Racing respiration to adapt to warming oceans

Microbial mixotrophs combine two forms of metabolism—photosynthesis and phagotrophy—in a single cell. Because of their dual metabolism, they can have diverging impacts on the carbon cycle either by drawing down carbon via photosynthesis or producing carbon dioxide via respiration of digested prey. Theoretical predictions argue that mixotrophs should become more phagotrophic at warmer temperatures, potentially contributing to a positive climate feedback loop, but experimental evidence for this prediction is mixed. We hypothesized that mixotroph metabolic plasticity (the extent to which mixotrophs can separately vary their investments in photosynthesis and phagotrophy) predicts the capacity for evolutionary responses because mixotrophs with more innate plasticity experience weaker natural selection. To test this hypothesis, we experimentally evolved eight mixotroph lineages at colder and hotter temperatures. We find that per-generation rates of evolution can be 3-5 times faster at colder temperatures, but that the magnitude of phenotypic change over the same absolute amount of time is often larger at warmer temperatures (e.g., up to a 3x increase in growth rate) due to a greater number of generations elapsed at warmer temperatures. These increases in growth rate were accompanied by decreases in mean chlorophyll content, suggesting that mixotroph adaptation may reduce respiratory demands previously met by photosynthetic activity. This is consistent with mathematical models of mixotroph evolutionary adaptation. Our preliminary results suggest that while evolutionary responses appear to be constrained by underlying metabolic traits, the underlying correlation with plasticity is nuanced.