Pretreatment and fermentation of salt-water grown algal biomass as a feedstock for biofuels and high-value biochemicals

Abstract

Utilization of halotolerant algal strains and saline water resources is necessary to increase the areas available for algal cultivation and to mitigate fresh water use issues. However, the added salt content from salt-water algae cultivation may impact downstream biomass pretreatment and conversion processes especially when combined with the salt that is generated as part of our Combined Algal Processing (CAP) scheme which includes pretreatment with sulfuric acid for cell disruption and carbohydrate hydrolysis. Here we compared the pretreatment, fermentation, and lipid extraction processes on salt-water versus fresh-water grown algal biomass. Response surface pretreatment plots showed that a broader range of conditions for the salt-water grown algae species yield >90% sugar yield compared to a narrower range for fresh-water species. Despite this, we anticipated that high salt content would inhibit fermentation of algal sugars, a key element the CAP scheme and sought to reduce the formation of additional salt from pretreatment and neutralization by substituting oxalic acid for sulfuric acid. The sugar release response surface for oxalic acid was of a different shape than that of sulfuric acid but achieved >90% sugar release at 2% acid. Fermentation results showed that for Saccharomyces cerevisiae, the added salts had minimal impact on sugar utilization rates or ethanol production. For Actinobacillus succinogenes however, a significant delay was observed due to the additional salts and fermentation was further delayed or inhibited by pretreatment with oxalic acid. This was remedied by removal of the oxalic acid either by the addition of CaCO3 or trioctylamine whereupon fermentation rates recovered. Differences in lipid extraction yields were observed from the fermentation broth between the salt-water grown and fresh-water grown algal biomass and was species dependent.