An investigation into factors influencing the production and degradation of microcystins.
Graham, Douglas J. L.
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Microcystins (MCYST) potently inhibit protein phosphatases and these cyclic heptapeptides are powerful hepatotoxins. They are known to be secondary metabolites produced by several species of cyanobacteria. The release of these peptides into water supplies poses a considerable threat to both humans and animal both as acute or prolonged exposure. The function and regulation of these peptides has yet to be elucidated as most results are inconclusive. This thesis examines MCYST levels in cultures of Microcystis aeruginosa grown under various growth conditions, by determining changes in intracellular MCYST in relation to biomass, cell number, chlorophyll a, protein and extracellular levels. Increasing inorganic carbon through the addition of sodium bicarbonate, strongly affected MCYST levels as concentration increased MCYSTs decreased. Both the intra and extracellular data confirmed that a decrease in production had occurred, not cellular excretion. Additional investigations also confirmed the reductions were not linked to elevated sodium ions. However, growth of M. aeruginosa in media sparged with 5% CO2 enriched air did not lead to reduced microcystin levels, therefore implying that MCYST levels are only reduced by bicarbonate levels. The persistence of these secondary metabolites in the environment is a major concern as potable water guidelines become more stringent. Identifying and understanding natural degradation processes which could be developed or used to predict degradation rates is essential, studies identified that rates of degradation for different MCYST variants are affected by the microbial diversity of the water body. However, prior exposure to multiple toxins increases a microbial population’s ability to degrade complex compounds like MCYST and nodularin. This thesis also highlights the need for developing an advance treatment approach, TiO2 photocatalysis has relatively recently been explored for industrial application of water management although the cost of using UV is prohibitive. Therefore work developing new and novel catalysts which have equal or better activity for degrading compounds while using only visible light as the activation energy and not UV is essential. This study identified a few novel visible light catalysts the best of which (KSH Burg) rival the degradation potential of the UV photocatalyst Degaussa P25 under UV light.