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Kendra Turk-Kubo:
Marine nitrogen fixation: Importance of enigmatic nitrogen-fixing microbes in open ocean and coastal ecosystems

Date: 28-05-2021    Supervisor: Jonathan Zehr




The oceans are central to life on Earth, playing a critical role in climate regulation and  biogeochemical cycling of nutrients. The ocean's "plants" - phytoplankton - are important primary producers but are largely limited by the availability of nitrogen. One important source of nitrogen is supplied by specialized single-celled organisms that use a metabolic process called nitrogen fixation to access nitrogen gas from the atmosphere and convert it to biologically available nitrogen. 

Marine nitrogen-fixing microbes inhabiting the well-lit surface ocean have diverse physiologies, from free-living autotrophs to particle-bound heterotrophs to symbiotic associations with eukaryotes, and few cultivated marine nitrogen-fixers are available to study in isolation. Their distributions and nitrogen-fixing activities are controlled by a complex interplay between physical and chemical controls and biological interactions that vary from species to species. 

In this thesis, cultivation-independent techniques and biogeochemical rate measurements were used to advance our understanding of the environmental drivers behind the distribution and activity of some of the most enigmatic nitrogen-fixing taxa. The work presented collectively advances our understanding of marine nitrogen fixation by linking the presence and activity of several diazotrophic groups to whole community nitrogen fixation and examining the influence of nutrient controls and grazing pressure on their growth. Together these findings underscore the importance of considering the environmental drivers of individual diazotrophic taxa in order to better predict their collective impact on the global nitrogen cycle in a changing ocean.