Di-nitrogen (N₂) fixation plays a crucial role in oceanic carbon and nitrogen cycles and is important for marine biogeochemistry on regional and global scales. N₂-fixing (diazotrophs) cyanobacteria are considered to contribute the most to marine nitrogen fixation, and thereby fuel the surrounding phytoplankton communities with bioavailable ammonia. Distribution and activity of phytoplankton, including diazotrophs, are largely driven by environmental conditions and temperature is often considered the main influence. A greater understanding of the environmental conditions that govern the diazotrophs, is vital to accurate predictions and estimations for the marine nitrogen budget, as well as understanding their impact on the nitrogen and carbon cycles.

Therefore, the primary aim of this thesis was to determine abundances and distribution patterns for various cyanobacterial diazotrophs in two different regions that in spite of their hydrological differences are prime regions for diazotrophy. Moreover, we attempted to identify the environmental conditions that govern cyanobacterial diazotrophs abundance.

In the first study we observed a clear separation of the unicellular diazotroph UCYN-A from the other diazotrophs, including the other unicellular types (UCYN-B, UCYN-C) in the Western Tropical South Pacific. The main driver of the vertical distribution of diazotrophs was based on a temperature-depth gradient, which was similar to the findings of our meta-analysis which included 11 additional datasets. Using newly and previously designed primers and probe sets for the UCYN-A1 and A2 hosts, we also observed discrepancies in detection and abundance for the two UCYN-A symbiotic strains (A1 and A2) and their respective hosts, which is in contrast to the current understanding of a highly specific and obligate partnership. Lastly, cross-hybridization tests revealed that the qPCR assay targeting the UCYN-A2 strain also enumerated UCYN-A1, demonstrating the difficulty in quantifying closely related strains.

In the second study we distinguished the environmental conditions favoring two closely related heterocystous cyanobacterial strains of Richelia intracellularis (het-1 and het-2) which associate with two different diatom hosts (Rhizosolenia and Hemiaulus, respectively) in the Western Tropical North Atlantic (WTNA). In general, the Amazon River (AR) plume heavily influenced het-1 and het-2 abundances; higher densities were quantified at stations with mesohaline sea surface salinities and maximum abundances were detected in the sub-surface, below the freshwater discharge. However, maximum abundances at oceanic sea surface salinity stations were observed nearer to the surface. A piecewise Structural Equation Model (SEM) was developed and identified turbidity as an important factor governing het-1 and het-2 abundance. In addition, het-1 and het-2 distribution pattern was influenced by dissolved inorganic phosphorus (DIP) concentration and salinity. Het-1 tended to penetrate deeper waters and was favored by increased salinity, while the opposite was true for het-2.

The results of this thesis contribute to a better understanding of marine cyanobacterial diazotrophs’ ecological niches and will prove useful in future predictions and research on nitrogen fixation, and the role of cyanobacterial diazotrophs in marine biogeochemistry.