Interactive effects of nitrogen and light on growth rates and RUBISCO content of small and large centric diatoms

Among marine phytoplankton groups, diatoms span the widest range of cell size, with resulting effects upon their nitrogen uptake, photosynthesis and growth responses to light. We grew two strains of marine centric diatoms differing by ~4 orders of magnitude in cell biovolume in high (enriched artificial seawater with ~500 µmol L^-1 µmol L^-1 NO₃^-) and lower-nitrogen (enriched artificial seawater with <10 µmol L^-1 NO₃^-) media, across a range of growth light levels. Nitrogen and total protein per cell decreased with increasing growth light in both species when grown under the lower-nitrogen media. Cells growing under lower-nitrogen media increased their cellular allocation to RUBISCO and their rate of electron transport away from PSII, for the smaller diatom under low growth light and for the larger diatom across the range of growth lights. The smaller coastal diatom Thalassiosira pseudonana is able to exploit high nitrogen in growth media by up-regulating growth rate, but the same high-nitrogen growth media inhibits growth of the larger diatom species.