Unravelling Groundwater Protist and Its Linkage to Geogenic Chromium: An Underexplored Microbial Group for Groundwater Contamination

Protists are pivotal yet understudied in groundwater systems. Linkage of groundwater protists to geogenic high Cr(VI) has remained elusive. Here, we profiled protistan composition, structure, trophic functionality, and Cr(VI)-associated clusters, alongside the interactive effect of Cr(VI) and hydrogeochemical factors. Protistan communities significantly diverged between low (<50 μg/L) and high Cr(VI) (>50 μg/L) groundwater. Elevated Cr(VI) favored phototrophs with facultative mixotrophic potential but heterotrophic consumers. Notably, Cr(VI)-tolerant phototrophs with facultative mixotrophic potential were strongly correlated with high dissolved oxygen. Four identified Cr(VI)-associated clusters displayed distinct trophic structures and were linked not only to Cr(VI) but also to the Cr(VI)-associated hydrochemical factors of groundwater depth, total dissolved solids, and dissolved oxygen. Path analysis revealed the effect paths of groundwater depth, total dissolved solids, dissolved oxygen, and Cr(VI) on protistan clusters, suggesting potential divergent, niche-specific responses of cross-trophic clusters to Cr(VI) and covarying hydrochemical factors. Additionally, highly abundant ciliates reflected the bioaccumulation potential for Cr(VI). Our findings highlight groundwater protists as an underexplored microbial group with potential significance in the ecological dynamics of Cr(VI)-contaminated aquifers potentially mediated via their multitrophic interactions.