Metabolomics Spatial Survey of Modern Stromatolites from South Africa and Characterization of Ibhayipeptolides

Evidence of life on earth dates back more than 3.4 billion years in the form of lithified layers of complex microbial mats known as microbialites, which are ubiquitous in the fossil record. Modern, extant microbialites are comparatively rare but have been documented globally. Multi-faceted, molecular investigations are required to understand community structure and function, factors that influence formation and growth, and how modern microbialite microbial communities differ from those in non-lithifying microbial mats. In this study we selected living, layered microbialites (stromatolites) in a peritidal environment near Schoenmakerskop in the Eastern Cape of South Africa as a study site for conducting a spatial survey to map the composition and small molecule production of the microbial communities across the system. Substrate cores and water samples were collected from nine sampling stations ranging from the upper point of the freshwater inflow to the lower marine interface where tidal overtopping takes place. Substrate cores provided material for parallel analyses of microbial community diversity by 16S rRNA gene amplicon sequencing and metabolomics using LC-MS2. Species diversity was correlated with metabolite diversity between the nine sampling stations and prominent specialized metabolites were targeted for characterization. A new series of cyclic hexadepsipeptides, named ibhayipeptolides, was most abundant in substrate cores of submerged microbialites, and we predict that this large molecular family is produced by cyanobacteria belonging to the family Geitlerinemaceae. These data contribute knowledge about, and facilitate future targeted studies of, specialized metabolite function and biosynthesis in microbialite communities.