Change in the uptake and cellular Si:N ratio in diatoms responding to the ambient Si:N ratio and growth phase

A change in the Si:N ratio of diatom cells during growth was examined for Chaetoceros socialis and Thalassiosira sp., with different initial silicate to nitrate (Si:N) ratios in the media. During exponential growth, C. socialis assimilated silicate and nitrate with a molar ratio of 0.5, independent of the ratio in the media, but after the depletion of nitrate, silicate continued to be taken up, and the Si:N ratio in the stationary phase increased to 2 as a function of the Si:N ratios in the media. In contrast, the ratio of silicate to nitrate taken up by Thalassiosira sp. increased with an increase in the Si:N ratio in the media. The Si:N ratio in the cells during the stationary phase increased in response to an increase in this ratio in the media. The Si:chl  a ratio also increased with the increase in the initial Si:N ratio in the media, while the N:chl  a ratio did not change to a great extent, indicating the changes in the cellular Si:N ratio was derived from changes in the Si content of the cells. These results indicated that the cellular Si:N ratio changed with the Si:N ratio in the medium, and the Si:N uptake ratio during the growth phase was different depending on diatom species. Thus, the dominance of different diatom species may affect nutrient composition and dynamics in the ocean.