The biogenic crusts inside two submarine caves of the Aegean Sea were studied to characterize the organisms involved in their formations. The walls and ceilings of the studied Fara and Agios Vasilios Caves (Lesvos Island, Greece) are mainly covered by bryozoans and serpulids and, to a lesser extent, coralline algae, scleractinian corals, sponges and vermetids. Often the skeletons of these organisms were cemented together to form small and flat bioconstructions (Fig. 1). The quantitative relationships among the skeletal components in the biogenic crusts vary according to sample location: coralline algae and corals dominate close to the opening of the caves, whereas serpulids, bryozoans and sponges are the main crust builders in the innermost cave sectors. Fine sediment (microcrystalline calcite) is a minor component of the crusts and it consists of two fractions: the autochthonous micrite, mineralized directly inside the crusts through organomineralizzation processes, and the allochthonous micrite, derived from the erosion of carbonate skeletons and/or substrate. Crusts typically show scarcity of microbial autochthonous micrite and abundance of endolithic sponges, whose organic tissue decay left some of the hollows filled by spicules. The abundance of sponges, competing with carbonatogenetic bacteria for the same living cryptic spaces, may have prevented the development of microbialites. This competition could explain the morphological differences between the studied biotic crusts and the large biostalactites, characterized by abundance of microbialites and shortage of sponges, that are common in other Mediterranean caves (i.e. Sicily, Apulia and Cyprus).
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