Ciliates: Key Organisms in Aquatic Environments

Ciliates are a diverse clade of microbial eukaryotes that are one of the most morphologically complex and highly differentiated taxa among single-celled organisms. These organisms exhibit a combination of unique characters, such as nuclear dimorphism (somatic macronucleus and germline micronucleus), sexual reproduction by conjugation, highly specialized organelles and complex cytoskeletal structures, that make them important model organisms in studies of cell biology, genetics and evolution. In addition, ciliates are key drivers of the biogeochemical cycling of nitrogen, carbon, and other elements. They play an essential role in energy flow in aquatic systems as the linker between primary production and higher trophic levels. Therefore, ciliates are cytologically, phylogenetically and ecologically important components in aquatic environments.

Despite their significance to aquatic ecosystems, our understanding of the diversity, evolution and ecological roles of ciliates is relatively limited. Over 8,000 nominal species have been reported to date, but the overwhelming majority of ciliate species are inadequately described by modern standards (e.g., molecular survey). It has been estimated that 83–89% of free-living ciliate species are undescribed, indicating that many species await discovery, especially from under-sampled habitats such as the deep ocean and tropical marine environments. Only a small proportion of ciliates have been studied phylogenetically to assess their evolutionary relationships. This is mainly due to the lack of accurate and easily verifiable taxonomy data upon which to build molecular studies. Therefore, phylogenetic relationships among many ciliate groups remain mostly unclear. Furthermore, ciliates ecophysiology and diversity can be impacted by environmental factors. However, they generally tend to be lumped together with other protist groups in ecological studies rather than being investigated as an independent group. Therefore, there is a pressing need for deeper exploring their ecological response to environment variations at the individual, population, and community levels.

This Research Topic will gather the latest information on the diversity, evolution and ecology of ciliates across marine and freshwater environments. We welcome all contributions that reveal their diversity and evolutionary relationships based on morphological, morphogenesis and phylogenetic analyses, and characterize their ecological function, community organization, biogeography, and interactions with organisms at various trophic levels in all aquatic habitats. We especially encourage contributions from the under-sampled area and using an integrative approach.