Production of Organic Acids from Seaweed Biomass (Laminaria japonica) using a Continuous Mixed Culture System

Abstract

Marine biomass is a potential green carbon resource for bio-based energy and chemicals. In this study, Laminaria japonica was chosen as a model marine biomass due to the fast growth rate and well-established cultivation technology. A mixed culture system was established by inoculating anaerobic sludge obtained from an anaerobic digester and operated in a continuous mode for over 200 days by feeding non-sterile suspension of dried kelp in tap water without additional nutrients. Various organic acids including butyrate, propionate and acetate were produced of which the profiles and yields were dependent to the change of pH: the portion of acetate in total organic acids increased according to the increase of pH, while the portion of butyrate was higher (> 85 %) at low pH (4.5~5.0). The maximum yield and productivity of total organic acids were as high as 0.42 g/g COD, 1.8 g/L/day, respectively at pH 7 with 64.4 % of acetate in total organic acids. The comparative DGGE analyses of mixed microflora provided information on the profiles of microbial population during the operation of the CSTR. Along with the changes of pH and substrate loading, major microbes contributing to acids production changed: a majority of microbes were identified as fermentative microbes at low pH, while saccharolytic microbes at high pH. Clostridium sp., a particular anaerobic fermentative microbe producing volatile fatty acids (VFAs) from sugar, and Prevotella sp., were commonly identified regardless of operational conditions. It is considered that pH sensitiveness of microbes could affect the yield and selectivity of VFAs according to pH changes. We successfully demonstrated a continuous biorefinery process using a mixed culture system for the production of organic acids from marine seaweeds and suggest consideration of this marine biorefinery process as a platform technology for next-generation bioenergy production.