Model lignin compounds affect lipid accumulation by the oleaginous bacterium Rhodococcus rhodochrous

Lignocellulosic biomass is the most abundant and renewable organic material in the world. The lignocellulosic complex is composed of cellulose, hemicellulose, and lignin, which can be pretreated to release sugars from the complex, and often recalcitrant, lignin polymer for microbial fermentation. Oleaginous microbes can accumulate over 20% of their cell dry weight as lipids that are stored as intracellular energy reserves. The characterization of other oleaginous bacteria creates opportunities for the development of alternative feedstocks and technologies. Rhodococcus rhodochrous is a gram-positive bacterium recently determined to be oleaginous when grown in glucose-supplemented media. The purpose of this study was to evaluate model lignin compounds as substrates for lipid accumulation by R. rhodochrous. Shake-flask experiments were performed using 4-hydroxybenzoic acid (HBA) and vanillic acid (VA) as model lignin compounds with and without glucose as a co-substrate in limited nitrogen conditions to promote lipid accumulation. Samples were taken daily and lipids were extracted from a freeze-dried pellet using a modified Bligh and Dyer method. R. rhodochrous accumulated over 40% of cell dry weight as lipid when grown in glucose with HBA and VA, but less than 20% when grown in HBA and VA without glucose. Cell dry weight increased significantly in all treatments suggesting R. rhodochrous could survive and grow in HBA and VA alone or with glucose as a co-substrate. Future research will aim to optimize culture conditions of lignocellulosic compounds for growth and lipid accumulation by R. rhodochrous.