Biofilms remember: Osmotic stress priming as a microbial management strategy for improving salinity acclimation in nitrifying biofilms

With increasing freshwater scarcity and greater use of seawater,fluctuating salinities are becomingcommon in water treatment systems. This can be challenging for salinity-sensitive processes like nitri-fication, especially in recirculating aquaculture systems (RAS), where maintaining nitrification efficiencyis crucial forfish health. This study was undertaken to determine if prior exposure to seawater (priming)could improve nitrification in moving bed biofilm reactors (MBBR) under salinity increase from fresh-water to seawater. The results showed that seawater-primed freshwater MBBRs had less than 10%reduction in nitrification activity and twice the ammonia oxidation capacity of the unprimed bioreactorsafter seawater transfer. The primed biofilms had different microbial community composition but thesame nitrifying taxa, suggesting that priming promoted physiological adaptation of the nitrifiers. Primingmay also have strengthened the extrapolymeric matrix protecting the nitrifiers. In MBBRs started up inbrackish water (12‰salinity), seawater priming had no significant impact on the nitrification activityand the microbial community composition. These bioreactors were inherently robust to salinity increase,likely because they were already primed to osmotic stress by virtue of their native salinity of 12‰. Theresults show that osmotic stress priming is an effective strategy for improving salinity acclimation innitrifying biofilms and can be applied to water treatment systems where salinity variations are expected