Nutrient storage links past thermal exposure to current performance in phytoplankton.

The growth of populations and organisms often depends on their previous history of environmental exposure: a phenomenon referred to as "phenotypic memory." The field of ecology presently lacks a mechanistic theory describing phenotypic memory and, as such, evaluating the ecological consequences of this phenomenon is a major challenge. Here, we show that internal nutrient storage connects past thermal experience to current growth in phytoplankton. We develop a mechanistic model showing that delays in the response of nutrient stores to changing temperatures produces phenotypic memory. By testing this model against experimental data of phytoplankton growth rates following temperature perturbations, we find general patterns in the population consequences of phenotypic memory: Prior exposure to warm temperatures depletes nutrient stores, and, in doing so, slows growth during subsequent temperature exposure and restricts the breadth of the thermal niche (i.e., the range of acute temperature exposures yielding a positive growth rate). Our model reveals how phenotypic memory produces temporal variation in critical thermal minima and maxima and predicts that the thermal niche is constricted by long-term exposure to warm temperatures (e.g., during summer months), but that high frequency temperature fluctuations can expand a population's thermal niche. This work provides a mechanistic framework for considering the ecological implications of phenotypic memory.