Dynamics of nitrogen fixation in Australia's marine environment

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NO FULL TEXT AVAILABLE. This thesis contains 3rd party copyright material. ----- Australia’s marine waters are critical to maintaining the health and prosperity of the nation, with ocean ecosystem services estimated to be worth ~$44 billion annually to the Australian economy. However, to date very little is known about the composition and function of the microorganisms inhabiting Australia’s marine waters, despite the fact that they are critically involved in regulating climate, biogeochemical nutrient cycles, and ocean productivity. Previously, Australia’s marine waters, have been shown to be deplete in bioavailable forms of nitrogen (N), indicating that N limitation may substantially impact biological productivity. While the biological activity of dinitrogen (N₂) fixing bacteria (diazotrophs) can mitigate N limitation, until this thesis, no in-depth analysis of the significance of this process in Australia’s marine waters had been undertaken. Therefore, the goal of this thesis is to characterise the dynamics of N₂ fixation, within a range of ecologically and economically important regions within Australia’s marine environment. Herein, studies were conducted in both temperate and tropical regions, within a range of environments, including an inverse estuary, coastal waters, shallow-shelf seas and the open ocean. The findings of this thesis demonstrate that biological N₂ fixation is indeed a significant process within Australia’s marine waters, occurring at globally significant rates, and driven by diverse assemblages of cyanobacterial and heterotrophic diazotrophs. Relatively high rates of up to ~ 90 nmol L⁻¹ d⁻¹ were observed in both tropical and temperate biomes, however, N₂ fixation demonstrated substantial heterogeneity over a range of spatial and seasonal scales. Specifically, N₂ fixation was found to peak in the tropics during the austral winter, due to an increase in heterotrophic diazotrophs, while maximum rates of N₂ fixation in temperate waters occurred during the austral summer and autumn, and coincided with increased abundances of cyanobacterial and heterotrophic phylotypes. The biogeography of N₂ fixation was found to be driven by region-specific shifts in physicochemical parameters (e.g. sea surface temperature, dissolved inorganic nutrients) and oceanographic processes. Significantly, the East Australian Current, the western boundary current of the South Pacific subtropical gyre was found to play a major role in driving high N₂ fixation rates and diazotroph abundances at temperate latitudes. Overall, data from this thesis will provide a vital frame-work within which we can begin to understand processes influencing N availability in Australia’s oceans, as well as to begin to predict how ecosystem changes may affect biogeochemical processes, and consequently primary productivity here, under future global-change.
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