Diatom cell-specific silica production as a morphological adjustment to high growth irradiance

Marine diatoms represent one of the most successful phytoplankton groups in the marine environment. Their ecological success has been attributed to their photosynthetic plasticity, and the protection afforded them by their rigid silica cell walls. In this study, we investigated the influence of light environments on the physiology and morphology of the centric diatom Thalassiosira oceanica . Specifically, we determined how irradiance influenced growth rates, cell stoichiometry, photobiology and silica production. We found that T. oceanica exhibited optimal growth at irradiance around 250 µmol photons m -2 s -1 , and showed a 15%–23% increase in frustule thickness at growth irradiances at or above 480 µmol photons m -2 s -1 . While we found no direct relationship between growth and silicification, we did find that heavier silicified cells showed reduced dependence on non-photochemical quenching and lower excitation pressure when exposed to high irradiances. The observed co-adjustment of cell properties and silicification in response to growth irradiance, suggests that T. oceanica uses both physiological and morphological plasticity in photo-acclimation. These findings shed light on the functionality of frustule morphology and plasticity on diatom photo-acclimation, whereby through coordinating cellular changes with frustule properties, T. oceanica may be able to modulate the intracellular light field of its cell to optimise growth. This light-induced frustule modulation may represent an important growth strategy for natural diatom assemblages found in environments where light exposure can be highly dynamic.