Top-down control on major groups of global marine diazotrophs

Dinitrogen (N-2) fixed by a group of prokaryotes (diazotrophs) is the dominant process adding bioavailable nitrogen into the ocean. Although it has been intensively studied how N-2 fixation is controlled by resources (bottom-up factors), it is unclear whether the grazing (top-down control) effectively impacts growth and distribution of different diazotroph groups. In this study, we evaluate this question by conducting log-log regression of diazotroph biomass onto corresponding N-2 fixation rates in the global ocean. The slope of the regression for Trichodesmium is similar to 0.8, indicating that a small portion of the increase in N-2 fixation does not accumulate as its biomass. This leads to a conclusion that Trichodesmium is under a substantial top-down control, although bottom-up control still dominates. We also analyze the residuals of the regression in the North Atlantic, concluding that free trichomes of Trichodesmium are subject to stronger top-down control than its colonies. The weak correlation between the biomass and N-2 fixation of unicellular cyanobacterial diazotrophs indicates that the degree of top-down control on this type of diazotrophs varies greatly. The analyses obtain unrealistic results for diatom-diazotroph assemblages due to complicated nitrogen sources of these symbioses. Our study reveals the variability of top-down control among different diazotroph groups across time and space, suggesting its importance in improving our understandings of ecology of diazotrophs and predictions of N-2 fixation in biogeochemical models. Measurements of size-specific N-2 fixation rates and growth rates of different diazotroph groups can be useful to more reliably analyze the top-down control on these key organisms in the global ocean.

Automated summary

This study reveals the variability of top-down control on different diazotroph groups in time and space, suggesting its importance in improving our understanding of diazotroph ecology and predictions of N-2 fixation in biogeochemical models. Measurements of size-specific N-2 fixation rates and growth rates of different diazotroph groups can be useful in analyzing the top-down control on these key organisms in the global ocean.