Description: Protists are single-celled organisms, which are very sensitive to changes in environmental parameters. They show a high diversity and occur under a huge variety of environmental conditions – also in polar regions. They live in and on the ice flows, as well as in the water column beneath. The knowledge about the interchange of marine protists between sea ice and the water surface is still insufficient, whereas more and more studies pay attention to the cryopelagic coupling of these microorganisms. Recently in the context of global change, where sea ice minima are observed more frequently - especially in the Arctic Ocean. The central hypothesis of this thesis refers to the coupling of the protist communities in the sea ice and the water column. During the freezing process, the salt leaves the ice through a channel system (“brine channels”), which contains high salinities and offers many habitats for different organisms to coexist on small scales. Therefore, we assume a higher diversity in the sea ice than in the under-ice water. Although the distance between both habitats is relatively small, results of other studies in the Arctic Ocean showed already differences in the community composition. To address this hypothesis, a molecular approach has been chosen. The protist community in the ice and the water shows a similarity of ~ 60-70%. This result indicates, that the exchange between ice and water is relatively high, which confirms former studies about cryo-pelagic coupling. The second part of this thesis is about the comparison of the cryo-pelagic coupling between the Arctic and the Southern Ocean, to get insides into potentially different mechanisms in both polar regions. Data for the Southern Ocean are still scarce in this context. Therefore, we include Antarctic samples from the equivalent season. A taxonomic overlap of ~ 60-70% between the sea ice and the under-ice water is remarkable. Therefore, we conclude similar mechanisms like in the Arctic Ocean. In total, ~ 60% of the taxa are found in both, the Arctic and the Southern Ocean. Consequently, a global exchange of marine protists is imaginable, but true bipolarity has to be proven by sampling in latitudes between both poles. The focus of the last part is on freshwater taxa and especially the comparison between the land-surrounded Arctic Ocean and the ocean-surrounded Southern Ocean. The Arctic Ocean is influenced by a higher amount of freshwater input (e.g. rivers), and our results confirm more freshwater taxa in the Arctic samples than in the samples of the Southern Ocean. The results of this study bring inside into a variety of aspects of cryo-pelagic coupling in the Arctic and Southern Ocean. The high exchange of taxa between the sea ice and under-ice water, as well as the occurrence of one taxon at both poles, might be more common than assumed by previous studies and need to get more attention in the future, when a further impact of climate change on ice extension takes place.

Description: Shellfish contamination with azaspiracids (AZA), which are lipophilic marine biotoxins produced by marine dinoflagellates, is a major and recurrent problem for the Irish shellfish industry. AZA are produced by certain species of Amphidomataceae, but the species diversity of this group of microalgae in Irish waters is poorly known. Here we present a morphological and molecular characterization of multiple new strains of non-toxigenic Azadinium isolated on an oceanographic survey in 2018. A lack of AZA production for all strains presented here was demonstrated by LC-MS/ MS analysis. One strain of Azadinium caudatum var. margalefii (first strain for the area) confirmed nontoxigenicity of Atlantic populations of this species. One strain designated as Azadinium cf. zhuanum was similar to Az. zhuanum described from China but differed from the type strain in nucleus position, by the dominant number of apical plates, and by significant differences in rRNA gene sequences. Finally, two new non-toxigenic Azadinium species are described from the North East Atlantic: Azadinium galwayense sp. nov. and Azadinium perfusorium sp. nov. Azadinium galwayense differed from other Azadinium by a characteristic combination regarding presence and location of the ventral pore (vp; on the right side of the pore plate), of a pyrenoid (located in the episome), and by a pentagonal shape of the median anterior intercalary plate 2a, and lack of contact between plates 1´´ and 1a. Azadinium perfusorium shared the same vp position as Az. galwayense and differed by a characteristic combination of a pyrenoid located in the hyposome, a tetragonal shape of plate 2a, and a relatively large size of the two lateral anterior intercalary plates. Molecular phylogeny confirmed the distinctiveness of these two new species and their placement in Azadinium. The present findings significantly increased knowledge on the diversity of Azadinium species in the North East Atlantic.