THE DIVERSITY, DISTRIBUTION AND POTENTIAL METABOLISM OF NON-CYANOBACTERIAL DIAZOTROPHS IN THE NORTH ATLANTIC OCEAN

Abstract

In large areas of the Atlantic Ocean, primary productivity is limited by fixed nitrogen. Biological N2 fixation, conducted by diazotrophs, contributes significantly to the input of new fixed nitrogen in those areas. Recent findings clearly showed the occurrence of non-cyanobacterial diazotrophs in oceanic areas outside of the most frequently investigated cyanobacteria-dominated tropical surface oceans. This thesis investigates the diazotrophic community in the coastal and open Atlantic Ocean spanning from surface to depth and from 40°S to 60°N. Using TaqMan assays, the abundances of several diazotrophic phylotypes were measured in the context of the US GEOTRACES program on a transect in the tropical Atlantic Ocean. Distribution of diazotrophs in the surface waters was significantly correlated with the deposition of Saharan dust in the Eastern North Atlantic. Below the surface, an association with the nutrient-rich North African upwelling waters was found. High-throughput sequencing of the nifH gene from 407 samples collected throughout the Atlantic, uncovered a broad array of new nifH sequences. It further demonstrated the shift from cyanobacterial-dominated diazotrophic communities in the tropics to non-cyanobacterial communities at higher latitudes and below the euphotic zone. To better understand the functional role of these marine diazotrophs, which have few ecologically relevant representative in culture, an analysis of 132 diazotrophic reference genomes including 112 non-cyanobacterial species revealed their diverse metabolic potential. Utilization of alternative organic carbon sources, iron acquisition and anaerobic respiration were some aspects found, indicating a role that exceeds that of N2 fixation. A novel heterotrophic diazotroph was isolated from the Bedford Basin using cell-sorting flow cytometry. The isolate could grow in artificial seawater depleted in fixed nitrogen and was actively transcribing the nifH gene. Genome sequencing revealed the presence of the full nif operon and pathways that suit diazotrophy. Clade-specific TaqMan qPCR assay showed the wide distribution of the isolate in the temperate North Atlantic Ocean. A one-year time-series explored the entire microbial community and the diazotrophs in the temperate Bedford Basin, where the novel diazotroph was isolated. Temperature, nutrients and O2 concentrations were the major drivers of microbial community structure. The diazotrophic community was very diverse and showed seasonal variation.