UC San Diego

The metabolic balance between photosynthesis and respiration imposes a biological control on the ocean’s ability to sequester carbon dioxide within the global carbon cycle. In the Arctic, however, the net trophic state of this balance is still uncertain due to sporadic regional data availability, methodological measurement biases, and poor mechanistic constraints on various physical and biological influences. In this dissertation, I used a combination of field observations and experimental approaches to identify mechanistic links between microbial (phytoplankton, bacterial, and archaeal) community structure and biogeochemical oxygen cycling, resolving uncertainties in the biological influences on net metabolic balance in the surface Arctic Ocean. First, I experimentally investigated the inherent biases of sample melting procedures on measurements of first year sea ice microbial community structure, such as cell abundance, photo-physiology, and community composition. I found that an equivalent-volume 35-ppt buffered melt was best at minimizing cell loss due to osmotic stress, but various buffers were enough to reduce bias in amplicon sequence-based community composition when comparing to direct melting procedures. Next, I explored direct linkages between water column microbial community structure and metabolic balance using nearly a year’s worth of amplicon sequencing and biological oxygen utilization observations collected during the MOSAiC International Arctic Drift Expedition (2019-2020). Applying predictive machine learning techniques to this paired timeseries, I identified both general successional patterns and specific taxa within the microbial community that tracked, and could reliably predict, biological oxygen consumption ( 5.32 mol kg-1). Finally, I characterized fine-scale environmental controls on near-surface microbial community structure and metabolic oxygen turnover during the Arctic melt season. Observations from the MOSAiC Expedition indicated that meltwater induced salinity stratification results in ecologically distinct microhabitats with unique microbial assemblages and significant variation in net biological oxygen consumption. Culture-based laboratory experiments further showed that while primary production decreased significantly with exposure to low salinity conditions, community respiration rates persisted indicating increased likelihood for net heterotrophy at the air-sea interface in stratified meltwater layers. Taken together, this dissertation highlights microbial contributions to surface ocean metabolic oxygen turnover and provides information to better study, and model, this key ecological process in a future Arctic.