Molecular Fingerprinting to Understand Diazotrophic Microbe Distribution in Oligotrophic Oceans

Abstract

In oligotrophic systems, where primary production is low and nitrogen is in short supply, nitrogen fixation process is intense. Although a few diazotrophs (eg. Trichodesmium) have been widely-studied, the rest of the diazotrophic community is still poorly understood. Furthermore, the global distribution of diazotrophs are yet to be clearly resolved. This dissertation assessed the distribution of diazotrophs in oligotrophic systems, particularly in the tropical and subtropical oceans, using genomics tools including next-generation sequencing. We first tested out a pair of nifH-specific primer that previously performed well in silico, but found that its application on seawater samples was biased towards paralogous, non-functional nitrogenase nifH genes. Instead, we found that the use of a nested PCR method using different primers sets to be more effective in amplifying functional nifH genes. Trichodesmium sp., UCYN-A and Pseudomonas sp. forms the core of the diazotrophic communities in oligotrophic oceans. Temperature is the primary driver of the abundances and distributions of these organisms in the Pacific, Atlantic and Indian Oceans, as well as in the oligotrophic Red Sea. Trichodesmium tends to dominate warm, surface waters, while UCYN-A prefers cooler environments and dwell in sub-surface waters in the Red Sea. Due to the dominance of Pseudomonas in the large-sized fraction samples, they are believed to be part of the Trichodesmium-associated consortia, although this requires further investigations. We also found non-cyanobacterial species of diazotrophs to be dominant previously-described hotspots of nitrogen fixation, and found evidence for the widespread of alternative nitrogenases (Cluster II). Using the Red Sea as an exemplar for future warming ocean, we found patterns of niche partitioning in the Red Sea diazotrophs, based on their distribution along seasons, latitude and depth. Our one-year observation of Red Sea Trichodesmium population witnessed the collapse of the population at temperatures above 32°C. This dissertation not only improve our understanding of the effects of future rising temperature on the natural populations of diazotrophs, but it also helps to establish a baseline understanding of the structure, spatial and temporal dynamics of Red Sea diazotrophs, which has not been discussed elsewhere.