Continuous culture of marine diatoms under silicon limitation. 3. A model of Si-limited diatom growth 1

A biochemically based model for simulating silicon-limited diatom growth has been developed in which silicic acid uptake is described by Michaelis-Menten kinetics. Utilization of silicon within the cell is a function of silicon available in internal pools, while growth is a function of silica in the frustule. Uptake is frequently limited by the rate of silicon utilization within the cell. That rate may be set by the enzymes, or more likely one key enzyme, which regulates the process of frustule formation, Alternatively, some other factor such as light or another nutrient may limit growth, and this in turn may limit silicon utilization within the cell and silicic acid uptake. The model was designed specifically to simulate steady state and transient behavior of a Si-limited diatom population growing in a chemostat. Comparison of simulations and actual chemostat results indicate that it is a realistic representation of diatom response under these conditions. Diatoms require silicon to synthesize their frustules, and it is reasonable to assume that the supply of silicon as orthosilicic acid, Si(OH)4, could at times limit diatom growth. During diatom blooms very low or undetectable levels of silicic acid have been observed in both fresh water (e.g. Tessenow 1966; Schelske and Stoermer 1972) and marine environments (e.g. Grant 1971; Ryther et al. 1971). These observations have led to an increased interest in the effects of silicon limitation on diatom growth in the natural environment (Kilham 1971; Dugdale 1972; Azam and Chisholm 1976) and in laboratory experiments (e.g. Davis et al. 1973; Harrison 1974; Paasche 1973a,b, 1975). Recently progress has been made with nitrogen uptake models for phytoplankton (Williams 1971; Grenney et al, 1973) r Contribution 991 from the Department of Oceanography, University of Washington. This work was supported by National Science Foundation grants GA 31093, GB 20182, and DES-7421434. a Present address: Great Lakes Research Division, University of Michigan, Ann Arbor 48109. 3 Present address: Bigelow Laboratory for Ocean Sciences, McKown Point, West Boothbay Harbor, Maine 04575. ’ Present address: Institute of Oceanography and Department of Botany, University of British Columbia, Vancouver, B.C. V6T lW5. which specify that growth and uptake are not directly coupled. In the Grenney et al. model, uptake is dependent on the extracellular nutrient concentration while growth is dependent on the internal nutrient pool concentration, in this case as amino acids available to form protein. There is considerable biochemical evidence to indicate that this approach is reasonable. Although there are numerous steps in the conversion of nitrate, nitrite, or ammonium to protein and nucleotides, the process seems to be regulated at a few key points: uptake, nitrate and nitrite reduction, catalysis by glutamic dehydrogenase, and the formation of protein; and only these steps need be included in the model. Werner (1966) showed that diatoms have small intracellular pools of metabolically active, low molecular weight or organically bound silicon; these forms constitute a few percent of the total silicon content of the diatom cell, the vast majority of the silicon being incorporated as silica in the frustule. This general picture was recently confirmed by Azam et al. (1974) using 31Si as a tracer. The present model for silicon uptake and utilization is based on the biochemical framework developed by Werner (1966) and on extensive experiments with Skeletonema costatum (Davis 1973; HarLIMNOLOGY AND OCEANOGRAPHY 41 JANUARY 1978, V. 23(l)