Fungal CO2 Fixation in Soil Systems: Distinct Roles of PEPC and PC in Carbon Allocation Under Varying CO2 and Thiosulfate Conditions

The role of filamentous fungi in soil CO2 fixation under elevated CO2 concentrations remains poorly understood, particularly regarding the regulatory mechanisms of CO2 fixation enzymes. In this study, a microcosm incubation experiment was conducted to investigate the effects of Glucose, Na2S2O3, and CO2 concentrations on the CO2 fixation capacity of Trichocladium uniseriatum. A full-factorial design with three Glucose levels (0, 100, and 200 mg·kg−1), three Na2S2O3 levels (0, 50, and 100 mg·kg−1), and two 13CO2 levels (5% and 10%) was employed. Fungal colony growth, activities of phosphoenolpyruvate carboxylase (PEPC) and pyruvate carboxylase (PC), and the accumulation of 13C in different soil carbon fractions were analyzed. Results showed that colony diameter was significantly influenced by Glucose and Na2S2O3 concentrations, with optimal growth observed under moderate levels (100 mg·kg−1 Glucose and 50 mg·kg−1 Na2S2O3) at 10% CO2. PC activity was negatively correlated with CO2 concentration but positively correlated with Na2S2O3 concentration, whereas PEPC activity showed positive correlations with both CO2 and Na2S2O3. The δ13C abundance in active carbon fractions (dissolved organic carbon, DOC, and microbial biomass carbon, MBC) was significantly higher than in soil organic carbon(SOC), with MBC showing the greatest enrichment. Random forest models identified distinct drivers for 13C accumulation across carbon fractions: CO2 and PEPC contributed to 13C accumulation in SOC, while PC activity drove 13C allocation to DOC and MBC. Partial least squares path modeling further revealed that PC and PEPC collectively mediated 13C allocation to DOC, with PC specifically driving 13C into MBC and PEPC channeling 13C into SOC. These findings demonstrate that T. uniseriatum mediates CO2 fixation through the differential regulation of PEPC and PC, thereby influencing the partitioning of fixed 13C among soil carbon fractions. This study provides mechanistic insights into fungal-driven CO2 fixation under changing atmospheric CO2 scenarios.