Ciliates as bioindicators of water quality: A case study in the neotropical region and evidence of phylogenetic signals (18S-rDNA)

The aim of our study was to evaluate the water quality of an urban stream in southeastern Brazil by analyzing epibenthic ciliates, and to investigate the existence of phylogenetic signal for saprobity in ciliates. However, before conducting this type of phylogenetic study, it is necessary to evaluate if the saprobic classification used frequently in the northern Hemisphere is suitable for neotropical ecosystems. Sediment samples were collected from five sampling stations: two in rural areas and three in urban areas. During the one-year study, with monthly collections, 39 ciliates species were found, of which 32 are included in the saprobic system. Physical, chemical and biological parameters of water confirm the spatial heterogeneity of the sampling stations, with a clear influence of organic pollution on the composition and structure of ciliates taxocenosis. The saprobic index and the saprobic valence index were used to evaluate the water quality of the sampling stations and demonstrated clear heterogeneity between the stations and high degree of pollution of the urban area. These sampling stations were dominated by ciliates indicators of polysaprobric environments. Since we were able to successfully use the saprobic index in a limnic ecosystem in Brazil, we applied the phylogenetic signal validation as a tool for saprobity prediction of the limnic ciliate species not yet analyzed. A phylogenetic tree containing only 18S-rDNA nominal sequences of freshwater ciliates was estimated and used to explore the existence of the phylogenetic signal, which showed that the sensitivity/tolerance of ciliates to organic pollution reflected evolutionary divergence. The results confirm the existence of phylogenetic signal for the saprobrity in Ciliophora. Also, our results suggest that evolutionary analysis is a potential method to predict lineages of ciliates not yet classified for saprobity.