Cyanobacterial Toxin Degrading Bacteria: Who are they?

Cyanobacteria are considered as one of the most significant groups of Bacteria with major roles in both aquatic and terrestrial ecosystems. Some genera within the Cyanobacteria also produce beneficial bioactive metabolites that can be extracted and used in fields such as medicine. However, cyanobacteria are also known for the production of several types of toxins (cyanotoxins) which affect many organisms, primarily in aquatic environments. A lot of cyanotoxins are generally persistent to physicochemical factors, but some bacterial genera have been found to be capable of successfully degrading these compounds.

Among five other types of cyanotoxins mentioned in this paper, microcystins (MCs) are the most ubiquitous consisting of a group of cyclic peptides, and are produced by a variety of genera such as Microcystis and Anabaena. Many approaches have tried to remove MCs from water systems, yet microbial degradation is considered as most effective. One cyanotoxins-degradation pathway for MCs involves the mlr gene cluster and has been identified in a Sphingomonas sp. strain (α-Proteobacteria). This gene cluster codes for enzymes that hydrolyse peptide bonds and are responsible for the enzymatic opening of the ring structure of MCs. 

For the identification of major taxa containing strains of bacteria that carry the mlr genes, or that can grow on MCs, 16S rRNA genes of these bacterial isolates were retrieved from GenBank and the phylogenetic relationship established. The authors recognised that the majority of cyanotoxins-degrading bacteria belong to α-/β-Proteobacteria and Actinobacteria, and most isolates carrying the mlr gene sequence were close relatives of the Sphinogomonadaceae family, which are lately studied for biotechnological purposes.

Sphingomonads are widely distributed in oligotrophic ecosystems and they are known to be able to transform hazardous organic compounds, although their ecological role is not well understood and cultivation failures are due to lack of appropriate culture media. The findings of this study suggest that Sphingomonad strains are one of the most active groups of bacteria degrading cyanotoxins. Some Actinobacteria were also found to occur where MCs were present although they lack the mlr genes, implying that are other genes responsible for the degradation of cyanotoxins which are yet to be identified.

So far, most identified MC-degrading bacteria are uncommon in ecosystems with high cyanotoxin levels, which could be a consequence of the usage of wrong media, potentially failing to culture many strains. Metagenomic analyses would be more suitable to study the diversity of bacterioplankton during blooms of MC-producing cyanobacteria so that more co-occurring strains could be identified and mlr genes, or genes coding for other cyanotoxin-degrading pathways could be discovered.

Kormas, K.A., Lymperopoulou, D.S., 2013. Cyanobacterial Toxin Degrading Bacteria: Who Are They? BioMed Research International, Volume 2013.

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