Abstract:
Increased nutrients entering aquatic systems are disruptive, sometimes causing eutrophication. In this thesis, I characterized microbial responses to nutrient availability across a freshwater-to-marine transect and temporal cyanobacterial bloom event. Additionally, I identified the nutrient acquisition strategies and the genetic traits that distinguish toxic and non-toxic benthic cyanobacteria of the widespread genus Microcoleus.
To explore microbial ecology across a spatial gradient, I collected water and sediment from nine subtidal sites across a 5 km stream-to-marine transect for DNA and RNA sequencing. Nutrient analyses confirmed a transition from phosphate-limitation in freshwater habitats to nutrient-rich conditions in the brackish zone, which corresponded to significantly higher expression of genes related to phosphate uptake in freshwater and higher primary productivity and nitrogen metabolism in the brackish zone.
To characterize the community distribution across a temporal freshwater cyanobacterial bloom event, I sampled Microcoleus-dominated benthic mats over a 19-day proliferation event for proteogenomics analysis. Microcoleus form visible proliferations that coat riverbeds when nitrogen is supplied in excess, but not phosphorus. Metagenomics coupled with proteomics approaches indicated that non-diazotrophic Microcoleus are equipped with diverse mechanisms for nitrogen and phosphorus acquisition, enabling them to proliferate and outcompete other taxa in low-phosphorus river waters.
Microcoleus bloom communities often comprise morphologically similar anatoxin-producing (toxic) and non-anatoxin-producing (non-toxic) strains. This thesis compared 42 Microcoleus genomes to elucidate the genetic differences between toxic and non-toxic strains. Comparative genomic analysis revealed significant genome streamlining and infers thiamine auxotrophy among toxic strains, which potentially rely on their metabolically versatile, non-toxic counterparts for thiamine. Toxic and non-toxic strains were differently equipped with additional nitrate transporter and cyanophycin genes, likely conferring distinct competitive advantages based on nitrogen availability. These results highlight the potential for cooperative and competitive interactions that contribute to the co-occurrence of Microcoleus strains in aquatic environments. This thesis advances our understanding of how microorganisms metabolise and transform nutrients across spatial and temporal gradients in aquatic systems. I provide novel insights into the metabolic differences of closely related toxic and non-toxic cyanobacterial species that help to explain their environment distributions and association with nitrogen, which may assist in predicting and managing future Microcoleus bloom events.