Cellular and extracellular production of carbohydrates and amino acids by the marine diatom Skeletonema costatum: diel variations and effects of N depletion

A detailed study was made of cellular and extracellular production of carbohydrates and amino acids by the marine diatom Skeletonema costatum (Grev.) Cleve during different growth phases. Batch cultures were run with a 14:10 h light:dark cycle in N-limited media at 2 different nutrient strengths. The exponential growth rate was 2.0 div. d super(-1), and balanced growth took place except for significant diel variation in chemical composition. Inorganic C and N were primarily assimilated during the photophase, and the elemental cell quotas increased accordingly. The level of storage polysaccharide, beta 1,3-glucan, oscillated between 17% (end of scotophase) and 42% (end of photophase) of cellular organic C, and the corresponding protein:glucan ratio alternated between 2.3 and 0.7. Cell wall polysaccharides constituted 6 to 10% of cellular organic C. Concurrently, the cellular free amino acid pool oscillated between 8% (end of scotophase) and 22% (end of photophase) of cellular organic N. Glutamine emerged as the principal amino acid during photosynthesis, increasing from 0.2 to 12 fmol cell super(-1), and the corresponding glutamine:glutamate ratio increased from 0.05 to 2. Upon NO super(-) sub(3) exhaustion, the glucan level increased rapidly for 3 to 4 d, and then stabilized at 75 to 80% of cellular organic C with little diel variation. In contrast, the cellular N quota decreased by 80%, and the cell wall polysaccharide quota decreased by 35%. Consequently, the protein:glucan ratio decreased to