Investigating the multiplicative effects of climate change on Southern Ocean phytoplankton

Abstract

Southern Ocean phytoplankton have been shown to overcome the low light and low iron (Fe) environment through genomic and physiological adaptations. Together, these adaptations allow the efficient use of light and Fe to photosynthesise optimally in this cold polar region. Through biological and physical processes, the Southern Ocean (SO) accounts for approximately 40% of global carbon fixation. Model projections indicate that light, temperature, Fe and CO2 (thus pH) in the Southern Ocean are likely to change simultaneously in the future due to changing climate. Although prior investigations have constrained the response of SO species to changes of individual environmental variables, multiple species responses to concurrent changes is unclear. This project utilises physiological measurements and molecular biological tools to explore adaptation responses and biochemical strategies of Southern Ocean phytoplankton to environmental changes. I quantified the thermal performance curves of three SO species under growth saturating and sub-saturating light in Fe replete and Fe limiting media to understand the capacity for species to withstand warming. I then measured the protein concentrations expressed under varying light and Fe conditions in three SO species compared to a temperate diatom to discover trade-offs to light and iron in photosynthetic proteins. To assess SO phytoplankton responses to climate change, two species were grown under changes in CO2, light, temperature and Fe. Finally I characterised the Rubisco from SO and a temperate diatom to understand differences in carbon acquisition strategies compared to a vascular land plant. This study adds to a growing research focus which aims to understand how marine biota will respond to climate changes over the coming century. It also aims to uncover underlying adaptations that allow SO phytoplankton to fix and export carbon in spite of cold temperatures, low light and growth limiting iron concentrations in this region. The evidence presented in this thesis discusses the evolutionary ramifications for dispersal into the SO and reveals different evolutionary histories of SO diatoms isolated from the same location. Furthermore, the results from this study agree with other findings to suggest that Chaetoceros spp will increase in abundance in response to climate change.