Light, nutrients, and P:C ratios in algae: Grazer performance related to food quality and quantity

Continuous cultures of the green algae Selenastrum capricornutum were grown at different concentrations of total phosphorus (1-50 μmol P/L) and different light levels (10-200 μmol quanta·m^-2·s^-1). Growth yields in terms of C, N, and P were positively correlated to total P level. Total biomasses in terms of C and N, but not P, were also positively correlated to light level. The cell quotas of C were positively correlated with light, whereas cell quotas of P were negatively correlated with light. The resulting elemental ratios gave a quite consistent pattern, where high light caused significant reductions in both N:C and P:C ratios, as well as strong reductions in chlorophyll to carbon ratios. The increase in P:C ratio with increasing total P and decreasing light can be interpreted as an adaptive response to self-shading. In that case, our results indicate that light adaptation not only involves a cost in terms of increased chlorophyll synthesis, but also in terms of increased P demands. This provides new insight not only into the physiological regulation of C and P uptake in algae, but it could also explain deviations from the Redfield ratio. The growth of juvenile Daphnia magna fed S. capricornutum from the different light and phosphorus treatments was studied in a series of short-term assays (7 d) covering a gradient of food concentrations. The response of Daphnia growth rate along this quantity (0.5-5.0 mg C/L) and quality (0.5-12 μg atomic P-[mg atomic C]^-1) gradient gave a close fit to a double hyperbola model. Changes in elemental ratios of the algae were reflected in the growth rate of Daphnia, such that up to 40% reduction in its growth rate could be attributed to increased C:P ratios. This study demonstrates that the physiological responses of phototrophs in terms of chlorophyll content and elemental composition depend strongly on ambient light and nutrient regimes. It also confirms that these patterns can yield contrasting responses on herbivore growth responses along the food quantity and quality axes.