Diversity of microbial plankton in the Southwest Pacific Ocean: a synopsis from carbon production to export

Abstract

Marine microbes drive carbon fixation and export. Despite diverse trophic strategies, size and taxonomy present in microbial populations, the influence of community composition on these processes are unresolved. Here, we used DNA metabarcoding to characterise microbial communities driving carbon fixation and export in oligotrophic subtropical and high-nutrient low-chlorophyll (HNLC) subantarctic waters, east of Aotearoa-New Zealand. First, we developed a nested PCR protocol targeting the petB gene to assess the composition of Synechococcus, especially from samples with low DNA content such as flow cytometry sorted populations. Second, we measured the cell-specific carbon fixation rates of small phytoplankton (Synechococcus, picoeukaryotes and nanoeukaryotes) populations and characterised their community composition. Distinct pico and nanoeukaryote populations maintained similar cell-specific carbon fixation rates in oligotrophic subtropical and HNLC subantarctic conditions, which could be due to phago-mixotrophic taxa dominating the communities in subantarctic waters, therefore alleviating iron stress. Third, we characterised the protistan community within sinking particles, the water column from surface to mesopelagic depths, and sediments, to determine the sources and loss of the exported community. Our results suggest about half of protistan amplicon sequence variants (ASVs) from the upper water column are present in sinking particles, but 90% of these ASVs are lost during export to the sediments, with minimal influence from the lower water column protist community. Overall, this thesis highlights the importance of characterising microbial communities.