Effects of nutrients on diatom skeletal silicification: Evidence from Neodenticula seminae culture experiments and morphometric analysis

The diatom Neodenticula seminae (Simonsen and Kanaya) Akiba and Yanagisawa is important as one of the main primary producers in the subarctic North Pacific Ocean ecosystem. Its valve morphology showed a marked spatial difference between populations in the NW margin of the North Pacific Ocean and those of oceanic regions including the Bering Sea. There were also seasonal fluctuations in the degree of valve silicification found in the former region. Determining the origin of this intraspecific variability in morphology will improve our understanding of the detailed relationships between (paleo-) hydrography and the ecological and evolutional responses of microalgae.We conducted time-series batch-culture experiments under different macronutrient conditions to evaluate the morphological responses of N. seminae. We used a cloned strain originally from the subarctic Oyashio Current off northeast Japan and made morphometric valve measurements. Under low ambient Si concentrations, we observed accelerated skeletal silicification, resulting in an increase of heavily silicified specimens when specific growth rates were high during logarithmic growth phases. In contrast, accelerated silicification continued throughout the entire incubation period (16 d) under high Si concentrations. These contrasting patterns of skeletal silicification under opposing Si conditions probably resulted from a balancing between Si availability and specific growth rates. There was an inconsistent relationship between the cell Si quota and skeletal silicification. This suggests that during logarithmic growth, Si was not always incorporated into the skeleton but was sometimes concentrated more in intracellular pools.