Constraints on microbial variability in the open ocean

Marine microorganisms – single-celled eukaryotic plankton, cyanobacteria, heterotrophic bacteria, archaea, and viruses – are essential members of the open ocean community. They are responsible for driving oceanic and global biogeochemical cycles, accounting for up to half of total global primary production. Their function can be constrained by chemical and physical factors that vary vertically down the water column and across differing oceanic regions. Understanding the environmental factors controlling microbial ecology, physiology and activity in the ocean is a major aim in the field of microbial oceanography, as this information can help to inform biogeochemical models in the context of climate change. Molecular techniques have greatly advanced our knowledge of the diversity and species-specific metabolic profiles in complex microbial communities within their natural environment, providing novel insight into oceanic microbial ecology that can be used to generate new and testable hypotheses. Metatranscriptomics, or the high-throughput sequencing of community RNA, can provide a snapshot of microbial activities of in situ microbial communities and under experimentally altered conditions, revealing specific molecular adaptations of phytoplankton and bacteria to environmental change. By using molecular techniques, the role of environmental factors on modulating microbial community activity across two open ocean regions was inferred throughout this dissertation. Using a metatranscriptomic approach, the influence of temporal, trace element and depth-resolved environmental gradients on phytoplankton and viruses inhabiting the sub-Antarctic Southern Ocean was resolved. A high resolution, multi-omics study in the Bermuda Atlantic Time Series of the Sargasso Sea revealed unique microbial and viral interactions and activity within a stratified oceanic water column. Together, these studies illuminate region-specific ecological determinants of microbial community structure and functions across the open ocean water column.