Transcriptomic analyses of bacterial growth on fungal necromass reveal different microbial community niches during degradation

ABSTRACT Bacteria are major drivers of organic matter decomposition and play crucial roles in global nutrient cycling. Although the degradation of dead fungal biomass (necromass) is increasingly recognized as an important contributor to soil carbon (C) and nitrogen (N) cycling, the genes and metabolic pathways involved in necromass degradation are less characterized. In particular, how bacteria degrade necromass containing different quantities of melanin, which largely control rates of necromass decomposition in situ , is largely unknown. To address this gap, we conducted a multi-timepoint transcriptomic analysis using three Gram-negative, bacterial species grown on low or high melanin necromass of Hyaloscypha bicolor . The bacterial species, Cellvibrio japonicus, Chitinophaga pinensis , and Serratia marcescens , belong to genera known to degrade necromass in situ . We found that while bacterial growth was consistently higher on low than high melanin necromass, the CAZyme-encoding gene expression response of the three species was similar between the two necromass types. Interestingly, this trend was not shared for genes encoding nitrogen utilization, which varied in C. pinensis and S. marcescens during growth on high vs low melanin necromass. Additionally, this study tested the metabolic capabilities of these bacterial species to grow on a diversity of C and N sources and found that the three bacteria have substantially different utilization patterns. Collectively, our data suggest that as necromass changes chemically over the course of degradation, certain bacterial species are favored based on their differential metabolic capacities. IMPORTANCE Fungal necromass is a major component of the carbon (C) in soils as well as an important source of nitrogen (N) for plant and microbial growth. Bacteria associated with necromass represent a distinct subset of the soil microbiome and characterizing their functional capacities is the critical next step toward understanding how they influence necromass turnover. This is particularly important for necromass varying in melanin content, which has been observed to control the rate of necromass decomposition across a variety of ecosystems. Here we assessed the gene expression of three necromass-degrading bacteria grown on low or high melanin necromass and characterized their metabolic capacities to grow on different C and N substrates. These transcriptomic and metabolic studies provide the first steps toward assessing the physiological relevance of up-regulated CAZyme-encoding genes in necromass decomposition and provide foundational data for generating a predictive model of the molecular mechanisms underpinning necromass decomposition by soil bacteria.