The seasonal succession of optimal diatom traits

Diatoms are a successful taxon of primary producers in the world's oceans with significant impact both on biogeochemical cycles and the structure of marine food webs. We present a trait-based model that captures key physical characteristics of diatoms: a silica exoskeleton and a large centric vacuole. We examine how these unique features contribute to the success of diatoms compared to similar-sized, nonvacuolated, shell-less phytoplankton with identical photosynthetic and nutrient uptake machinery. Our predicted seasonal succession of optimal diatom traits and growth rates are consistent with observed annual time series. The respective environmental conditions where the diatom attributes of shell and vacuole confer an advantage over nonvacuolated, shell-less phytoplankton are typical during the transition phases of seasonal cycles-during the early phase when vacuolation provides an advantage in terms of light affinity and protection against protist grazers and late phase when specific nutrient affinity and protection from metazoan grazers are of benefit. Although the model reproduces high light and nutrient affinities as well as observed seasonal patterns of diatom dominance, it fails to predict the high growth rates generally observed in diatoms compared to other phytoplankton taxa. This exposes a conundrum in our mechanistic understanding of phytoplankton trade-offs; namely, if the high growth rate of diatoms is independent of their physical attributes (shell and/or vacuole), why has it not been exploited by nondiatom taxa?