An investigation on the effects of cyanopeptides on the growth and secondary metabolite production of Microcystis aeruginosa PCC7806.

Cyanobacteria are one of the oldest forms of photosynthetic life and may have contributed significantly to the evolution of oxygen into the then anoxic environment. Cyanobacteria are also one of the best sources of natural secondary metabolites (cyanopeptides) some of which have harmful effects on the ecosystem, while others may be beneficial. It is known that these secondary metabolites are continuously produced during growth, however, it is not known whether the producing cyanobacteria actually benefit from these metabolites. The overarching aim of this study was to answer the question Why do cyanobacteria produce secondary metabolites?. With this aim in mind, preliminary work focused on understanding the growth and secondary metabolite production characteristics of Microcystis aeruginosa PCC7806. The technique of labelling secondary metabolites with 15N was successfully employed in differentiation and quantification of ex-novo and de-novo metabolites. The effect of exogenous cyanopeptides such as microcystins, aerucyclamides, anabaenopeptins, aeruginosamide, cyanopeptolin and aeruginosin on M. aeruginosa PCC7806 was evaluated using a rapid bioassay approach along with an automated cell enumeration technique. The results indicate that at least some cyanopeptides (microcystins-LR, microcystin-LF, aeruginosamide, anabaenopeptin B and aerucyclamide A) induce significant changes to cell division and metabolite production rate. In an ecological scenario, the release of such secondary metabolites by lysing cells (such as when blooms collapse), may be perceived as an alarm signal by surrounding live cells, which may in turn slow cell division and prepare for re-invasion. This may be a strategy for species survival and dominance. While the results from this study do not confirm a role for cyanopeptides, it is thought that the results are clearly indicative of the role played by cyanopeptides for the producing organism. In order to confirm a role, it is recommended that monitoring ribosomally synthesised metabolites (e.g. aerucyclamides) along with chlorophyll-a gene expression, with sophisticated techniques such as qPCR are used