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 -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, β-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 -1 , and the corresponding glutamine:glutamate ratio increased from 0.05 to 2. Upon NO 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 < 0.1. The cellular free amino acid pool decreased by 90% within 24 h of N depletion, and continued to decrease slowly throughout the stationary phase. Glutamine decreased most rapidly, and constituted <1% of the free amino acids in the stationary phase. Extracellular production accounted for 4% of total photosynthetic production during both exponential and stationary growth phase, but the absolute excretion rate (per cell) was markedly higher in the exponential phase. A transient high release occurred in the transition phase in 1 case, which was probably caused by cell leakage. Extracellular production by 'healthy' cells contained 33% polysaccharides, 15% monosaccharides and 5% free amino acids (as C). The composition of the extracellular amino acids differed from the intracellular ones, and changed considerably from exponential to stationary growth phase. This study illustrates the rapid response of carbohydrate and amino acid dynamics to ambient N and light conditions at the cellular level.

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