Production of Organic Acids via Autofermentation of Microalgae: A Promising Approach for Sustainable Algal Biorefineries

We demonstrate a facile and simple method for the direct conversion of microalgae biomass into organic acids via self-anaerobic metabolism in the dark (autofermentation). This one-step method does not require the use of additional catalysts (chemicals or enzymes), occurs at ambient temperature, and uses wet microalgae biomass as feed. We studied the effect of pH, salinity, and biomass concentration on the organic acid yield, rate of fermentation, and the ability of microalgae to ferment free sugars amended to the microalgae feedstocks. Our results show that when autofermentation of microalgae biomass was performed at extreme alkaline pH (>10), ∼90% of the degraded organic carbon was recovered as organic acids. Also, relative to mildly alkaline pH (8.4), a 4- to 6.5-fold enhancement in fermentation rates was observed at extreme alkaline pH (10.4) for the microalgae Chlorella sorokiniana SLA-04 and Scenedesmus obliquus UTEX 393 as well as the cyanobacterium Synechococcus elongatus UTEX 2973. The results also showed that both cellular carbohydrates and proteins were degraded during autofermentation, although the yields of organic acids from carbohydrates were higher. The lipids appear to be conserved in the residues and could potentially be extracted after autofermentation for further conversion. Externally added sugars could also be fermented in addition to cellular organic carbon. Overall, the autofermentation approach is suitable for conversion of lipid-lean, but carbohydrate- and protein-rich microalgae biomass that is typically produced in mass cultivations that target high biomass productivity.