Distributions of fungal melanin across species and soils

Abstract

Soil is one of Earth's largest carbon (C) sinks, and the diverse community of fungi it houses may affect soil C storage through the biosynthesis of recalcitrant cell wall polymers like melanin. We tested the hypotheses that (1) specific biological features of fungi - evolutionary history, functional guild, growth rate, and functional gene abundance in fungal genomes - predict fungal melanin content across species and (2) the abundance of melanin-producing fungi in soil is related to soil C storage and oxidative enzyme activity by colorimetrically assaying melanin in hyphal tissue of 62 fungal species. We found no phylogenetic signal for melanin content across the species used in our study. Instead, hyphal melanin content varied across fungal species, correlating with 177 protein domains encoded in fungal genomes. Melanin concentrations were positively correlated with protein domains involved in biosynthesis of phenolic melanin precursors, as well as primary and secondary metabolite biosynthesis, cell signaling, and oxidation-reduction reactions. By contrast, hyphal melanin content was negatively correlated with protein domains involved in DNA replication processes and stress response, as well as hyphal growth rate, suggesting a physiological tradeoff between melanin biosynthesis and cellular growth. Estimated melanin content of soil was positively correlated with total soil C and soil peroxidase activity, suggesting that fungal melanin may influence soil C cycling processes.