Diatoms are a successful taxon of primary producers in the world’s oceans, contributing about 40% to the total marine primary production with significant impacts both on biogeochemical cycles and the structure of marine food webs. In this paper, we develop a trait-based model that captures the key physical characteristics of diatoms; a silicate exoskeleton and a large centric fluid-filled vacuole. These traits provide diatoms with several advantages. In particular the vacuole gives diatoms a much greater surface area allowing them to be more efficient in the uptake of nutrients and the capture of light. We examine how these unique features contribute to the success of diatoms compared to similar-sized, non-vacuolated, shell-less phytoplankton with identical photosynthetic and nutrient uptake machinery.
Our predicted seasonal succession of optimal diatom traits and growth rates is consistent with observed annual time series. Specifically, the respective environmental conditions where the diatom attributes of shell and vacuole confer an advantage over non-vacuolated, shell-less phytoplankton are typical during the transition phases of seasonal cycles; namely 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. We point to the evolution of diatoms as having occurred during a geological period when tectonic processes vastly expanded the extent of high latitude oceans subject to high seasonal variations.
While 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 non-diatom taxa?
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Hansen AN, Visser AW (2019). The seasonal succession of optimal diatom traits. Limnology and Oceanography. doi: 10.1002/lno.11126
Image Credit: Prof. Gordon T. Taylor, Stony Brook University, NOOA