Drought Drives Extracellular Polymeric Substances Accumulation and Functional Shifts in Streambed Biofilm Communities.

This study investigates the adaptive response of streambed microbial biofilms to water scarcity, focusing on the role of extracellular polymeric substances (EPS) production across a gradient of hydrological conditions. Sediment samples from 37 streams in the north-eastern Iberian Peninsula, encompassing both permanent and intermittent flow regimes, were analysed for EPS-polysaccharide content, microbial biomass, chlorophyll-a, and biofilm function (carbon substrate utilization profiles). Drought conditions were characterized based on the number of dry days over the eight months preceding sampling. Results revealed that EPS production increased significantly in intermittent streams, particularly under long-term drought, reaffirming that EPS synthesis is a key microbial strategy to mitigate desiccation stress. Notably, when normalized to prokaryotic density, EPS content exhibited a significant positive correlation with drought duration, emphasizing the dominant role of heterotrophic bacteria over algae in EPS secretion. However, EPS content alone was not a universal indicator of water scarcity, which showed a large variability in permanently flowing streams. Functional profiling showed clear shifts in carbon substrate utilization associated with stream hydrology. Intermittent streams exhibited a broader metabolic range, and particularly a capacity to use phenolic compounds, suggesting an adaptation to terrestrial organic matter inputs. Contrary to expectations, functional diversity increased in drier conditions, challenging assumptions derived from controlled experiments and underscoring the resilience of Mediterranean microbial biofilm communities to drought. These findings provide empirical support for EPS-mediated drought adaptation in natural biofilms and highlight functional diversity as a potential mechanism maintaining ecosystem processes under increasing aridity due to climate change.