Speaker
Description
Marine diazotrophs convert atmospheric nitrogen gas into bioavailable nitrogen that can fuel up to 50% of the primary productivity in oligotrophic subtropical and tropical seas. Despite their importance, little is known about their global biogeography and diversity since global studies have been hampered by scarce data observations. This limitation prevents us from understanding the link between diazotroph richness and ecosystem function, especially rates of nitrogen fixation as this is the main bioavailable nitrogen source in oligotrophic oceans. Here, we analyse the correlation between global diazotroph species richness and nitrogen fixation rates by integrating -omics and microscopy-based observations into species distribution models that were developed to cope with uneven sampling effort and sparse observations. Global diazotroph richness is generally higher in subtropical and tropical regions and declines towards the poles. Our results show that sea surface temperature and nutrient-related environmental parameters rank as the most important predictors on a global scale and biogeographic pattern strongly overlap in tropical regions supporting concepts such as niche complementarity. Additionally, we provide the first global biogeographic pattern of non-cyanobacterial diazotrophs that show increased habitat suitability within upwelling regions when contrasted to cyanobacterial diazotrophs. This result further links to the hypothesized niche of non cyanobacterial diazotrophs to be found in nutrient rich waters containing higher concentrations of particulate organic carbon, where oxygen poor microniches provide a sheltered environment for the oxygen sensitive nitrogenase enzyme. We found a positive relationship between global diazotroph richness and nitrogen fixation rates (R = 0.74, p < 0.001; R = 0.66, p < 0.001) supporting the resource use efficiency hypothesis.
