Biofilm dynamics under salt exposure: insights from irrigation piping systems

Global agricultural dependence on blended saline and freshwater irrigation mandates a mechanistic understanding of how salinity influences microbial biofilms within distribution networks, which are pivotal mediators of water quality and pathogen viability. Here, we examine the architectural, mechanical, and operational reactions of multi-species biofilms to saline exposure (0.6% NaCl) utilizing a regulated laboratory-scale irrigation model. Through a cohesive methodology combining confocal microscopy, atomic force microscopy, 16S rRNA sequencing, and meta-transcriptomics, we elucidate that salinity instigates a pivotal trade-off in biofilm maturation. While salt stress consistently suppressed live and dead cell biovolumes, it induced a significant enhancement of extracellular polymeric substances (EPS), leading to a thicker, EPS-rich biofilm architecture. These saline biofilms exhibited a lower adhesive force and Young's modulus, indicating a softer and less sticky surface. A community analysis revealed a reduction in taxonomic heterogeneity, along with an increase in specialized taxa associated with hydrocarbon decomposition functionalities, such as Hydrogenophaga and Nakamurella. Consequently, transcriptomic characterization revealed substantial upregulation of genes implicated in lipid distribution, ionic equilibrium, and oxidative stress mitigation, in conjunction with a downregulation of intercellular adhesion pathways. Our findings reveal that salinity drives biofilm adaptation towards a protected, EPS-dominated state with a functionally specialized community, suggesting a potential increase in the resilience of biofilms and risk of pathogen shielding in saline irrigation systems.