Comparative Protein Expression in Different Strains of the Bloom-forming Cyanobacterium Microcystis aeruginosa

Abstract

Toxin production in algal blooms presents a significant problem for the water industry. Of particular concern is microcystin, a potent hepatotoxin produced by the unicellular freshwater species Microcystis aeruginosa. In this study, the proteomes of six toxic and nontoxic strains of M. aeruginosa were analyzed to gain further knowledge in elucidating the role of microcystin production in this microorganism. This represents the first comparative proteomic study in a cyanobacterial species. A large diversity in the protein expression profiles of each strain was observed, with a significant proportion of the identified proteins appearing to be strain-specific. In total, 475 proteins were identified reproducibly and of these, 82 comprised the core proteome of M. aeruginosa. The expression of several hypothetical and unknown proteins, including four possible operons was confirmed. Surprisingly, no proteins were found to be produced only by toxic or nontoxic strains. Quantitative proteome analysis using the labelfree normalized spectrum abundance factor approach revealed nine proteins that were differentially expressed between toxic and nontoxic strains. These proteins participate in carbon-nitrogen metabolism and redox balance maintenance and point to an involvement of the global nitrogen regulator NtcA in toxicity. In addition, the switching of a previously inactive toxin-producing strain to microcystin synthesis is reported. Molecular & Cellular

Microcystin Production and Regulation under Nutrient Stress Conditions in Toxic Microcystis Strains

“…Changes in the cell redox state are processes that can be intimately linked to the generation of reactive oxygen species and oxidative stress in cyanobacteria (21). Therefore, we suggest that the increase of microcystin observed in our experiments is linked to oxidative stress caused by severe nutrient limitation.…”

“…It was also shown that the addition of hydrogen peroxide had less detrimental effects on toxic Microcystis strains than on nontoxic ones (20), suggesting protection by microcystin. Another report (21) demonstrated the increase of microcystin synthesis, but not of the gene transcription for toxins, in cells exposed to severe limitation of iron. Also, experiments run under iron deficiency (22) found increasing transcription levels of the mcy gene and of microcystin synthesis.…”

Microbial communities reflect temporal changes in cyanobacterial composition in a shallow ephemeral freshwater lake

“…There is an evident difficulty in correlating many environmental parameters with cyanobacterial community composition, toxicity or toxigenicity across such studies (Rinta-Kanto et al, 2009;Al-Tebrineh et al, 2011;Otten et al, 2012;Lee et al, 2014;Ngwa et al, 2014). In part, this lack of correlation can be attributed to genomic variability among closely related strains (Humbert et al, 2013;Sinha et al, 2014), giving rise to differing growth optima and potential to produce a myriad of toxic and non-toxic metabolites (Humbert et al, 2013), changes in the genome copy number throughout the growth phase (Griese et al, 2011) and uncertainty regarding the control of regulatory systems that direct the expression of toxic metabolites (Kaebernick et al, 2000;Alexova et al, 2011;Carneiro et al, 2013;Neilan et al, 2013;Rzymski and Poniedziałek, 2014;Makower et al, 2015).…”

Transcriptomics-Aided Dissection of the Intracellular and Extracellular Roles of Microcystin in Microcystis aeruginosa PCC 7806

“…The facts that microcystin binds covalently to cysteines of proteins under stress conditions and that RubisCO is a predominant binding partner provide a mechanistic clue to understanding these observations (17). Differences in the carbon-nitrogen metabolism and redox maintenance also were discussed in a comparative proteomic study of toxic and nontoxic strains (18). Evidence for the binding of the global nitrogen regulator NtcA protein in the bidirectional mcy promoter region suggested regulatory links between the carbonnitrogen metabolism and microcystin biosynthesis (18,19).…”

“…Differences in the carbon-nitrogen metabolism and redox maintenance also were discussed in a comparative proteomic study of toxic and nontoxic strains (18). Evidence for the binding of the global nitrogen regulator NtcA protein in the bidirectional mcy promoter region suggested regulatory links between the carbonnitrogen metabolism and microcystin biosynthesis (18,19). At present, it is not clear how much of the phenotypic and proteomic differences are governed on the transcriptional level, whether the intra-or the extracellular portion of microcystin has a larger impact on cellular physiology, and to what extent the intra-and extracellular roles are connected.…”