Simultaneous depolymerization and fermentation of lignin into value-added products by the marine protist, Thraustochytrium striatum

Abstract

Biodegradation of lignin has been focused on bacteria and fungi, but little information is available on lignin biotransformation with marine protists. In this paper, kraft pine lignin was used as a carbon source for a newly recognized and patented lignin degrader, Thraustochytrium striatum HB. The fermentation medium was optimized in terms of nitrogen source and trace elements. The results showed NH4Cl was the best nitrogen source to enhance lignin consumption from 3 (for yeast extract or peptone) to 17%, while the utilization of yeast extract or peptone was found to inhibit lignin consumption by T. striatum HB. The presence of trace elements and vitamins improved lignin utilization from 17 to 25% by inducing the production of lignolytic enzymes. Different lignin depolymerization methods, including laccase, fungal secretome and Fenton reagent were used to enhance lignin fermentation. In the presence of laccase, both cell growth and lignin consumption were improved, and it was found that different laccases had different working mechanisms. Enzymatic depolymerization resulted in 25–30% lignin degradation, slightly higher than that (22%) from Fenton reagent. NMR analysis indicated that the fungus-derived laccase primarily worked on the breakdown of inter-unit linkages in lignin molecules, while plant-derived laccase possibly attacked the aromatic ring structures. With the function of laccase, more low-molecular-weight lignin fragments can be generated, and laccase-catalyzing reaction would direct the equilibrium toward depolymerization by preventing repolymerization of small fragments. On the contrary, lignin peroxidase-dominant fungal secretome and Fenton reagent facilitated lignin degradation but not cell growth, possibly due to the generation of inhibitory compounds from lignin depolymerization by fungal secretome and Fenton reagent. The results from this research demonstrate that marine protists, such as T. striatum HB are a class of promising platform microorganisms for lignin valorization into valuable bioproducts.