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 within the marine environment. This limitation prevents us from understanding the link between diazotroph richness and ecosystem function, especially nitrogen fixation as this is the main bioavailable nitrogen source in oligotrophic oceans.   To analyse the correlation between global richness and nitrogen fixation rates, we integrate -omics and traditional microscopic based observations and make use of Species Distribution Models that have been developed to cope with datasets that suffer from uneven sampling efforts and scarce observations.   Global diazotroph richness is generally high in subtropical and tropical marine regions declining polewards. Our results show that sea surface temperature (mean adjusted R² of 0.17) and nutrient-related environmental parameters (mean adjusted R² of 0.11 - 0.14) 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 probability of occurrences 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. Finally, to assess the relationship between global diazotroph richness and ecosystem function, we analysed the correlation between our global richness projections and global nitrogen fixation rates from two separate publications and 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.  

Primary Presenter: Dominic Eriksson, ETHZ (deriksson@ethz.ch)

Authors:

Dominic Eriksson, ETH Zurich  (deriksson@ethz.ch)

Damiano Righetti, DTU  (drig@aqua.dtu.dk)

Fabio Benedetti, ETHZ  (fabio.benedetti@usys.ethz.ch)

Lucas Paoli, ETH Zurich  (lucas.paoli@biol.ethz.ch)

Guillem Salazar, ETH Zurich  (uillem.salazarguiral@biol.ethz.ch)

Nicolas Gruber, ETHZ  (nicolas.gruber@env.ethz.ch)

Shinichi Sunagawa, ETHZ  (ssunagawa@ethz.ch)

Meike Vogt, ETHZ  (meike.vogt@env.ethz.ch)