The dynamics of Microcystis blooms in hypereutrophic ponds

Metagenomic studies have revealed the vastness of the microbial world with systems like cyanobacterial bloom complexes, revealing more diversity than previously anticipated. Cyanobacteria thrive within the hypereutrophic conditions of wastewater treatment plants (WWTPs). Their presence, along with their specialised metabolites, including toxins, reduce treated water quality and represent a risk to both public and environmental health. This thesis investigated the ecology of cyanobacterial proliferation in WWTP lagoons using next generation sequencing technologies and identified the intricate diversity within cyanobacterial bloom complexes. This diversity confers an adaptive capacity to the cyanobacterial communities that enables persistent surface blooms. Amplicon sequencing of waste stabilisation ponds within the treatment conveyance demonstrated that Microcystis preferentially inhabits ponds downstream of the nutrient reduction intervention and remains conserved throughout the treatment process. Whole metagenome sequencing was employed to characterise temporal stability in the Microcystis community. Two high quality Microcystis species representative genomes (SRGs) were recovered, showing perennial proliferative episodes correlated with environmental parameters such as solar radiation, temperature and lagoon pH. The sequencing also revealed that mcy+ and mcy- genotypes of the Microcystis SRGs were abundant during different bloom events, indicating that toxin synthesis may play a role in adaptive community response to environmental stimuli. Amplicon sequencing identified the WWTP sediment as an inoculum source for the seeding of recurrent Microcystis blooms. A novel in-vitro propagation methodology was established to generate toxigenic Microcystis blooms from WWTP sediments, contributing significantly to the mechanistic understanding of Microcystis bloom formation. The application of next-generation sequencing technologies in this thesis provides deep insight into cyanobacterial dynamics within waste stabilisation ponds. This is critical for improving knowledge into the spatial and temporal dynamics of Microcystis blooms. Further, by elucidating bloom formation mechanisms and identifying persistent genetic reservoirs in sediment, this research informs targeted management practices essential for sustainable water resource utilisation and offers crucial insights for cyanobacterial risk assessment strategies.