Biodiversity and evolution of marine planktonic ciliates

The marine plankton comprises a speciose and functionally diverse assemblage of protists (unicellular eukaryotes). Molecular analyses demonstrate that only a small fraction (~ 1520%) of the entire diversity is represented by known species; the majority of protists have still to be described. Likewise, there are only preliminary data on the global distribution of planktonic protist species. However, understanding of ecosys- tem functioning in a world faced with climate change and ocean acidification urgently requires biodiversity and biogeography estimates in the main plankton components. The Oligotrichea, the dominant and ecologically important marine planktonic ciliates, connect the microbial loop (polysaccharides secreted by algae via bacteria via flagellates to ciliates) with the phytoplankton-based food web, transferring energy to higher trophic levels, such as minute crustaceans and fish larvae. Assess- ments of their species richness suggest that the South China Sea (northwest Pacific) is a hotspot not only concerning their total diversity, but also because of the huge number of species apparently restricted to this region. In this first joint project, Austrian and Chinese ciliate experts will discover numerous new Oligo- trichea species in the South China Sea. Investigations on the ciliates morphology, ultrastructure, cell divi- sion patterns, and marker genes will provide characters for inferring their phylogenetic relationships. Of spe- cial interest are the endemic species whose placements in the genealogies are often uncertain owing to con- tradicting morphologic and genetic data. Eventually, the findings shall contribute to discover previously misinterpreted, e.g., not inherited from common ancestor, but convergently developed, cell features. Analyses of short marker gene sequences of all Oligotrichea enclosed in water samples collected at different seasons and climate zones (North Sea, Mediterranean Sea, and northwest Pacific) together with data from previous sampling campaigns available in the web will enable estimates of the species global distributions and predictions about changes in the community composition of Oligotrichea and ecosystem functioning with increasing water temperatures, information essential for many regions worldwide whose food supply largely depends on the productivity of the oceans.

Ciliates are fascinating unicellular organisms of microscopic size (< 1 mm). The dominating ciliate group in the marine plankton are the Oligotrichea, of which approximately 1,000 shell-forming tintinnid species and about 200 naked aloricate species are known. Oligotrichea are an essential component of the marine food web. They feed on bacteria and other unicellular organisms and are preyed upon by small crustaceans, a crucial food source of fish larvae. Many important aspects of these ciliates are, however, still unknown. In the joint project with Chinese collaboration partners (NSFB project 317 611 330 01), we compared the genetic diversity of Oligotrichea in European and Chinese coastal waters based on DNA sequence data. We discovered a biodiversity hotspot in Chinese coastal waters and revealed that European and Chinese ciliate communities are quite different. Despite connecting water masses and currents, we could detect dispersal limitations for several groups, which included many currently unknown oligotrichean species. We further contrasted geographic patterns in marine and freshwater plankton by focussing on an ecologically important group of lake ciliates. Genetic traces of these species, which again included unknown ones, were recorded in 252 freshwater lakes in 17 countries worldwide. On the contrary, a comprehensive literature survey suggested a tintinnid species to be endemic to Asian rivers and lakes. In total, we redescribed six ciliate species including tintinnids and established a new tintinnid genus. Based on the latter findings, we could reconstruct the likely appearance of the first tintinnid that lived about 350 Mya. Further, we developed a user-friendly web-based tool that detects features in DNA sequences characterising particular groups of organisms like the traditionally used morphological features. Thereby, we obtained molecular features of several oligotrichean groups. The present project could also shed light on further interesting aspects of this ecologically important group of marine planktonic protists by means of transmission electron microscopy, i.e., we reconstructed the evolution of the beautiful and highly elaborate tintinnid loricae, investigated the minute harpoons of tintinnids, and studied for the first time a tintinnid resting cyst.