Fungal-bacterial interactions and environmental factors contributing to production of extracellular polymeric substances

Abstract

Microbial extracellular polymeric substances (EPS) are gaining recognition as critical components in soil biogeochemical processes and drought resilience. This thesis investigates the ecological and environmental drivers of EPS production in soils, with a focus on how fungal-bacterial interactions and soil properties such as mineralogy and organic C content shape the quantity and composition of EPS. Two complementary experiments were conducted: a laboratory-based fungal-bacterial co-culture study and a field-based soil mesocosm drought experiment. The co-culture experiment re-vealed that soluble EPS production was primarily driven by bacteria, with limited influence from fungal partners. Specific fungal-bacterial pairs modified EPS composition, most notably involving Fusarium and Stenotrophomonas. However, overall, the presence of fungi did not consistently promote EPS production. In the mesocosm study, short-term drought and exclusion of plant roots or fungi did not significantly impact EPS production, while soil mineralogy strongly influenced EPS quantity and composition. The most notable soil evaluated was an ustox. Despite its low total organic C content, this soil produced disproportionately high EPS-associated carbon, suggesting pedogenic properties such as manganese toxicity may stimulate microbial EPS production. These findings emphasize that intrinsic soil properties and taxon-specific physiology of microbes are more influential in stimulating EPS production than moderate water limitation impacting plant growth. The study highlights methodological limitations in EPS extraction based on soil organic C content and mineralogy, and calls for improved protocols to accurately compare EPS production across mineralogically diverse soils. This work provides foundational insights into microbial EPS dynamics in soil systems and informs future strategies to leverage endogenous microbial traits for enhancing agricultural drought resilience.