Phytoplankton fitness landscapes across gradients in temperature and nutrient concentration

Background/Question/Methods

Phytoplankton fitness is strongly influenced by environmental conditions, including nutrient concentration and temperature. These factors share two important features: they have highly non-linear effects on fitness and are rapidly changing in natural environments as a result of human activity. Traits determining performance across temperature and nutrient gradients are frequently used to predict performance in the environment without careful consideration of how these factors interact to influence fitness. We examined these interactions in order to improve our ability to predict the effects of environmental change on phytoplankton species and communities.

We measured fitness (population growth rates) of a marine diatom, Thalassiosira pseudonana, in two 5 x 5 factorial experiments. We used 5 temperatures (20, 25, 27.5, 30 and 32.5°C) crossed with 5 phosphate levels (1, 2.5, 5, 15 and 36.2µM) in one experiment and with 5 nitrate levels (1, 5,  15, 25, 100µM) in another. At each temperature, we used these data to estimate the maximum growth rate and half-saturation constant for phosphate and nitrate using the Monod model and examined the effect of temperature on these parameters. Similarly, we estimated the optimum temperature for growth and temperature niche width at each nutrient level and examined the effect of nutrient concentration on these parameters. We then united these models to examine the fitness landscape of this species across these important environmental factors.

Results/Conclusions

We found that temperature and nutrients interact strongly to determine fitness. The maximum growth rate and half-saturation constant for growth are highly temperature-dependent, with the latter being at its minimum near the optimum temperature for growth. Similarly, the optimum temperature for growth and the temperature niche width are strongly dependent on nutrient concentration, with both parameters saturating at intermediate concentrations. Optimum temperature varied by ~2°C over the range of nutrient concentrations tested, while half saturation constant varied by approximately an order of magnitude.

Therefore, optimum temperatures for growth and nutrient parameters estimated under typical laboratory conditions may not reflect performance relevant to an organism’s natural environment. Further studies characterising these fitness landscapes are need in order to test models of how these factors interact, as well as assess constraints and trade-offs in performance across axes. The strong interactions between these factors suggests that predicting the effects of environmental change on the performance of individual species, let alone communities, will be a challenge.