The Evolution of Mixotrophic Nanoflagellates in Response to Increasing Ocean Temperatures

Marine microbial mixotrophs use a combination of photosynthesis and heterotrophy to gain energy and nutrients. As the dominant carbon dioxide fixers and the primary consumers of bacteria in oligotrophic subtropical gyres, which are expected to expand under warmer ocean temperatures, mixotrophs play a key role in global biogeochemical cycling. Rising temperatures may also fundamentally affect the physiology and metabolic function of these organisms. Metabolic theory predicts that respiration will increase with temperature at a faster rate than photosynthesis, which suggests mixotrophs will become more heterotrophic. However, fast growth rates and short generation times may allow for these organisms to adapt to temperature stress by rebalancing their dual forms of metabolism. The balance between these two metabolic modes determines whether they are a source (more heterotrophy) or sink (more photosynthesis) of global carbon dioxide, prompting this investigation into how mixotrophy may change with changing ocean conditions.

This study aims to better understand how mixotrophs will react to increased temperature by experimentally evolving two strains of the nanoflagellate mixotroph, Ochromonas, at three temperatures: its ancestral temperature, as well as a warmer and a cooler temperature. Results from the first two years of evolution show that the photosynthetic responses to increased temperature differ in the two strains, with one strain increasing the photosynthetic contribution to its metabolism at higher temperatures, and the other decreasing it. This suggests that while some mixotrophs will be able to maintain photosynthetic contributions to their metabolism, others will become more heterotrophic, reducing their contribution to the biological carbon pump. Changes in respiration rates and physiological changes in carbon content and stoichiometry will also be discussed