Production of carbohydrates by marine planktonic diatoms. II. Influence of the [formula omitted] ratio in the growth medium on the assimilation ratio, growth rate, and production of cellular and extracellular carbohydrates by Chaetoceros affinis var. willei (Gran) Hustedt and Skeletonema costatum (Grev.) Cleve

Abstract

The N P ratio of the growth medium was varied in six steps from 0.4 to 100 in batch cultures of the marine diatoms Skeletonema costatum (Grev.) Cleve and Chaetoceros affinis var. willei (Gran) Hustedt. The influence of these changes on growth rate, assimilation ratio of nitrogen and phosphorus, production of cellular and extracellular carbohydrates, protein, cellular phosphorus and cell size was studied during exponential and stationary growth phase. A decrease in the growth rate from ≈ 1.7 to 1.1 div. day −1, and a decrease in the rate of uptake of nitrate from the growth medium, were observed at N P ratios in the medium over the range of 10 to 100. The content of phosphorus/cell was almost constant for the lowest N P ratios both in the exponential and stationary growth phase, and then decreased for both species with an increasing N P ratio. The cellular N P ratio is not constant, but changes in the direction of the composition of the medium. The range of the cellular N P ratios was from ≈ 2 to 59. Rapidly growing cells of both species produced relatively little carbohydrate while slow-growing cells produced relatively much more, and this production is influenced by the N P ratio of the medium. An increase in the N P ratio of the medium leads to a decrease in the ratio of cellular protein to carbohydrate in the exponential phase and an increase in the same ratio in the stationary phase. The major increase in cellular carbohydrate was due to an increase in the β-1,3 glucan content. The ratio of cellular carbohydrate to cellular phosphorus varied from 15 to 675 and this ratio is suggested as another useful indicator of the physiological state of marine diatoms. Particularly at high N P ratios, C. affinis produces large amounts of an extracellular polysaccharide, and under extreme P-deflciency this production is likely to constitute the main photosynthetic activity. S. costatum produced only small amounts of extracellular polysaccharides. The two species also show an important difference in the N P balance point — the value found being ≈ 12 for S. costatum and 24 for C. affinis i.e., S. costatum is likely to use more of the available phosphorus than C. affinis and is more able to divide and produce cellular carbohydrate when under N-limitation. The potential ecological significance of these findings is discussed.