Microbes form a fundamental part of the terrestrial carbon cycle by respiring CO2 into the atmosphere as a byproduct of decomposition. The efficiency by which microbes use carbon for growth versus respiration – termed ‘microbial growth efficiency’ (MGE) – is a critical lynchpin in carbon-cycle climate models. MGE is commonly assumed to be a byproduct of environmental conditions, yet this belies a large body of evidence highlighting that direct competitive interactions (i.e., combat) strongly alter ecosystem process rates. When bacteria and fungi compete, they produce secondary chemicals and compounds intended to kill their competitors and prevent displacement. To date no study has investigated how combat alters growth efficiencies across environmental conditions. Here, we conducted a laboratory experiment to disentangle the relative importance of individual physiology versus combative interactions as drivers of MGE. Individual MGE was measured on 10 wood-rot fungi grown across three temperature conditions (16, 22, and 28°C) crossed with two nitrogen levels (high vs. low). Sixty unique three-species communities were subsequently selected at random, with ten communities assigned to each of the six treatment levels. Community-level MGE was measured over the course of 10 days, and the observed MGE of the community was then compared to the expected community-weighted MGE.
Results/Conclusions
Temperature, nitrogen, and combat all had significant effects on MGE. At low-nitrogen, temperature had negligible impacts on individual MGE, whereas at high-nitrogen temperature had a strong negative influence (22% decrease in MGE per 12°C increase; p=0.002). At low temperatures, N-addition increased individual MGE by 9.5%, whereas at high temperatures N-addition decreased MGE by 14.7%. These results suggest that nitrogen is a primary control on individual MGE, with temperature emerging as a secondary control only after nutrient-limitation is alleviated. Across all communities, combat reduced MGE by 13.9% relative to expected MGE. High-N communities exhibited 10.5% greater losses in MGE relative to low-N communities (p<0.001), and warmer communities (28°C and 22°C) exhibited 13.0% greater losses in MGE than 16°C communities. The largest reductions in MGE were observed in high-N, 28°C communities (28.2% reductions; p<0.001) and the smallest reductions were observed in low-N, 16°C communities (0.03%; p=0.56). Across all microcosms (both individuals and communities), temperature, nitrogen, and combat explained 8%, 6%, and 12% of the variability in MGE, respectively. Combined, these results suggest that combat is as important as abiotic conditions in determining community-level MGE, supporting the need to include microbial community dynamics in existing carbon-cycle models.