The effect of pH on growth rate, biochemical composition and extracellular carbohydrate production of the marine diatom Skeletonema costatum

The marine diatom Skeletonema costatum was grown at different pH values in a semicontinuous cultivation system which kept the pH at a pre-set value. Growth rate was nearly constant at pH 6·5-8·5 (average 2·4 divisions day[minus sign]1) and declined at pH > 9. Organic carbon (C) production decreased from an average of 5·3 mg l[minus sign]1 at pH 6·5-8·5 to 2 and 1 mg l[minus sign]1 at pH 9·0 and 9·4, respectively. Storage [beta]-1,3-linked glucan decreased from a high of 7·1 mg l[minus sign]1 at pH 6·5 to 0·2 mg l[minus sign]1 at pH 9·4, and, concomitantly, the percentage of total organic C as glucan C decreased from 60% to 10%. Cellular concentration of amino acids varied from about 62 to 8 mM over the pH range 8·0-9·4. At pH 9·4 the concentration of glutamine was below the detection limit. The extracellular production of carbohydrates in the pH range 6·5-8·5 was only 3·7% (average) of total C production and increased at pH 9·0 and 9·4. The decrease in growth rate at pH > 9·0 might be caused by a decrease in the rate of some important biochemical reactions, as well as by a change in cell membrane properties in this pH range.