Pendant bioconstructions cemented by microbial carbonate in submerged marine caves (Holocene, SE Sicily).

Unusual pendant bioconstructions occur within two submerged caves in the Plemmirio Marine Protected Area,south of Syracuse (SE Sicily). These cm- to dm-sized bioconstructions project downward from the cave roofs.The main framework builders are serpulids belonging to the genera Protula, Semivermilia and Josephella.Encrusting bryozoans, corals, hydrozoans and sponges are subordinately present,whereas bivalves and brachiopodsare overall rare. These faunal associations are characteristic of the studied caves, but only in some placesthey formsmall bioconstructions that are cemented by microbial carbonatesmade up by autochthonous, peloidalto clotted peloidal and aphanitic micrite. The autochthonous micrite stabilizes the pendant bioconstructions,herein referred to as biostalactites. The microbial carbonate reveals high contents of bacterial lipid biomarkers.Among the most abundant compounds are mono-O-alkyl glycerol ethers (MAGEs) and branched fatty acids(10-Me-C16:0; iso- and anteiso-C15:0 and -C17:0), interpreted as biomarkers of sulfate-reducing bacteria. Othercompounds preserved in the autochthonous carbonates include lipids derived frommarine zoo- and phytoplankton(brassicasterol, dinosterol and monounsaturated short-chain fatty acids), unspecified marine bacteria (saturatedand monounsaturated short-chain fatty acids) and land plants (long-chain fatty acids, long-chain alcohols,stigmasterol, sitosterol and campesterol). The observed lipid biomarker signatures are remarkably similar tothose of post-glacial reefal microbialites, where microbialite formation in cavities of the coral framework wasalsomediated by sulfate-reducing bacteria. In the submerged caves of PlemmirioMarine Protected Area, serpulidcolonies apparently occur preferentially where freshwater is seeping through crevices in the limestone. The consequentnutrification is believed to favor serpulid aggregation and the growth of other skeletal (i.e., bryozoansand corals) and soft-bodied organisms (i.e., sponges and ascidiae). The resultant accumulation of biomass andits taphonomy provide niches for sulfate-reducing bacteria, which induce carbonate precipitation cementingand stabilizing the biostalactites. The finding of pendant, cemented bioconstructions in the Mazzere and Granchicaves of the Plemmirio Marine Protected Area reveals that sulfate-reducing bacteria are involved in microbialiteformation in various cryptic environments. The formation of such peloidal to clotted peloidal microcrystallinecarbonates in cryptic ecosystems is a significant factor for the stabilization of different kinds of bioconstructions,spanning from small biostalactites to large reefs.