Comparison of microbial strains as candidate hosts and genetic reservoirs for the valorization of lignin streams†

Abstract

Bioconversion of lignin-rich streams requires microbial hosts capable of utilizing and tolerating heterogeneous mixtures of monomeric and oligomeric compounds. Promising strains such as Novosphingobium aromaticivorans F199, N. aromaticivorans JMN2, Pseudomonas putida KT2440, Rhodococcus opacus PD630, Rhodosporidium toruloides NBRC0880, Sphingobium lignivorans B1D3A, and S. lignivorans SYK-6 possess inherent catabolic abilities to utilize lignin-related compounds (LRCs). In this work, we compared the cellular fitness and catabolic capabilities of these six bacteria and one yeast on a lignin-rich stream, alkaline pretreated liquor (APL) from corn stover, and on representative aromatic and aliphatic compounds. First, a minimal medium recipe that supported the growth of all seven strains on LRCs was selected for the comparison. Using this minimal medium, P. putida KT2440 was found to have the fastest growth and greatest tolerance when grown on guaiacyl-type compounds, p-hydroxyphenyl-type compounds, aliphatic acids, corn stover APL, and a model chemical mixture, whereas the S. lignivorans strains had the fastest growth on the syringyl-type compound. After 120 h on APL, the change in total lignin was 10–12% and aromatic and aliphatic compound usage was 85–96% for all the strains except R. opacus PD630, which had minimal utilization of APL components. Although substantial conversion of high-molecular-mass lignin was not observed by any strains, the S. lignivorans strains showed detectable modification β-ether units. Additionally, the N. aromaticivorans strains liberated aromatic compounds, potentially from lignin oligomer modification. This work serves as a comparison of seven promising microbial strains for bioconversion of lignin-enriched streams, providing a foundation for evaluating suitable microbial platforms for lignin valorization and genetic reservoirs to source unique metabolic capabilities.