The effect of different light regimes on diatom frustule silicon concentration

Significant changes in frustule morphology of Coscinodiscus granii induced by different light regimes have previously been observed, but whether these changes are companied by similar changes in cellular silicon concentration or are solely due to reorganization is unknown. The influence of six different light regimes, blue (B), green (G), yellow (Y), red-orange (RO), red (R) and white (W), at two intensities (100 and 300 μmol photons m−2 s−1) on cellular silicon content was therefore assessed for C. granii. Both the cellular Si content and the Si concentration per surface area were higher at 300 than at 100 μmol photons m−2 s−1 for all tested wavelengths, except Y light. At the same light intensity, cells grown at B light had the highest cellular Si content (except W at 300 μmol photons m−2 s−1) and Si concentration per surface area. At R, OR, G, B and W light, high intensity led to higher cellular Si content although the mean frustule size was smaller (opposite for Y light). As the higher Si content was not due to a larger cell size, we hypothesize that the effect is due to changes in valve thickness. The underlying adaptive significance of this phenomenon might be that at low irradiance (100 μmol photons m−2 s−1), diatom buoyancy is modulated by accumulating less silicon in the cells, as the less silicified cells may have lower sinking rate and thus remain longer at the higher light in the upper part of the water column, which could partly contribute to the ecological success of the diatom.