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As a major component and link between trophic levels, protists fulfil a variety of different ecological functions in microbial food webs and are essential components in all ecosystems. In particular, one group of heterotrophic protists, the choanoflagellates play an important role in aquatic habitats, as they are efficient filter feeders, able to handle several food particles at the same time. As choanoflagellates resemble morphologically the choanocytes of sponges, they got already early attention regarding the evolution of multicellularity. The close relation could be proven by phylogenetic analyses, in which choanoflagellates cluster within the Opisthokonta as a sister group to the metazoans, making them the closest unicellular relatives to the animals. Classical taxonomical studies on this group revealed a high morphological diversity and later on phylogenetic analyses were able to depict the relationship among a variety of species resulting into two major orders, the Craspedida possessing only an organic covering like a theca or a glycocalyx and the Acanthoecida with an inorganic siliceous basket-like covering (lorica). But still, we face a high discrepancy between the number of morphologically described and sequenced species. In addition, investigations on ecological properties proofed to be essential to further extend our knowledge on choanoflagellates concerning evolutionary questions. Moreover, the significance of a comprehensive integrative taxonomy with accurate species descriptions is nowadays undeniable, as modern molecular surveys using high-throughput sequencing methods produce massive amounts of molecular information, which can only be analysed and interpreted with the scaffold of a verified reference database founded on taxonomical data. Within this study, several diverse habitats from freshwater to hypersaline environments were investigated to extend our current knowledge on the diversity and ecology of choanoflagellates. From freshwater and marine habitats, exemplified by studies from the River Rhine at Cologne and a transect of the Atlantic Ocean respectively, new craspedid choanoflagellate species were described based on detailed morphological (light and scanning electron microscopy) and molecular (SSU and LSU rDNA sequencing as well as transcriptomic) data. In addition, by an empirical survey, insights into salinity tolerances of several craspedid choanoflagellates were gained for a better understanding of their ecological dispersal potential, in particular the potential to cross the marine-freshwater boundary. Furthermore, an unexplored and hostile environment, the Atacama Desert in Northern Chile was investigated. This region is characterized by hypersaline water bodies, heavy metal contamination and extreme UV radiation. Studying these extreme environments resulted in the description of seven new craspedid choanoflagellates and ecological experiments revealed that these species were able to tolerate a broad range of varying salinities. Molecular data of the SSU and LSU rDNA pointed towards high genetic distances to their closest marine relatives and phylogenetic analyses underlined their isolation-driven speciation derived from biogeographical restrictions. Further multigene analyses based on protein level revealed high mutation rates of aquatic organisms from this area. The biological divergence estimates correlated with geological events, e.g. the formation of the salt flats, and help to understand colonization and radiation processes. The establishment of choanoflagellate cultures and their transcriptomes from this area will be the basis to get further insights into the adaptation strategies also from a molecular perspective. By studying one of the salt flats, a new lorica-bearing species, Enibas tolerabilis gen. et sp. nov. could be isolated and described. This species is euryoecious regarding salinity, able to survive freshwater and hypersaline conditions, a characteristic never observed before for acanthoecids. The newly established genus Enibas clustered within a separated sister clade to all previously identified nudiform reproducing species in the family of Acanthoecidae, which comprised so far only four genera. Using video microscopy, its nudiform lorica reproduction could be proven by life observation of the cell division. In a targeted resampling approach, a phylogenetic closely related amplicon from molecular surveys was identified and a morphological description to this prior deposited sequence provided. By including other environmental eukaryotic sequences, it became obvious that the species richness within this family was underestimated. To summarize, several choanoflagellate species belonging to the order of Craspedida and Acanthoecida from diverse aquatic habitats were isolated and described. In particular, the amount of comprehensive transcriptomes of choanoflagellates, which will be the basis for further studies on gene functionality, was doubled. The use and combination of different methodologies gave detailed species descriptions for a comprehensive integrative taxonomy which can elucidate the ecological and evolutionary role of choanoflagellates. The present thesis emphasizes the urgent need of taxonomy to generate reliable reference data for further molecular meta-analyses investigating the biodiversity of protists. |
