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Controls on marine community respiration


Microorganisms engineer their surroundings through their metabolisms, and some of the most important metabolic reactions for multicellular life involve oxygen. The supply and removal of oxygen in aquatic systems depends on many physical, chemical, and biological processes, but controls on the biological processes involving oxygen (particularly respiration) are the least well understood. Different communities of microorganisms may produce and consume oxygen at different rates, but chemical and physical conditions also affect microbial activity. An understanding of all the factors affecting respiration is paramount during the era of climate change as the carbon dioxide produced during respiration can serve as a positive feedback. In this work, I sought to better understand the controls on marine community respiration (CR) using a range of study sites and methods across the Pacific Ocean. Each location was selected based on its ability to help untangle some of the possible variables that affect CR. First, I examined how CR varied with coincident measurements of temperature, turbidity, and chlorophyll concentrations (a proxy for phytoplankton biomass) across the entire Pacific Ocean using data collected by wave gliders (drones) that left from San Francisco, CA and arrived in Australia and Japan. CR was weakly related to different explanatory variables across the Pacific, but more strongly related to particular variables in different biogeographical areas. The results indicate that CR is not a simple linear function of chlorophyll or temperature, and that at the scale of the Pacific, the coupling between primary production, ocean warming, and CR is complex and variable as nutrients and the microbial community change. To further understand the contribution of the microbial community to CR I measured the community, changes in CR, and several environmental variables throughout the upwelling season in Monterey Bay, CA—a dynamic nearshore environment. CR varied significantly over time as a function of temperature, dissolved oxygen (DO), upwelling, and chlorophyll—but also varied with a subnetwork of the microbial community. One subnetwork was associated with higher CR and warmer temperatures, while another was associated with lower CR and DO. Although some microbial taxa were represented in both subnetworks (e.g. Flavobacteriaceae), there were also taxa unique to each network—including the Arctic97B-4 marine group and SAR324 clade (in the subnetwork that was negatively correlated with CR) and Planktomarina and eukaryotic phytoplankton (in the subnetwork positively correlated with CR). The results indicate that multiple microbial interactions regulate CR and carbon cycling in the coastal ocean, and that the presence of particular eukaryotic phytoplankton contribute to CR. Finally, to understand the contribution of nutrients and photosynthesis to CR I conducted a series of experiments in the marine lakes of Palau. The lakes served as natural mesocosms, each with slightly different communities and physical and chemical properties. By experimentally adding nitrogen, a labile carbon source, and transplanting bottles to manipulate light and temperature I was able to tease apart which environmental factors were the most important in determining GPP and CR in different types of systems (lakes) in addition to how the two rates are related across systems. While nutrient rich lakes had higher GPP, this was not the case for CR, showing decoupling between the two rates. However, CR spiked when both nitrogen and labile organic matter were added, shifting the metabolic balance of the community from autotrophic to heterotrophic. Together, these results show that both the heterotrophic microbial community and the photosynthetic community are major contributing factors to CR, so models projecting future rates of CR in a changing world should start to include changes in the microbial community in addition to changes in temperature and nutrients.

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