Dr. G. Charles Dismukes

Sodium concentration cycling was examined as a new strategy for redistributing carbon storage products and increasing autofermentative product yields following photosynthetic carbon fixation in the cyanobacterium Arthrospira (Spirulina) maxima. The salt-tolerant hyper-carbonate strain CS-328 was grown in a medium containing 0.24 to 1.24 M sodium, resulting in increased biosynthesis of soluble carbohydrates up to 50% of the dry weight at 1.24 M sodium. Hypoionic stress during dark anaerobic metabolism (autofermentation) was induced by resuspending filaments in low sodium (bi)carbonate buffer (0.21 M), which resulted in accelerated autofermentation rates. For cells grown in 1.24 M NaCl, the fermentative yields of acetate, ethanol and formate increase substantially to 0.75, 1.56 and 1.54 mmol/(gDW*day), respectively (36, 121, and 6-fold increases in rate relative to cells grown in 0.24 M NaCl). Catabolism of endogenous carbohydrate increased by approximately 2-fold upon hypoionic stress. For cultures grown at all salt concentrations, hydrogen was produced but its yield did not correlate with increased catabolism of soluble carbohydrates. Instead, ethanol excretion becomes a preferred route for fermentative NADH reoxidation together with intraceullar accumulation of reduced products of acetyl-CoA formation when cells are hypoionically stressed. In the absence of hypoionic stress, hydrogen production is a major beneficial pathway for NAD+ regeneration without wasting carbon intermediates such as ethanol derived from acetyl-CoA. This switch presumably improves the overall cellular economy by retaining carbon within the cell until aerobic conditions return and the acetyl unit can be used for biosynthesis or oxidized via respiration for much greater energy return.