Different modes of calcification control the preservation of organic matter in a recent microbial mat

Microbial mats probably represented the earliest complex ecosystems on Earth. Fossil, mineralized microbial mats (microbialites) are as old as the Archean ( 3.5 Ga), and some contain putative remains of organic matter (OM). However, the exact processes of mineralization and preservation of OM are still poorly understood. Here, we analysed lipid biomarkers, combined with petrographic and histologic investigation, in a depth profile of a recent calcifying mat ( 1500 years) from a hypersaline lake in Kiritimati, Central Pacific. Our results revealed two major phases of microbial mat development, separated by a distinctive, laterally continuous mineral crust. The primary OM in these deposits was preserved in very different way compared to the mineral crust. The bulk microbialite was formed due to the inhibition function of exopolymeric substances (EPS). Their functional groups bound to divalent cations, forming authigenic carbonates when EPS degraded, protecting the microbial communities from a fast metabolic driven calcification. During further growth of the mat, OM was gradually degraded, as reflected by a decline in total organic carbon and bacteria-derived hopanoids and fatty acids in the deeper parts of the mat. In contrast, petrographic features suggest the carbonate crust at the boundary horizon was a thin biofilm rather than a multilayered microbial mat, formed during an interruption at ca. 1100 years BP. The observed μm-scale superposition of radial-fibrous botryoidal carbonate crust indicates that the entire biofilm was calcified due to a very fast collapse of the EPS inhibition function. This hydro-chemically driven, event-like mode of calcification was accompanied by a significant biotic change, as inferred from abundant filamentous microbes and diatoms. It also caused a fast and efficient inclusion of lipid biomarkers, as reflected by a remarkable increase in the abundances of fatty acids and, to a lesser extent, hopanoids. These results suggest that episodic environmental changes could have induced a rapid entombment of OM in microbial mats and its enhanced preservation within distinctive mineral precipitates. Such rapidly formed precipitates may have preserved OM better than normal mineralization and represent excellent targets for the search of authentic OM in the ancient microbialites.