Impact of lignin constituents on the bacterial community and polycyclic aromatic hydrocarbon co-metabolism in an agricultural soil

Abstract

Lignin is a complex biopolymer comprising phenolic monomers with different degrees of methoxylation and may potentially enhance the degradation of soil pollutants such as polycyclic aromatic hydrocarbons (PAHs) through co-metabolism. However, the contribution of lignin constituents, including phenolic and methoxy subunits, to PAH biodegradation remains unclear. Here, p-hydroxybenzoate (pHBA), vanillate and methanol were selected to simulate phenolic units and methoxy groups of lignin. Soil microcosms receiving these compounds were established to evaluate their regulation on the bacterial community and PAH co-metabolism. There were different effects of different components on the biodegradation of a four-ring PAH, benzo(a)anthracene (BaA), as characterized using an isotopic tracer. Only vanillate significantly stimulated BaA mineralization to CO2, with pHBA and methanol leading to no appreciable change in the allocation of BaA in soil compartments. The lignin constituents had differential impacts on the soil bacterial community, with substantial enrichment of methylotrophs occurring in methanol-supplemented microcosms. Both vanillate and pHBA selected several aromatic degraders. Vanillate caused additional enrichment of methylotrophs, suggesting structure-dependent stimulation of bacterial functional guilds by lignin monomers. Compared with its constituents, lignin produced more extensive responses in terms of bacterial diversity and composition and the fate of BaA. However, it was difficult to link BaA co-metabolism to any specific bacterial taxa in the presence of lignin or its subunits. The results indicate that the co-metabolism effects of lignin may not be directly associated with phenolic or methoxy metabolism but with its regulation of the soil microbiome.