Structure of the microbial communities in marine hotspots for climate change

Marine microbes (viruses, bacteria and eukaryotes) produce and consume a major portion of organic matter in the ocean. Their metabolic activity contributes to a complex carbon cycle that sustains life in the Ocean. Advances in high-throughput molecular techniques have shed light on the enormous genetic diversity and metabolic potential of microbes that underpin ocean productivity. However, our ability to translate this knowledge into a quantitative understanding of how marine communities function, and how they will respond to environmental change is limited. This thesis focusses on the integration of quantitative methods with molecular techniques to elucidate the mechanisms controlling the transfer of energy from primary producers to others trophic levels. This work was carried out in the context of two oceanographic voyages encompassing contrasting ocean regions, the East Australian Current (EAC) System, and the Dalton polynya on the Antarctic coast. The physical effects of changing climatic conditions are well documented in both regions, although the ecological consequences are poorly understood. This study provides the first systematic description of microbial communities in the EAC system and Dalton polynya. Changes in microbial community structure across environmental gradients were determined using high resolution flow cytometry. The contribution of functional groups to the elemental Carbon (C) budget was experimentally determined. Phytoplankton rates of growth and pathways of C and energy transfer were quantified using predator - prey interaction experiments. These studies highlight the microbial contribution to the C budget in each system, with cyanobacteria representing major contributors (75%) to the photosynthetic biomass in the EA C. In contrast heterotrophic bacteria likely play a more prominent role as recyclers in the Antarctic. Viruses were an important source of mortality in the EAC, and likely represent an important driver of carbon flux in the EAC region. Quantitative analyses were combined with metagenomics studies to also investigate the previously uncharted DNA virus diversity within the EAC system, and to provide benchmark data on the diversity of bacterial and eukaryotic community that inhabit the remote area of the Dalton polynya. This work integrates comprehensive datasets that provide a baseline on the abundance, genetic diversity of microbial groups and infection rates to define the forces structuring microbial community in two contrasting hotspots for climate change . And provide a baseline to predict the future of production in these two regional ecosystems.