Do mixotrophs modulate a positive climate feedback loop? Modeling the evolutionary response of mixotrophs to future ocean conditions

Marine planktonic mixotrophs combine photosynthesis and heterotrophy to gain the carbon and energy required for their growth and reproduction. The balance of these two different forms of metabolism shapes the impact of these organisms on the upper ocean carbon cycle: More heterotrophy results in more local respiration of organic matter, while more photosynthesis results in greater drawdown of carbon dioxide. Because mixotrophs are dominant community members in the oligotrophic gyres, which are predicted to expand with climate change, this metabolic balance can influence the biological carbon pump across vast swaths of the ocean. Furthermore, metabolic theory predicts that mixotrophs should, on average, become more heterotrophic with increasing temperatures, potentially leading to a positive climate feedback loop accelerating carbon dioxide accumulation in the atmosphere.

Here, we use results from a two-year experiment evolving mixotrophs to new temperatures to parameterize a mathematical model of mixotrophy in the upper ocean. Our experimental data suggest that mixotrophs diverge in their evolutionary responses: While some evolve rapidly to re-balance the relative contributions of photosynthesis and heterotrophy to their energy budget, others’ evolutionary responses exacerbate the expected response by further downregulating photosynthesis. However, using simple ecosystem models, we show that prey limitation may ultimately restrict mixotroph heterotrophy, bounding impacts on the biological carbon pump and decelerating climate feedbacks, even if mixotroph physiology shifts towards increasing heterotrophy. In addition to representing, to our knowledge, the first attempt at accounting for mixotroph evolutionary responses to climate change, these findings highlight the importance of including mixotrophs in upper ocean biogeochemical frameworks.