Microbial Life in Playa-Lake Sediments: Adapted Structure, Plastic Function to Extreme Water Activity Variations

Saline shallow lakes in arid and semi-arid regions frequently undergo drying episodes, leading to significant variations in salinity and water availability. Research on the impacts of salinity and drought on the structure and function of biofilms in hypersaline shallow lakes is limited. This study aimed to understand the potential changes of biofilms in playa-lake sediments during the drying process. Sediments were sampled at different depths (surface, subsurface) and hydrological periods (wet, retraction, and dry), which included a decrease in water activity (a_w, the availability of water for microbial use) from 0.99 to 0.72. a_w reduction caused a greater effect on functional variables compared to structural variables, indicating the high resistance of the studied biofilms to changes in salinity and water availability. Respiration and hydrolytic extracellular enzyme activities exhibited higher values under high a_w, while phenol oxidase activity and prokaryote biomass increased at lower a_w. This shift occurred at both depths but was more pronounced at the surface, possibly due to the more extreme conditions (up to 0.7 a_w). The increased levels of extracellular polymeric substances and carotenoids developed at low a_w may help protect microorganisms in high salinity and drought environments. However, these harsh conditions may interfere with the activity of hydrolytic enzymes and their producers, while promoting the growth of resistant prokaryotes and their capacity to obtain C and N sources from recalcitrant compounds. The resilience of biofilms in hypersaline lakes under extreme conditions is given by their resistant biochemichal structure and the adaptability of their microbial functioning.