Light:dark (12:12 h) quantification of carbohydrate fluxes in Crocosphaera watsonii

Light:dark (12:12 h) quantification of cellular carbohydrate fluxes was conducted with the unicellular, N-2-fixing cyanobacterium Crocosphaera watsonii WH8501 grown in continuous cultures under conditions of diazotrophy. As commonly observed in photoautotrophs, C. watsonii accumulates photosynthetic products as carbohydrates, which thereafter serve as an internal energy reserve for energy-demanding processes. Our goal was to further discriminate between cellular pools with different sizes to observe their respective dynamics. Total cellular carbohydrate as well as the cellular carbohydrate pools showed a diel cycle of accumulation and decrease. Total cellular carbohydrate contents represented 45 and 32% of the cellular carbon content at the end of the light and dark period, respectively. The observed decrease in cellular carbohydrate was not only due to carbohydrate catabolism but also to their release in the environment as extracellular polymeric substances (EPS), including soluble EPS and transparent exopolymeric particles (TEP). The occurrence of such excretion during the exponential growth of C. watsonii appears as part of the metabolic regulations that participate in maintaining a daily carbon: nitrogen balanced growth. About 10% of the total carbon cell content is excreted daily as soluble EPS. TEP, which are formed from dissolved precursors (including some soluble EPS) released by C. watsonii, increased during the dark period, when they represented up to 3.6% of total carbon cell content. Such recurrent release of carbohydrate by populations of C. watsonii is expected to stimulate the microbial loop in warm, oligotrophic oceans.