Relating Biological Rate Measurements and Microbial Processes Across Diverse Ocean Ecosystems

Abstract

Marine microbes play key roles in driving patterns of important biogeochemical processes including primary production across the global ocean. Despite the importance of such interactions between the marine microbial community and ocean biogeochemistry, oceanographers have yet to attain a deep understanding of the ecological mechanisms underlying these connections. Due to the vast scale of ocean ecosystems, however, large-scale yet high-resolution surveys are necessary to uncover specific relationships between biology and elemental cycling for more detailed study.With this need in mind, this dissertation takes advantage of recent advances in both underway techniques to measure in situ biogeochemical rates—most notably the dissolved O2/Ar method for measuring net community production (NCP)—as well as molecular sequencing methods to directly investigate relationships between marine microbial community structure, productivity, nitrogen (N2) fixation, and nutrient availability across large ocean regions. At the same time, this work also improves our understanding of the O2/Ar technique by evaluating its performance and key assumptions in a dynamic upwelling environment and by presenting recommendations to improve the accuracy of productivity estimates generated using this approach.Presenting data and measurements from the most comprehensive survey of marine microbial community structure and patterns of productivity and N2 fixation in the western North Atlantic to date, this manuscript highlights intriguing connections between regional peaks in productivity and N2 fixation, the mixotrophic algae Chrysophyceae and Aureococcus anophagefferens, and Braarudosphaera bigelowii, a eukaryotic host organism for N2-fixing bacteria. In addition, we report a strong negative relationship between eukaryotic marine microbial diversity and productivity across the region. We further highlight the importance of considering diel cycles of productivity/respiration, other non-steady-state conditions, and vertical fluxes of O2/Ar when calculating and interpreting NCP rates obtained from surface O2/Ar measurements. Ultimately, these findings contribute to our ability to evaluate community production using surface ocean dissolved gas measurements and provide important insights into patterns of marine microbial activity and community structure into the western North Atlantic.