A test of the gleaner-opportunist trade-off among photosynthetic traits in Cryptophyte algae

As photosynthetic producers, phytoplankton form the foundation of aquatic food webs. Understanding the relationships among photosynthetic traits in phytoplankton is essential to revealing how diversification of these traits allows phytoplankton to harvest energy from different light environments. We investigated whether the diversification of 15 species of cryptophytes, a phylum of phytoplankton with diverse light-capturing pigments, showed evidence of trade-offs among photosynthetic performance traits as predicted by the gleaner-opportunist resource exploitation framework. We constructed photosynthesis vs. irradiance (P-E) curves and rapid light curves (RLCs) to estimate parameters characterizing photosynthetic performance and electron transport rate. We inferred the evolutionary relationships among the 15 species with ultraconserved genomic elements and used a phylogenetically controlled approach to test for trade-offs. Contrary to our prediction, we observed a positive correlation between maximum photosynthetic rate, P_max, and P-E α, an indicator of a species' sensitivity to increases in light intensity when light is a scarce resource. This result could not be explained by electron transfer traits, which were uncorrelated with photosynthetic rate. Together, our results suggest that ecological diversification of light exploitation in cryptophytes has escaped the constraints of a gleaner-opportunist tradeoff. Photosynthetic trade-offs may be context- or scale-dependent, thereby only emerging when investigated in situations different from the one used here.