Environmental variability is ubiquitous, but its effects on ecological communities are neither fully understood nor readily predictable. Understanding the proximate mechanisms underlying how populations respond to environmental variation remains a critical concern, particularly as climate warming and anthropogenic activity impact essential abiotic characteristics (e.g., mean environmental temperatures, spatio-temporal thermal patterns, and nutrient availability). One common response organisms have for coping with environmental variability is acclimation, which is often characterized by reversible, short-term phenotypic plasticity. Here, we investigate the population-level response of Chlamydomonas reinhardtii, a freshwater green algae, to temporal patterns in its thermal environment. As algal growth rates noticeably deviate in constant versus variable thermal environments, we investigate whether such changes in growth rate might be explained by corresponding changes in stoichiometry. To achieve this, we first measured the exponential growth rate of C. reinhardtii populations in varying nutrient conditions (nutrient-replete, nitrogen-limited, and phosphorus-limited) across a thermal gradient (ranging from 15-40OC) that had either been previously acclimated to each respective temperature (fully-acclimated) or 15OC only (cold-acclimated). Second, we acclimated C. reinhardtii populations to 15, 17, 28, 34, and 37OC and measured total nitrogen, phosphorus, and carbon using elemental and spectrophotometric analyses.
Results/Conclusions
Our results indicate that C. reinhardtii populations exhibit a strong thermal dependence of exponential growth, and populations acclimated to cold conditions experience meaningful, transitory deviations from growth rates experienced by populations acclimated to warmer conditions. The impact of thermal dependence on exponential growth was most pronounced in nutrient-replete conditions, exemplified through high deviations from fully-acclimated performance. Correspondingly, transitory deviations in growth rate from acclimated conditions were diminished in P-limited environments and non-existent in N-limited conditions. C. reinhardtii populations also displayed a strong thermal dependence of stoichiometry for all three nutrient environments. Notably, the C:N ratio of both the nutrient-replete and N-limited algal populations experienced an increase by 10% and 12%, respectively, from the lowest (15OC) to the warmest (37OC) temperature. Additionally, while on average 35% higher than nutrient-replete populations, N-limited populations demonstrated the greatest variation in the C:N ratios across temperatures, ranging from 10.2-15.2. We conclude by presenting a nutrient storage model of exponential growth that demonstrates how the thermal dependence of algal stoichiometry can alter expectations of exponential growth for algal populations in fluctuating environments. These results suggest the thermal dependence of stoichiometry as one potential mechanism explaining unanticipated phytoplankton growth rates in variable thermal environments.