Biological nitrogen fixation is a globally important process on the earth. In oceanic waters, it is an important bioavailable nitrogen source for supporting phytoplanktons in nitrogen limited conditions, and was suggested as one of the potential negative feedback mechanisms of the earth to the global warming. This process is carried out by diverse prokaryotes called diazotrophs, in which many diazotrophs in the marine ecosystems are uncultivated and their ecophysiology is unclear. In this thesis, the genetic diversity, ecophysiology and major determinants of marine diazotrophs were explored with applying molecular techniques in large scale field studies.

This thesis started with a survey conducted in the understudied Western North Pacific Ocean (WNPO), in which the Kuroshio in the WNPO...[ Read more ]

Biological nitrogen fixation is a globally important process on the earth. In oceanic waters, it is an important bioavailable nitrogen source for supporting phytoplanktons in nitrogen limited conditions, and was suggested as one of the potential negative feedback mechanisms of the earth to the global warming. This process is carried out by diverse prokaryotes called diazotrophs, in which many diazotrophs in the marine ecosystems are uncultivated and their ecophysiology is unclear. In this thesis, the genetic diversity, ecophysiology and major determinants of marine diazotrophs were explored with applying molecular techniques in large scale field studies.

This thesis started with a survey conducted in the understudied Western North Pacific Ocean (WNPO), in which the Kuroshio in the WNPO was first discovered as a hotspot of diversity of unicellular cyanobacterial diazotrophs (UCYNs). By further analyzing the diazotroph communities along the whole path of the Kuroshio, we showed that the diazotrophs were actively transported by the Kuroshio, which was predicted to be enhanced under the global warming. Besides that, a new biomarker (rpoC1 gene) was adopted to further explore the phylogeny of the UCYNs in the Kuroshio, which provided new insights to the newly defined UCYN-B2 and the understudied UCYN-C group of diazotrophs. In order to identify the major determinants of diazotrophs in the oceanic waters, basin-scale and seasonal variations of diazotroph populations were examined in the North Pacific Ocean, which were found to be well explained by the ocean circulations. The dynamics of a globally significant diazotroph, UCYN-A1, was found to be mainly mediated by physical forcing (high temperature associated with El Niño event and warm current), instead of chemical factors. Moreover, the previously proposed niche differentiation of the UCYN-A sublineages was confirmed; and the fidelity of the unusual symbiotic relationship between UCYN-A sublineages and their hosts was evaluated in the basin-scale study. Besides the cyanobacterial diazotrophs, by summarizing the potentially active diazotrophs in varied regions of the open oceans, we have suggested that the members of Marine I group and Gamma-4 are the major non-cyanobacterial diazotrophs in oceanic waters. Besides that, we have first discovered the differential distributional pattern of the sublineages of Gamma-A (the most abundant member of Marine I group), which was mainly mediated by temperature. By analyzing the genome of Gamma-4, we have provided new insight of the potential strategies of the marine heterotrophic diazotrophs to overcome nutrient limitation and oxygen stress in oceanic waters.