Protists exhibit community-level adaptation and functional redundancy under gradient soil salinity

One of the most important climate change-related issues that has extremely negative impacts on terrestrial life is soil salinization, especially in lowland paddy fields. Despite the enormous impact of salinity on microbial life, the majority of research focused on bacteria and fungi, neglecting the vast majority of eukaryotic diversity, the protists. Here we aimed to understand the sole impact of the soil salinity on protist communities in paddy field soil. To exclude the variations in other environmental factors that co-varies with the soil EC, we conducted a controlled in vitro experiment to study the direct effect of gradually increased salinity levels (ranging from 0.1 dS m^-1 to 12 dS m^-1) on protists in three non-saline (<0.3 dS m^-1) paddy field soils. Then, our in vitro results were confirmed in a field study, in which seawater intrusion caused the accumulation of sea salts in paddy fields along a river. The results of the in vitro and field studies were consistent, showing that alpha and beta diversities of protists are affected by soil salinity. While protist alpha diversity exhibited inconsistent patterns across soil types, beta diversity showed strong and consistent clustering by the salinity gradient. Although salinity significantly shifted protist communities and caused a 10-fold decrease in 18S rRNA gene abundances of protists, protists maintained functional stability, suggesting that even with the compositional shifts, the critical ecosystem functions, such as predation and primary production, remained intact. These results underscore the importance of functional redundancy in sustaining ecosystem functions under salinity stress.