The ecological effects of the cyanobacterial toxin cylindrospermopsin

The occurrence of nuisance growths or blooms of cyanobacteria in reservoirs and other waterbodies poses a potential risk to the quality of water as many species are capable of producing secondary metabolites that may be toxic or impart unpleasant taste and odours. Cylindrospermopsin, a potent cytotoxic alkaloid produced by numerous cyanobacteria including Cylindrospermopsis raciborskii, has gained increasing attention following its implication as the causative agent in a mass human intoxication event and numerous livestock mortalities. Cylindrospermopsin was also considered to pose significant risks to aquatic organisms, although the role of the cyanotoxin in aquatic ecosystems had received little attention. This thesis aimed to address this significant knowledge gap through a series of toxicity assays to determine the effects of the cyanotoxin in the reservoir and it receiving waters downstream within an ecological risk assessment framework. The cosmopolitan benthic cyanobacterium Lyngbya wollei, collected from Yabba Creek and the Brisbane River, was found to produce cylindrospermopsin (33 <g g-1 dry weight) and its naturally occurring analogue deoxy-cylindrospermopsin (550 <g g-1 dry weight). This is the first report of the production of the cylindrospermopsin analogues by a benthic cyanobacterium or any species of the Oscillatoriales. Screening of three other benthic cyanobacteria also identified, for the first time, toxicity associated with an Australian strain of Phormidium aerugineo-caeruleum. No known cyanotoxins were detected, although the observed effects were similar to that of neurotoxins. The comparative toxicity of the three cylindrospermopsin alkaloids; cylindrospermopsin, epi-cylindrospermopsin and deoxy-cylindrospermopsin was assessed using the brine shrimp nauplii bioassay. The toxicity of epi-cylindrospermopsin was an order of magnitude greater than cylindrospermopsin, although deoxy-cylindrospermopsin was substantially less toxic. These results compared favourably with the mouse bioassay and indicate that the hydroxyl group is required for the uptake or toxicity of the metabolite in whole organisms. The bioconcentration and bioaccumulation of cylindrospermopsin and deoxycylindrospermopsin was identified in a range of aquatic flora and fauna. Of particular interest was the discovery of cylindrospermopsin in the muscle and liver tissue of the eeltailed catfish (Tandanus tandanus) but not other fish species collected at the same site. This indicates that rather than extracellular uptake of the toxin, the source of cylindrospermopsin is dietary and possibly taken up through the consumption of freshwater bivalves. Importantly it suggests that cylindrospermopsin may be transferred through the aquatic food web. The accumulation of cylindrospermopsin in the tissue of a popular angling fish species also has implication for the exposure of humans to contaminated seafood. The toxicological effects of cylindrospermopsin and extracts of C. raciborskii were investigated through a series of acute exposure direct toxicity assessments using a floating macrophyte, green algae and a range of aquatic invertebrates from various trophic levels. Additionally, the effect of sub-lethal exposure was assessed using the growth and photosynthesis of green algae and an aquatic macrophyte, and the survival and reproductive success of the cladoceran Ceriodaphnia dubia. The effect of pure cylindrospermopsin was generally less that of C. raciborskii containing a comparable concentration of the toxin. An exception to this was the survival and reproduction of the cladoceran, Ceriodaphnia dubia, which may have benefited from increased levels of nutrition, in the form of bacteria and other micro-organisms, present in the extract. This may also explain the dominance of smaller daphnids during blooms of toxic C. raciborskii. Overall, concentrations of cylindrospermopsin required to elicit adverse effects were typically well above those which can be expected to occur in reservoirs. There is little evidence for significant ecological effects from exposure to cylindrospermopsin alone at concentrations below 100 <g L-1. However, it is likely that other bioactive compounds present in C. raciborskii extracts and cells contributed to the toxicity of the cyanobacterium. The risk posed by cylindrospermopsin alone to aquatic organisms in reservoirs and their receiving waters is therefore considered low. Water quality guidelines or trigger levels for the protection of aquatic ecosystems based solely on the cylindrospermopsin concentration may therefore not capture the potential risk of exposure to entire C. raciborskii cells. Nevertheless, a total (i.e. intra- and extracellular) concentration of cylindrospermopsin of 100 <g L-1 is suggested as an interim trigger level for the protection of aquatic ecosystems, although this should be revised following additional studies into the sub-lethal effects of chronic exposure to cylindrospermopsin and C. raciborskii