Light and growth in marine phytoplankton: allometric, taxonomic, and environmental variation

Light-dependent growth of phytoplankton is a fundamental process in marine ecosystems, but we lack a comprehensive view of how light utilization traits vary across genotypes and species, and how this variation is structured by cell size, taxonomy, and environmental gradients. Here, we compile 308 growth-irradiance experiments performed on 119 species of marine phytoplankton from all major functional groups, and characterize growth-irradiance relationships in terms of the initial slope of the growth-irradiance curve ($), the optimal irradiance above which growth declines (Iopt), and the maximum growth rate ($max). We find that $ declines with increasing cell size, although cell size appears to be a weak constraint on this trait. There are significant differences across taxa in $ and $max, with dinoflagellates, raphidophytes, and diazotrophs having the lowest values for both traits, and Phaeocystis spp. and diatoms having relatively high values. Iopt does not vary among taxonomic groups, and all traits exhibit large variation within most groups. Open-ocean isolates tend to have higher $, lower Iopt, and lower $max than coastal isolates, implying adaptation to low light and low productivity. The three traits are correlated across species such that $ and Iopt are negatively related while $max is positively correlated with both of these traits. There is some evidence that high $ carries a cost of high N demand even when nitrogen (not light) is limiting. The results elucidate contrasting light-related ecological strategies across phytoplankton and should help improve the parameterization of major functional groups in biogeochemical models.