Diversity of Microscopic Green Algae (Chlorophyta) in Calcifying Biofilms of Two Karstic Streams in Germany

For the first time the diversity of microscopic green algae (Chlorophyta) from calcified biofilms of karstic streams was analyzed using a combined approach based on pure cultures, i.e., 18S rRNA gene sequencing and microscopic analyses. Our study focused on two creeks in Germany. A considerable diversity of 34 species of green microalgae comprising three classes, the Trebouxiophyceae, Chlorophyceae and Ulvophyceae, was recovered. The biofilms of both streams were rather different in their species compositions which may reflect that they are exposed to differed hydrochemical conditions. The shallow Westerhöfer creek harbored predominantly Trebouxiophyceae and exhibited higher Mg²⁺ and SO₄²⁻ concentrations. In contrast, the deeper, longer and spatially more heterogeneous Deinschwanger creek harbored numerous species of Chlorophyceae. A lower number of species from the Ulvophyceae were spread on both studied streams. The closest relatives of the identified species were from other freshwater habitats, but mostly from phytoplankton. However, also several species we recovered from freshwater for the first time; so far they have been known from terrestrial habitats only. Less than half of the recovered species could be identified with names at the species level based on high sequence identities with available sequences from named reference strains. Most recovered species could not be identified with names to species level, i.e., they may represent species for which no 18S rRNA gene sequence have become available yet or which have been retrieved for the first time. The genus <i>Gongrosira</i> Kützing, often reported from freshwater tufa-stromatolites, was found to represent most likely a collective morphotype formed by several genera nested within the Ulvophyceae.