Physical and Nutrient Stresses for Stimulating Algal Biofuel Production: Quantitative Control of Fermentation
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Microalgae and cyanobacteria are phototrophs that have important advantages over land-based crops as potential feedstocks for energy production. They do not compete with food-crops for arable land, can often be cultured in saline or brackish waters, and produce non-recalcitrant biopolymers. One strategy for conversion of these feedstocks to fuel molecules is through anaerobic fermentation. We have studied the filamentous cyanobacterium Arthrospira (Spirulina) maxima for its ability to produce hydrogen and ethanol via fermentation of its own internal energy storage molecules, mainly carbohydrates. Growth of filaments in highly saline medium increases A. maxima's production of carbohydrates from 20% to 50% of dry weight. Subsequent fermentation of these filaments in hypotonic buffers stimulates by 50-fold the fermentative carbon products excreted relative to cells cultured in less saline growth medium. This stress stimulates production of ethanol at a rate of 1.56 mmol / gDW / day, or 121-fold higher than cultures that are not hypotonically stressed during fermentation. Unexpectedly, dark hydrogen production does not correlate with the increased rate of catabolism of soluble sugars. A second strategy for increasing fermentative yields from cells was investigated. Replacement of sodium with potassium in the fermentation buffer of A. maxima stimulates by 2-fold ethanol and hydrogen production, apparently due to increased energy demand arising from loss of energy-coupled sodium ion gradient. Supported by the AFOSR-MURI program.&'