Citric acid treatment of Tetradesmus obliquus biomass reduces dry matter loss in handling, queuing, and long-term storage, while stimulating auto-fermentative production of succinic acid.

Abstract

Post-harvest algae biomass is prone to degradation, resulting in mass loss and compositional changes, and preservation is vital to economic viability of algal products. Effective storage solutions are needed to mitigate for seasonal productivity variations (long-term storage) and to keep post-harvest biomass stable until processing. Ensiling has emerged as a long-term storage solution capable of preserving biomass up to 6 months with little loss without the energy demands of drying. Organic acids produced during ensiling lower biomass pH and prevent growth of degradative bacteria such as Clostridia. However, losses are front-loaded with a majority occurring within the first week before organic acids can accumulate. Currently, there is no information on the stability of algae biomass within the first 24 hr post-harvest or methods available to ensure stability during this period. Freshly harvested Tetradesmus obliquus UTEX 393 biomass was stored in three conditions: ambient atmosphere, anaerobic atmosphere without treatment, and anaerobic atmosphere with citric acid amendment. Citric acid treatment limited mass loss to 1% after 28 days, while untreated biomass experienced 4% mass loss after just 4 hr and 18% mass loss after 4 weeks. The carbohydrate fraction was most affected, with minimal changes to the elemental composition of biomass across treatments. Bacilli bacteria, including lactic acid bacteria, increased in abundance under all storage conditions. Untreated biomass showed a rise in Clostridia, but none were found in citric acid-treated biomass. After 28 days, organic acid composition differed significantly among treatments, with succinic acid being accumulated to 30% of dry cellular weight in citric acid treated UTEX 393 biomass. Citric acid treatment effectively mitigates biomass loss and, surprisingly, promotes substantial production of succinic acid. The unexpected autofermentation of UTEX 393 biomass to a versatile intermediate chemical such as succinic acid at high titers with minimal energy input could contribute to the economic viability of algae cultivation for fuels and chemicals. One-Sentence Summary  Tetradesmus obliquus biomass is susceptible to degradation immediately after harvest; citric acid treatment preserves biomass while stimulating succinic acid accumulation.