Intra-specific Variation, Selection, and Climate Change in the Seas

Marine ecosystems will be impacted by climate change over the next century, where increased temperature and ocean acidification are already changing the physical and chemical nature of the seas. How the marine biota will respond to these impacts is unclear, in particular we lack understanding of organisms' expected evolutionary responses to climate change. Microbes (i.e. bacteria and protists) are particularly important components of marine ecosystems, due to their roles in marine food webs, and their ability to influence climate via biogeochemical cycles. The majority of research into biological impacts of climate change is performed using short term perturbation experiments, where organisms adapted to present conditions are cultured in conditions predicted to occur up to 90 years in the future. Eco-physiological and/or life-history traits are then used to infer species responses, and in some cases fitness. This thesis suggests why this could be a misleading approach, and places emphasis on the importance of intra-specific variation in adaptation to climate change, using a series of experiments in the heterotrophic dinoflagellate Oxyrrhis marina. I quantify variation in eco-physiological responses to elevated temperature and pCO2, among geographic isolates of Oxyrrhis, develop a novel molecular method to quantify strain frequencies in mixed populations, and explicitly test the relationship between eco-physiology and competitive fitness. I reveal substantial variation in eco-physiological responses, and show that often this is not indicative of fitness. I then investigate frequency- and density-dependence of selection, and quantify how selection responds to environmental change. I conclude that the importance of intra-specific variation should never be overlooked, and that the best way to quantify selection from this variation is looking for relative changes in selection rates between isolates under experimental climate change conditions.