Variable kinetics of silicon-limited growth in<i>Thalassiosira pseudonana</i>(Bacillariophyceae) in response to changed chemical composition of the growth medium

The diatom Thalassiosira pseudonana Hasle & Heimdal was grown in batch culture in one artificial sea-water medium and one fresh-water medium over a range of orthosilicic acid concentrations. Growth rates and cellular silicon quotas were measured in the exponential growth phase. The half-saturation constant of silicon-limited growth was about 0·04 μm Si(OH)4 in the marine medium and about 8·6 μm Si(OH)4 in the freshwater medium, the maximum growth rate being about 2·75 d-1 in both media. Possible reasons for the difference in affinity for silicon in salt and fresh water include differences in pH, ion composition, and osmotic pressure. The distribution of T. pseudonana in nature is in agreement with the experimental data, but the investigation shows the importance of a systematic evaluation of the influence of water chemistry on silicon utilization especially in fresh-water plankton. The cellular silicon content was greater in fresh water and did not vary with substrate concentration as in the marine medium. The present batch-culture results did not conform to a previously established model relating growth rate in continuous culture to cell silicon quota in the same species.