Bacterial biological control of toxic cyanobacteria and the resulting eco-toxicity
Thesis (PhD)--Stellenbosch University, 2019.
ENGLISH ABSTRACT: The increased frequency and toxicity of freshwater cyanobacterial blooms is a cause for concern globally. Not only do these blooms result in reduced levels of oxygen in the water column and unpleasant odour, they also produce cyanotoxins as secondary metabolites. These toxins have resulted in the death of humans, aquatic organisms and wildlife. Bloom occurrences have been mapped globally and pose a challenge to water quality management due to toxin release, oxygen depletion, unpleasant water taste and odour. Biological control as a mitigation measure of these blooms has been explored using microorganisms and higher order grazers. A need exists for low cost, passive treatments through environmentally favourable control agents. In this dissertation, the biological control of filamentous Oscillatoria and unicellular Microcystis was investigated using predatory bacteria. Bacterial isolates from the phylum Proteobacteria were isolated and found to be effective in the reduction of microcystins released by both cyanobacteria. These were compared to a Lysinibacillus related isolate, which is from the phylum Firmicutes. The research conducted indicated a need for more molecular identification of wild strains of cyanobacteria, especially the less studied filamentous isolates to expand the database of sequences deposited for identification to be more conclusive through 16srRNA gene sequencing. Microcystis was positively identified through 16srRNA identification and showed similarity to a species collected from another province in the same country. The exposure of cyanobacteria to bacterial numbers at lower ratios (2:1 ratios of cyanobacteria to bacteria) indicated a stress response from cyanobacterial cell morphology and a reduction in toxicity. The research found that Pseudomonas rhodesiae (isolate 3w) and the Lysinibacillus fusiformis related isolate (isolate B) resulted in a greater reduction of toxins (microcystins) in both the filamentous and unicellular isolates. Measurement of toxicity was through the ELISA (enzyme linked immunosorbent assay) and confirmed through HPLC (high performance liquid chromatography), which indicated the presence of two microcystin variants, microcystin -LR and -RR. The bacterial isolates readily reduced the more toxic microcystin -LR as opposed to microcystin R-R, indicating that there may be microcystin degrading capacity in the isolates. Screening of eco-toxicity from the resultant bacterial treatment in the water indicated that reduced toxicity resulted in higher survival from the bio-indicator organisms overall. Thamnocephalus platyurus, a freshwater crustacean, was the most sensitive to changes in toxicity, proving to be a more suitable confirmation method of microcystin reducing treatment interventions. Isolate B was the best isolate from both a toxin reducing and eco-toxicity response perspective. This study also compiled all the findings from cyanobacterial bloom research in the African context as well as gaps in the area of bacterial biological control globally. In summary, the present study confirmed the potential to optimize passive bacterial control of cyanobacterial blooms on a larger scale, within a mixed bloom population.