| Summary: | Di-nitrogen (N2) fixers, also called diazotrophs, are able to reduce atmospheric N2 into bioavailable nitrogen, giving them an advantage in open ocean regions with low dissolved inorganic nitrogen concentrations. The focus of this thesis are three lineages of symbiotic heterocystous filamentous types (het-1, het-2 and het-3), that associate with several genera of microalgae called diatoms (collectively referred to as Diatom Diazotroph Associations, DDAs). Other major cyanobacterial diazotrophs in the ocean are the filamentous Trichodesmium spp., and the unicellular UCYN-A, UCYN-B and UCYN-C. Although widespread in the tropics and subtropics, and first described in the early 20th century, the DDAs are an understudied group of diazotrophs. Hence, our knowledge of their distribution, abundance, activity, and how these are constrained by the environment is limited. Initially we investigated the abundances and distributions of eight cyanobacterial diazotrophs, and two proposed micro-algal hosts of UCYN-A1 and A2, in the western tropical south Pacific (WTSP), using quantitative polymerase chain reaction (qPCR). Trichodesmium spp. was the most abundant diazotroph and het-1 was the most abundant DDA symbiont. Using correlation analysis a distinct vertical separation was observed between UCYN-A and the other diazotrophs (Trichodesmium spp., UCYN-B and DDA symbionts). The most influential environmental parameter on the diazotroph abundances in the WTSP was temperature, and in order to investigate this further we compiled qPCR data from 11 publicly available datasets from four ocean basins. Using a weighted meta-analysis we found that temperature was a robust factor governing the diazotroph abundances (except for UCYN-A) across ocean basins. Attempting to identify differences in environmental impacts on two of the DDA symbiont strains (het-1 and het-2), we applied a new statistical tool called piecewise structural equation model, on qPCR abundance data from the western tropical North Atlantic. We saw that the two strains ...
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