COS 120-5 - Beyond overflow metabolism: Physiological mechanisms to deal with elemental imbalances

Abstract: Soil microbial decomposers constitute a vital ecosystem component due to their role in decomposition, nutrient cycling, carbon sequestration, and plant-soil feedbacks. Future environmental change will alter the distribution of plant species and the quality of their litter (e.g. carbon to nitrogen (C:N)). We currently lack a strong mechanistic understanding of how soil microbial decomposers will respond to these changes. It has been widely accepted that overflow metabolism (i.e. increase in respiration with higher litter C:N) is the main mechanism to overcome the elemental imbalance between microbial biomass (C:N ~8) and plant litter (C:N 15-100). However, we hypothesize that overflow metabolism is less important than previously thought and that other physiological mechanisms play a key role in reducing the elemental imbalance. In order to understand how soil microbial decomposers reduce elemental imbalances we used three fungal strains with contrasting traits (Alternaria tenuissima, Cladosporium cladosporioides, and Trichoderma koningii) and exposed them to different C, N, and phosphorus (P) availabilities in minimal media. Each species was grown in the dark for up to 1 week at 23˚C. N supply was manipulated in two treatments, C:N = 5 and C:N = 80, maintaining C:P at 100. P supply treatments were set at C:P = 20 and C:P = 3000 while keeping C:N at 20. We quantified traits that contribute to reducing elemental imbalances, including extracellular enzymatic activities, uptake and excretion rates of N, and respiration. We also quantified extension rates, biomass C, N, and P contents, and total biomass produced to evaluate potential tradeoffs among these traits. Our preliminary data shows that our three fungal species had consistently different extension rates (Trichoderma > Alternaria > Cladosporium), but all three species had lower extension rates in high C:N treatments. On the other hand, respiration rates increased for all species when C:N and C:P ratios where high, which is consistent with overflow metabolism. We conclude that overflow metabolism is an important mechanism for soil microbes to reduce the elemental imbalance with their resources, though other mechanisms may also contribute.