Indigenous microbial communities as catalysts for early marine cements: An in vitro study

Early marine cementation is a fundamental process for many characteristics of carbonates, like the stabilisation of steep slopes. The genesis of early cements is often attributed to physicochemical processes but there is evidence for microbial mediation. To elucidate the role of microbes and associated organic material, in vitro experiments were undertaken in the presence and absence of indigenous microbiota in ooids from Schooner Cays, Bahamas and compared with native grapestones from Joulter Cays, Bahamas. Microscopic examinations by stereomicroscopy, scanning electron microscopy and thin section analysis of in vitro incubations with native flora document rapid grain fusion, resulting in the formation of grapestones within 30–60 days. The initial binding of the grains is primarily facilitated by exudates of extracellular polymeric substances and microbial communities acting as catalysts in the formation of micritic bridges, cements and encrusted aggregates. In vitro grapestones are similar to native grapestones from Joulter Cays with intergranular areas infested with extracellular polymeric substances, microbes, micritic cements, amorphous calcium carbonate nanograins and micritised outer surfaces. These similarities suggest that incubations with native flora follow similar mineralisation mechanisms as in the natural environment. In contrast, sterilised grains remain loose with little crystal formation after 60 days and are devoid of microbes and organic exudates. Owing to the near absence of precipitates, abiotic precipitation is not the driving force promoting early cements. In contrast, grain fusion is microbially mediated via both a passive mechanism, where extracellular polymeric substances and cell surfaces function as templates for crystal nucleation and generation of micritic cements, and through an active mechanism by which biofilm heterotrophs and autotrophs induce chemical alterations of a local environment, facilitating precipitation. This study underscores that microbially mediated cementation can occur at fast rates and that firmground to hardgrounds and slope stabilisation take place shortly after deposition of carbonate grains.