Ciliated protozoa from industrial WWTP activated sludge: a biodiversity survey including trophic interactions and redescription of Bakuella subtropica (Spirotrichea, Hypotrichia) according to Next Generation Taxonomy

Optimization of wastewater treatment with biological processes is a fundamental challenge of modern society. During past years new technologies have been developed for the purpose and prokaryotic organisms involved in the process extensively investigated. Nevertheless, relatively few studies so far analysed the protozoan community in these systems using modern integrative approaches, despite its obvious role in shaping ecological dynamics and, possibly, process efficiency. In the present study, we characterized the ciliate community in biological reactors of an Italian industrial (tannery) wastewater treatment plant (WWTP) applying modified Ludzack-Ettinger (MLE) process. This plant is characterized by moderate salinity, high solids retention time and high concentration of organic compounds, including a significant recalcitrant fraction. We performed the morphological and 18S rDNA characterizations of almost all the 21 ciliates retrieved along a one-year sampling period, and provided preliminary data on species occurrence, community dynamics, and trophic interactions. Only 16 species were observed on the sample collection day and most of them had an occurrence higher than 50%. The most frequently occurring and highly abundant organisms were Aspidisca cf. cicada, Euplotes spp., Paramecium calkinsi, and Phialina sp. Cyclidium cf. marinum was only found on a single date and its presence was possibly related to a summer break-induced perturbation. All the species showed the capability to survive the short oxic/anoxic cycling typical of the studied WWTP process. Intriguingly, some of them (i.e., Bakuella subtropica and Trochiliopsis australis) turned out to be species isolated from brackish natural environment rich in organic load as well. As for B. subtropica, we provided an emended redescription according to the most recent taxonomy standards that include also mitogenomic sequencing.