Ciliates as next-generation bioindicators in pollution-impacted freshwaters: Integrating community signatures with stress-response endpoints

Aquatic ecosystems are increasingly threatened by diverse pollutants that compromise ecological integrity and impair ecosystem functioning. Conventional physicochemical monitoring methods are insufficient to detect early biological responses or cumulative stress. Bioindicators offer a biologically integrative, cost-effective, and ecologically meaningful alternative. This review synthesizes current knowledge on established and emerging bioindicator groups, with a critical appraisal of ciliates (Phylum Ciliophora), whose short generation times, trophic versatility, and suitability for both morphological and molecular assays position them as promising next-generation indicators of freshwater ecosystem health. We identify a clear research gap: although numerous primary studies document dose-dependent cytotoxic, oxidative-stress, and behavioural responses in ciliates, but no comprehensive review has consolidated these findings or outlined how ciliates can be operationalized in biomonitoring frameworks. The objectives of this review are to (i) compile and evaluate ciliate-based assays ranging from microscopy to qRT-PCR and multi-omics approaches, (ii) compare ciliates with other bioindicator taxa, and (iii) introduce a novel, empirically testable Ciliate Community Index (CCI) that integrates community composition, morphological integrity, and fold-change–based molecular biomarker responses into a single, unified assessment framework. Literature synthesis reveals consistent early-warning signals, such as motility impairment, vacuolization, and antioxidant responses, demonstrating the feasibility of multi-tiered monitoring while also underscoring the need for standardized protocols and field validation. We conclude that, with appropriate calibration and methodological harmonization, the CCI has strong potential to serve as a sensitive and scalable tool for assessing metal-impacted freshwater ecosystems.