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, Pmax, 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.