INSIGHTS INTO THE INTERACTIVE EFFECTS OF TEMPERATURE AND IRON ON SOUTHERN OCEAN PHYTOPLANKTON

Abstract

The photosynthetic conversion of CO2 into organic matter by phytoplankton sustains marine food webs, influences the long-term burial of atmospheric carbon, and underpins many other critical Earth system processes. Seasonal phytoplankton blooms in the Southern Ocean contribute to a large portion of global phytoplankton metabolism, but are substantially limited by low iron availability and cold temperatures. In this thesis, I investigate how changes in iron and temperature influence Southern Ocean phytoplankton metabolic processes and explore implications for marine primary productivity and biogeochemistry. I first conducted laboratory culture experiments using a model Southern Ocean diatom to examine the effects of iron and temperature on growth rates and physiology. I found an interactive iron-temperature relationship, where warming decreased iron demand in this species, allowing cells to maintain half maximal growth at lower iron concentrations. I then used metatranscriptomic data from field experiments in the Ross Sea to ask questions about the cellular mechanisms that underpin enhanced growth under warming and low iron. I found that some diatom taxa grow better under warmer low-iron conditions due to their ability to use iron-conserving metabolic processes, thereby reducing cellular iron demand. Lastly, I combined metaproteomics with elemental measurements from two different shipboard experiments in the Weddell Sea to examine how iron and temperature-driven changes in phytoplankton growth could influence ecological and biogeochemical processes. Consistent with the previous laboratory and field experiments, here concurrent warming and increased iron availability amplified phytoplankton growth and nutrient consumption. I also identified taxon-specific and community-wide proteomic characteristics that connect phytoplankton growth and physiology with the stoichiometry of macro and micro-nutrient consumption. In sum, the findings presented in this thesis show that iron and temperature interactively affect phytoplankton growth through specific cellular mechanisms that have consequences for primary productivity and elemental stoichiometry in a changing ocean.