Euglenophyceae represent a group of unicellular eukaryotic organisms that have gained during their evolution the ability to photosynthesize. They aquired plastids via secondary endosymbiosis with a green alga as the plastid donor. Phylogenetic studies searching for the origin of this organelle revealed the green alga Pyramimonas parkeae from Prasinophytes as the closest known relative to euglenid plastids. Pyramimonas parkeae and Euglena share several genes clusters with unique order of genes in their plastid genomes, which also point to the Pyramimonadales as the donor of the plastids. However, it is posible, that organisms more closely related to euglenid plastids than P. parkeae, occur in the environment. In my diploma thesis I focused on the exploration of diversity of Pyramimonadales and Euglenophyceae in environmental samples. I used several approaches to perform this task. I amplified parts of the plastid genomes in environmental samples by using specific PCR and determined their position in the phylogenetic tree. I also made large-scale phylogenetic analyses based on 16S rRNA and 18S rRNA sequences from representatives of the groups Euglenophyceae, Prasinophytes and environmental samples. The results revealed the presence of a large number of environmental sequences relative to the...