<p>Saline lakes in arid and semi-arid regions experience recurrent drying and rain events that cause intense salinity fluctuations. Microorganisms are key actors in saline lakes carbon cycling, yet the development of resistant strategies to cope with extreme and fluctuating salinity conditions may compromise their functioning. Here, we investigate the impact of natural salinity variations on the heterotrophic activity of planktonic microbial communities in the hypersaline lakes of the Monegros area. To this end, a wide range of extracellular enzyme activities together with dissolved organic matter (DOM) quantity and quality were measured at the water column of four playa-lakes under different hydrological conditions. Salinity was the primary factor driving changes in enzyme activities across samples, whereas DOM optical properties remained relatively stable and did not correlate with enzyme activities. At intermediate salinity levels, enzyme activities fell within the range of observed values in freshwater ecosystems, suggesting a full adaptation of the microbial community to hypersalinity. However, when fluctuating towards both extremes of the measured salinity gradient, a concomitant decrease in prokaryote viability and hydrolytic enzyme diversity and activity was observed. We suggest that, under extreme salinity levels, prokaryotes are forced to reduce the energy allocated to enzyme production due to the energy expenditure on osmolytes’ synthesis to counteract osmotic stress. On the other hand, when rain events sharply decrease lakes salinity, prokaryotes must face a fast increase in turgor pressure that reduces their viability and energy allocation to enzyme production. In contrast, phenol oxidase activity steadily increased with salinity, probably as a protection mechanism.</p>