Functional molecular diversity of dissolved organic matter explained by predicted genome size of soil microbial communities

Abstract

Microbial decomposition of plant litter has been shown to decrease the molecular diversity of dissolved organic matter (DOM), but it remains unclear whether these DOM formation processes are conserved across ecosystems with differing microbial communities. To fill this knowledge gap, we investigated plant litter decomposition and DOM formation over a period of 150 days using materials sourced from five different ecosystem types. We tested whether microbial diversity alters DOM molecular diversity by manipulating the microbial community structure through serial dilution to produce “high” (10°) and “low” (10⁻¹⁰) diversity communities from each ecosystem. We measured microbial richness, microbial diversity and predicted genome size using 16S rRNA gene sequencing and DOM molecular diversity using tandem liquid-chromatography mass spectrometry (LC-MS/MS). We found that the functional molecular diversity of DOM across ecosystems was correlated with the average predicted genome size, a proxy for the metabolic potential of microbial communities (Pearson's r = 0.61, p = 0.008). Ecosystem type explained the most variation in DOM composition (ADONIS PERMANOVA; R² = 0.89, p-value = 0.001), but specific microbial phyla (e.g., Firmicutes, Bacteroidota, Chloroflexi) and genera (e.g., Sphingomonas, Stenotrophomonas, Leifsonia) were also correlated with DOM composition across the five ecosystems. Although DOM molecular richness was most strongly correlated with ecosystem type (F_(4,19) = 6.21; p-value = 0.002), DOM functional molecular diversity had a stronger relationship with the predicted genome size of the microbial community (F_(4,19) = 21.18; p-value = 0.0002). The positive relationship between DOM functional molecular diversity and microbial community genome size suggests that a larger microbial metabolic capacity produces a greater diversity of functionally dissimilar DOM compounds.