Phytoplankton are the unicellular photosynthetic microbes that form the base of aquatic ecosystems, and their responses to global change will impact everything from food web dynamics to global nutrient cycles. Some taxa respond to environmental change by increasing population growth rates in the short-term and are projected to increase in frequency over decades. To gain insight into how these projected 'climate change winners' evolve, we grew populations of microalgae in ameliorated environments for several hundred generations. Most populations evolved to allocate a smaller proportion of carbon to growth while increasing their ability to tolerate and metabolise reactive oxygen species (ROS). This trade-off drives the evolution of traits that underlie the ecological and biogeochemical roles of phytoplankton. This offers evolutionary and a metabolic frameworks for understanding trait evolution in projected 'climate change winners' and suggests that short-term population booms have the potential to be dampened or reversed when environmental amelioration persists.

Phytoplankton are the unicellular photosynthetic microbes that form the base of aquatic ecosystems, and their responses to global change will impact everything from food web dynamics to global nutrient cycles. Some taxa respond to environmental change by increasing population growth rates in the short-term, and, based on this, are projected to increase in frequency over decades. To gain insight into how functional traits in these projected "climate change winners" change over different timescales, we evolved populations of microalgae in ameliorated environments for several hundred generations. While populations initially responded to environmental amelioration by increasing photosynthesis and population growth rates as expected, this response was not sustained. Instead, most populations evolved to allocate a smaller proportion of carbon to growth while increasing their ability to tolerate and metabolise reactive oxygen species (ROS). This diversion of fixed carbon from growth to catabolism underlies a quality-quantity tradeoff in daughter cell production which drives the evolution of population growth rates and of functional traits that underlie the ecological and biogeochemical roles of phytoplankton. There is intraspecific variation in the trait combinations that evolve, but all are consistent with mitigating ROS production and accumulation in ameliorated environments over hundreds of generations. This offers both an evolutionary and a metabolic framework for understanding how functional traits can change in primary producers projected to be "climate change winners", and suggests that short-term population booms and associated trait shifts have the potential to be dampened or reversed if environmental amelioration persists.

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