Distinct types of microbial communities supported by iron oxidation

Microbial biostalactites and streamers growing at iron seepages in abandoned mines were submitted to 16S rRNA gene sequencing. They were collected at water seepages in the pH range of 2.4 to 6.6 in 6 different mines in the Czech Repulbic and Slovakia. The analysis of 85 communities showed that pH around 3.2 is a critical point, where alpha and beta diversity change discretely. At a pH below 3.2 the average number of ASVs per sample was 2.91 times lower than above this boundary. Autotrophs, heterotrophs and symbionts of eukaryotes originated from nearly non-overlapping species pools at the two habitat types differing in pH only. Communities from pH below 3.2 further divided in two distinct groups differing in diversity, taxonomic, and functional composition. Both types of communities coexist within the same stalactites. These types probably correspond to zones where the capillary structure of the stalactite is perfused or clogged, respectively. These findings show that the microbial community structure can be largely driven by hardly predictable details of the ecosystems spatial organization and not only by the measurable environmental parameters.