Abstract Description

Microbialites are organosedimentary structures formed by the metabolic activity of diverse communities of microorganisms. They occur in a wide range of environments ranging from temperate to extreme conditions. In South Africa, microbialites are found along the coast at the interface between terrestrial and marine systems (supratidal microbialites) where they are fed by freshwater dune seeps and impacted by marine overtopping. This study set out to characterise the microbial communities of the South African supratidal microbialites to identify conserved taxa that may be keystone species required for the formation of these microbialites. A comparative study of microbial communities at twelve supratidal microbialite formations along the coastline of the Eastern Cape Province of South Africa, between Oyster Bay and Gqeberha, was conducted. In general, the microbialite formations could be classified into three broad groups: the first group included formations that experienced strong and consistent rates of freshwater inflow; the second group included formations that were impacted by variable freshwater inflow, such as desiccation during the summer months and the third group was regularly impacted by sea water during marine overtopping. Morphological analysis using Scanning Electron Microscopy showed the presence of Extracellular Polymeric Substance layers, filamentous bacteria, vertical sheaths, opened castes and radial calcium carbonate crystal structures typical of these microbialite formations. The study sites were characterised by highly variable inorganic nitrogen concentrations in their freshwater inflows that did not appear to be a major driver of microbial community structure and diversity. Instead, 16S rRNA gene metabarcoding of bacterial communities showed that the abundance of the bacteria was influenced by strong freshwater inflow (Group 1) or desiccation and marine overtopping (Group 2). Across all the formations, seven Operational Taxonomic Units (OTUs) were conserved. These included OTU1 (Microcoleus sp.), OTU2 (Plectonema sp.), OTU19 (Ramlibacter sp.), OTU45 (Defluviimonas sp.), OTU56 (uncultured bacteria), OTU51 (Erythrobacter colymbi) and OTU67 (Hyphomicrobium sp.). Other OTUs that were conserved between this study and the previous study at Schoenmakerskop included, OTU4, OTU7, OTU13, OTU21, OTU23, OTU24, OTU34, OTU38, OTU47 and OTU50. Taken together, this study has demonstrated that the South African supratidal microbialite communities are likely formed by functionally conserved bacterial species that include seven keystone taxa conserved across all formations.