The cellular content of nuclear DNA varies up to 200,000-fold between eukaryotes. These differences can arise via different mechanisms. In particular, cell size and nutritional mode may influence evolution of the nuclear DNA content. Chrysophytes comprise organisms with different cell organizations and nutritional modes. Heterotrophic clades evolved independently several times from phototrophic or mixotrophic ancestors. Thus, chrysophytes are an ideal model taxon for investigating the effect of the nutritional mode on cellular DNA content. We investigated the genome size of heterotrophic, mixotrophic, and phototrophic chrysophytes. We demonstrate that cell sizes and genome sizes differ significantly between taxa with different nutritional modes. Phototrophic strains tend to have larger cell volumes and larger genomes than heterotrophic strains do. The investigated mixotrophic strains had intermediate cell volumes and small to intermediate genome sizes. Heterotrophic chrysophytes had the smallest genomes and cell volumes compared to other chrysophytes. In general, genome size increased with cell volume, but cell volume only partially explained the variation in genome size. In particular, genome sizes of mixotrophic strains were smaller than expected based on cell sizes.

Here, we analyzed patterns of taxon richness and endemism of freshwater protists in europe. even though the significance of physicochemical parameters but also of geographic constraints for protist distribution is documented, it remains unclear where regional areas of high protist diversity are located and whether areas of high taxon richness harbor a high proportion of endemics. further, patterns may be universal for protists or deviate between taxonomic groups. Based on amplicon sequencing campaigns targeting the SSU and itS region of the rDnA we address these patterns at two different levels of phylogenetic resolution. Our analyses demonstrate that protists have restricted geographical distribution areas. for many taxonomic groups the regions of high taxon richness deviate from those having a high proportion of putative endemics. in particular, the diversity of high mountain lakes as azonal habitats deviated from surrounding lowlands, i.e. many taxa were found exclusively in high mountain lakes and several putatively endemic taxa occurred in mountain regions like the Alps, the pyrenees or the Massif central. Beyond that, taxonomic groups showed a pronounced accumulation of putative endemics in distinct regions, e.g. Dinophyceae along the Baltic Sea coastline, and chrysophyceae in Scandinavia. Many other groups did not have pronounced areas of increased endemism but geographically restricted taxa were found across europe.

Chrysophyceae are a diverse group of planktonic protists widely distributed in freshwater. They encompass a variety of orders, whereby heterotrophy has evolved independently in several phylogenetic lineages. Therefore, closely related taxa evolved that developed different feeding strategies (photo-, mixo-, heterotrophy). In high-throughput sequencing studies, the Chrysophyceae were usually addressed as a group rather than split in individual phylogenetic orders. Also because of the close relationship of the distinct nutritional strategies, no functional statements were made about nutritional patterns. Based on an extensive phylogenetic tree and phylogenetic placement, we link OTU diversity of Chrysophyceae from 218 freshwater lakes with phylogenetic affiliations. This provides information on the relative importance of lineages affiliated with different nutrition modes. Our study demonstrates that Chrysophyceae are one of the most common groups in freshwaters. We found Chrysophyceae in 213 out of 218 sample sites across Europe and in several sites they belong to the most commonly retrieved taxa. Ochromonadales and a Chrysosacca-Apoikiida clade (including Apoikiida, Chrysosaccales, Chrysastrella) are the most widespread Chrysophyceae groups and show a high degree of OTU diversity. Most detected and assignable OTUs were affiliated with mixotrophic Chrysophyceae. Niche width differs only slightly between members of different clades and between the different trophic modes. We found several OTUs within the Ochromonadales, Synurales, and Chrysosacca-Apoikiida clade, that show a wide distribution and large tolerance ranges concerning ecophysiological factors.

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Biogeography in Europe is known to be crucially influenced by the large mountain ranges serving as biogeographical islands for cold‐adapted taxa and geographical barriers for warm‐adapted taxa. While biogeographical patterns are well‐known for plants and animals in Europe, we here investigated diversity and distribution patterns of protist freshwater communities on a European scale (256 lakes) in the light of the well‐studied post‐glacial distribution patterns of macroorganisms. Thus, our study compared 43 alpine protist communities of lakes located in the Alps, Carpathians, Pyrenees, and the Sierra Nevada with that of surrounding lowland lakes. We verified altitudinal diversity gradients of freshwater protists with decreasing richness and diversity across altitudes similar to those observed for plants and animals. Alpine specialists and generalists could be identified differing significantly in richness and diversity, but hardly in occurrence and proportions of major taxonomic groups. High proportions of region‐specific alpine specialists indicate an increased occurrence of distinct lineages within each mountain range and thus, suggested either separated glacial refugia or post‐glacial diversification within mountain ranges. However, a few alpine specialists were shared between mountain ranges suggesting a post‐glacial recolonization from a common lowland pool. Our results identified generalists with wide distribution ranges and putatively wide tolerance ranges toward environmental conditions as main drivers of protist diversification (specification) in alpine lakes, while there was hardly any diversification in alpine specialists.