Biodiversity of terrestrial algal communities from soil and air-exposed substrates using a molecular approach

Terrestrial green microalgae are important primary producers in various habitats like soils, stone (epilithic and endolithic), tree bark and artificial hard substrates. In this thesis, the molecular diversity of green algae (Chlorophyta) in terrestrial habitats was analyzed and assessed by mainly a culture-independent approach using molecular phylogenetics. This allowed the unambiguous re-identification of the same species or molecular operational taxonomic units (OTUs) and, therefore, changes in the taxonomic composition of the algal communities could be reliably assessed. The algal communities in soil and aerophytic algae in phototrophic biofilms on building stone, monuments, on tree bark and on man-made substrate from various sampling sites in Germany were investigated and compared. On building stones and stone monument surfaces also cyanobacteria and fungi were abundant and were analyzed as well. The soil and tree bark samples were obtained from defined research plots of the three German Biodiversity Exploratories, i.e. Hainich-Dun, Schorfheide-Chorin and Schwabische Alb, which comprised different land use types (forest versus grassland) of different management intensities each. From all sampling sites environmental DNA was isolated and 18S rRNA genes were amplified by PCR with a newly developed green algae preferring primer. Followed by cloning and sequencing the analyses resulted in a dataset of more than 3000 green algal partial sequences. Operational taxonomic units (OTUs) were calculated on the basis of partial sequences (including the hypervariable V4 region) followed by phylogenetic identification of representative full length sequences. For some subsets of samples also enrichment cultures were established and analyzed. In one example, the green algae dominated cover of a man-made hard substrate in an urban environment the different diversities obtained by the culture-independent approach and a culture-based method were compared. All air exposed surfaces, like stone and tree bark, investigated in this study were clearly dominated by members of the green algal class Trebouxiophyceae. Regarding the studied building stones, the composition of microalgal communities including cyanobacteria differed markedly between apparently similar substrata of two wall areas. This is mainly due to differences in the exposure to sunlight (and hence water availability) and the occurrence of gypsum crusts. Thus, the green algae composition on the daylight-exposed walls was dominated by Trebouxiophyceae, whereas OTUs in samples taken from a sampling site under low irradiance comprised also members of the Chlorophyceae. Interestingly, members of Chlorophyceae were preferably detected after enrichment in liquid crude cultures. The most abundant algae discovered in phototrophic biofilms on air exposed substrata were various OTUs representing the trebouxiophycean genera Apatococcus, Stichococcus, Trebouxia, and Coccomyxa. The green algal diversity in soils was extensively higher than in aerophytic green biofilms. Significant differences between green algal communities in 27 grassland and 30 forest soils were detected. Besides other factors soil pH explains this observation at best. In relation to management intensities the effects on algal communities were indirect, i.e. soils in unmanaged forests had a markedly lower OTU richness than that of managed forest sites. At the latter, presumably a low light intensity caused by high cover of litter and understory was available to the algal communities which may explain the differences. However, this was not evident for the grassland plots. The soil algal communities comprised a high diversity of members from both classes, the Trebouxiophyceae and Chlorophyceae. The most common OTUs in soil with particular dominance in grassland were species/OTUs of Chloroidium, Stichococcus, Muriella, Tetracystis and Bracteacoccus. The most abundant OTU in the studied forest soils was a so far unidentified Prototheca-related trebouxiophycean alga. This presumably heterotrophic alga was exclusively retrieved from forest soils.