Diversity, impact and fate of cyanobacterial toxins in freshwater ecosytems

Massive proliferations of cyanobacteria (bloom) are common in aquatic environments worldwide. These blooms are often toxic due to the presence of hepatotoxins or neurotoxins and have become a worldwide environmental problem. Various incidents of animal and human poisonings have been attributed to these toxins. Therefore, monitoring of potentially toxic cyanobacteria and the associated toxins need to be investigated routinely in each water body. In the first part of present study, LC-MS methods were applied for identifying and quantifying cyanotoxins diversity in Lake Garda. Anatoxin-a (ATX) and microcystins (MC) were always present in this lake with a different seasonal pattern. ATX represented an early summer peak, while MC showed a typical late summer-early autumn peak.The results of toxin analysis also revealed the presence of 5 variants of MC in this lake, but the variants MC-RRdm was always dominant over the others. In another chapter of this thesis the kinetic aspects of MC transfer from Planktothrix rubescens to Daphnia magna was investigated. Models of MC accumulation obtained from this part of study differed largely as a result of the duration of exposure and initial MC concentrations used. Within the first 24 h of exposure, MC accumulation in D. magna was linear, irrespective of the initial densities of toxic P. rubescens and MC concentrations. After 48h of exposure, MC accumulation in D. magna showed an exponential pattern. In the last part of this study, the taxonomic identification of new Oscillatoriales was carried out adopting a polyphasic approach and new potential ATX producers were screened through chemical characterization and identification of specific toxins encoding genes. The analyses were made on several strains isolated from environmental samples collected in Lake Garda. The results allowed identifying a new ATX producer, Tychonema bourrellyi. This is the first discovery of a planktonic genus belonging to the Oscillatoriales able to produce ATX