Fungal co-inoculation enhances drought tolerance and soil functioning during cyanobacterial biological soil crust development.

BACKGROUND: Soil moisture availability strongly constrains the establishment of cyanobacteria-based biological soil crusts (BSCs). This study investigated whether fungal co-inoculation could improve drought tolerance during biocrust development. METHODS: Cyanobacterial crusts (C) and cyanobacterial-fungal composite crusts (CF) were cultivated for 60 days under five soil-moisture levels (0-20%) using sand from the Hobq Desert, China. Chlorophyll a, chlorophyll fluorescence, soil nutrients, enzyme activities, EPS fractions, and GC-MS-based exometabolomics were measured. RESULTS: CF crusts maintained higher chlorophyll a content and photosynthetic activity (Fv/Fm) under drought, particularly at 5% soil moisture. Fungal co-inoculation increased soil C and N contents, enhanced sucrase, urease, and phosphatase activities, and markedly promoted extracellular polymeric substances, especially tightly bound EPS. Exometabolomic analysis revealed greater metabolic differentiation across the moisture gradient in composite crusts, with higher diversity of carbohydrate metabolites. Under equivalent drought conditions, composite crusts accumulated higher levels of drought-responsive carbohydrates (fructose, glucose, maltose, mannobiose, trehalose), while TCA-cycle intermediates generally decreased with increasing drought severity. CONCLUSIONS: Fungal co-inoculation enhances drought resilience of engineered BSCs through enhanced extracellular matrix development and drought-responsive changes in extracellular carbohydrate profiles, providing a promising microbial consortium for improving biocrust establishment in water-limited drylands.