Microbial communities in Messinian evaporite deposits of the Vena del Gesso (northern Apennines, Italy).

The Vena del Gesso (Northern Apennines) is a 230 m-thick succession consisting of up to 16 gypsum-shale cycles belongingto the “Lower Evaporites” formed during the Messinian salinity crisis in theMediterranean. The study of the microbial communitiespreserved in the gypsum crystals of one complete cycle (6th cycle at Monte Tondo quarry) showed abundant, regularly arranged filamentous forms that resemble morphologically modern obligate phototrophes, cyanobacteria colonizing modern photic, shallow-water gypsum basins.At least four different bacterial populations have been recognized:a) filamentous type cyanobacteria with characteristic inserted funnel shaped structure resembling the modern Scytonematacean;b) Type 1 organisms consisting of filamentous structures impregnated by clay minerals containing pyrite grains in the outer sheath;c) Type 2 filaments filled by clay minerals with dolomite in the outer sheath;d) Type 3 filamentous organisms with a central hollow tube and an encrusted outer sheath mainly composed of calcium carbonate.These organisms were probably associated with other heterotrophic bacteria as suggested by the presence of dolomite and pyrite structures.The size and preservation suggest that most of these cyanobacteria were likely conducting oxygenic photosynthesis as presently observed in modern solar salt works. It follows that they were living in shallow water settings or settled down from the water column to the bottom of a relatively deep evaporite basin.