Microbial carbon use efficiency (CUE), the balance between carbon (C) used for growth versus CO2 production, is an important control on how global soil C stocks will respond to climate change. Microbial CUE is generally expected to decrease as microbes are exposed to increasing temperatures associated with changing climate, resulting in a loss of soil C to the atmosphere. However, this response is not well constrained, partly due to a lack of information on how the dominant microbes in soil modulate CUE. Fungi are the dominant decomposers of soil organic matter in temperate forests in terms of both biomass and activity in decomposition, making fungal CUE-temperature responses of particular interest. Here we explore how CUE, growth rates, and respiration rates of ten fungal species respond to a 10°C increase in temperature. We isolated species representative of the diversity of decomposer fungi from forest litter, including species of the phyla Ascomycota, Basidiomycota, and subphylum Mucoromycotina. We used measurements of growth rate and respiration rate in liquid culture, and whole-genome sequencing to address the hypotheses 1) that CUE decreases when fungi are exposed to increased temperatures, and 2) that fungi from different taxonomic groups or functional types have different intrinsic CUE.
Results/Conclusions
Carbon use efficiency of the ten fungal species we measured had varied responses to increased temperature. CUE of the four species belonging to the phylum Ascomycota decreased by 7-9% when grown at 15 versus 25°C, whereas CUE of the two Mucoromycotina species increased 5-20%. The response of the Basidiomycota species was mixed – two had decreased CUE but the other two species had no response to increased temperature. CUE of the Ascomycota species was higher overall, averaging 0.82 versus 0.65 and 0.62 for the Basidiomycota and Mucoromycotina, respectively. Growth rate generally increased with increasing temperature. This response was most pronounced for the two Mucoromycotina species which had 160% greater growth rates at 25°C – the Ascomycota species increased by 33% and the Basidiomycota by 4% on average. Preliminary results from genome sequencing indicate that genome size was negatively correlated with growth rate. The copy number of ribosomal RNA genes normalized for genome size was negatively correlated with CUE and positively correlated with respiration rate. Together our results suggest that species responses are dependent on taxonomy and intrinsic genomic features – indicating that temperature-driven changes in fungal communities will result in altered community function that drives changes in soil C stocks.