Humic substance turnover by bacterial decomposers in the maritime Antarctic soil

Context Soil fungi and bacteria play a crucial role in decomposing soil organic matter (SOM) and providing nutrients to terrestrial ecosystems. However, in polar environments, their relative contributions to decomposition remain unclear. Aims This study aimed to determine whether fungi or bacteria contributes more to humic substances (HS) decomposition (the largest constituent of SOM) in maritime Antarctic soils under controlled laboratory conditions with elevated temperature and moisture levels. Methods Soil culturing method was used to select for soil microbes that efficiently degrade HS. During culturing at 18°C for 30 days, Antarctic soils were treated with selective antimicrobials to manipulate microbial communities. After culturing, HS degradation and bacterial communities were analysed. The effects of soil culturing on plant growth were also evaluated using Arabidopsis thaliana as a surrogate for Antarctic plants. Key results HS decomposition and solubilisation were more pronounced in antifungal-treated soils compared to antibacterial-treated soils. Antifungal treatment reduced bacterial alpha diversity, altered bacterial composition, and increased the abundance of rhizosphere-associated Saccharibacteria. Bacterial growth resumed quickly after antibacterial treatment indicating resilience to antimicrobials. Soil culturing resulted in a higher water-soluble HS fraction, which significantly enhanced the fresh and dry weights of A. thaliana. Conclusions Antarctic bacteria exhibit higher degradative activity and resilience to antimicrobials compared to fungi, promoting HS decomposition and solubilisation. This indicates that bacteria have a greater impact on Antarctic soil functioning than fungi. Implications These findings highlight the critical role of bacteria in nutrient cycling and plant growth in polar ecosystems.