Felizitas Bajerski (Braunschweig / DE), Johannes Sikorski (Braunschweig / DE), Selma Vieira (Braunschweig / DE), Christiane Baschien (Braunschweig / DE), Anika Methner (Braunschweig / DE), Anastasia Roberts (Salzburg / AT), Martin Gossner (Birmensdorf / CH; Zurich / CH), Jana Petermann (Salzburg / AT), Joerg Overmann (Braunschweig / DE)
Water-filled tree holes are naturally formed semi-enclosed cavities in which environmental conditions such as water availability strongly fluctuate. These small aquatic ecosystems feature island-like characteristics that render them suitable models to test island biogeography theory and to study the mechanisms of microbial community assembly.
Our study was conducted within the German Biodiversity Exploratories framework and is one of the first targeting both bacterial and fungal communities present in tree holes. Microbial biomass collected from 55 water-filled tree holes (predominately Fagus sylvatica) in three different geographic regions were analyzed by Illumina high-throughput amplicon sequencing (bacteria: V3 16S rRNA gene, fungi: ITS2 rRNA gene region). Community composition was used to elucidate the factors shaping biogeographic patterns and to examine the extent to which tree holes represent microbial islands.
Tree holes were found to harbour complex microbial communities. We observed bacterial species known to be associated with freshwater and sediment (Limnohabitans, Flavobacterium, Polynucleobacter), soil (Pseudomonas) and plant-associated (degrading) (Duganella, Paludibacter) environments, as well as phytopathogenic (Fusarium), insect-associated (Zoophthora), and aquatic fungi (Alatospora, Flagellospora, Gorgomyces) including yeasts (Camptobasidium). Overall, we observed a limited overlap of bacterial or fungal taxa between individual tree holes. In contrast to previous reports, species richness of the bacterial communities decreased with increasing tree hole volume. However, we found evidence that deterministic processes had little overall influence on microbial community assembly, whereas stochastic processes were predominant in these systems. Especially dispersal limitation was important, which had a greater impact on bacteria than on fungi. Since community assembly was largely mediated by stochastic processes, our results support the conclusion that tree holes act as islands for microbial communities.
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