proteins under oxidative stress conditions.
Microcystin molecule conserved structure and its variation. Meyer et al. (2016), Cell Chemical Biology, DOI: https://doi.org/10.1016/j.chembiol.2016.03.011
Analogous to microcystins the anabaenopeptides are bioactive hexapeptides produced by NRPS and are comprised of a ring of five amino acids fused to an exocyclic amino acid through the ureido linkage at the conserved D-Lys. As a group the anabaenopeptides show variability in bioactivity, such as acting as inhibitors of carboxypeptidases A and B, protein phosphatase 1, and serine proteases such as (chymo)trypsin.
Anabaenopeptide molecule general structure and the genetic basis of its synthesis. Entfellner et al. (2017) Front. Microbiol. https://doi.org/10.3389/fmicb.2017.00219
These peptides both are found widely spread among aquatic cyanobacteria occurring in freshwater and marine habitats as well as lower invertebrates such as sponges. Among terrestrial cyanobacteria prominent representatives are found in lichens and soil-dwelling cyanobacteria. Understanding the regulation of toxin or bioactive peptide synthesis at the (sub)cellular level of various genotypes is considered essential to understand the factors leading to the genetic variability observed in blooms, such as the co-occurrence of toxic and nontoxic genotypes in water (e.g. Chen et al. 2019, BMC Microbiology, https://doi.org/10.1186/s12866-016-0639-1 ). In general these peptides occur at high intracellular concentrations (i.e. one promille to half percent of cellular dry weight) and are potentially actively or passively released into the water. The conditions when those peptides are transported out of the cell are not known but would be also relevant to protection and water management.