MICROBIALITE VS SPONGES IN CRYPTIC BIOCONSTRUCTIONS FROM SUBMARINE CAVES OF THE AEGEAN SEA (EASTERN MEDITERRANEAN)

Fara and Agios Vasilios caves (Lesvos Island, Greece) hold bioconstructions growing in confined marine environments. Like other Mediterranean examples, the studied caves were observed to host cryptic communities with a strong dominance of soft-bodied sponges and skeletonized metazoans which form biogenic crusts. Following the environmental parameter variations in the caves, the biogenic crusts are composed mainly of scleractinian corals and red algae near cave openings, and of serpulids and bryozoans in the innermost cryptic cave sectors. The diffuse primary crust porosity, due to exoskeleton types, was enhanced by the boring activity of demosponges, whose spicules and organic matter remains partly filled microcavities. Microbial autochthonous micrite, abundant in the small cavities of the skeletal framework of biostalactites from Mediterranean (Sicily, Apulia and Cyprus: Belmonte et al., 2009; Guido et al., 2013, 2017) and Belize (Gischler et al., 2017) submarine caves, represents a minor component in the biogenic crusts of the Lesvos caves. Most cryptic micro-niches of the studied biogenic crusts, suitable for the carbonatogenetic sulfate-reducing bacteria development, were occupied by demosponges. Here the little amount of autochthonous micrite, around siliceous spicules, likely has resulted from induced and/or supported organomineralization, rather than microbial mediation. The abundance of sponges in the biogenic crusts of Lesvos caves may have limited the development of carbonatogenetic bacteria, preventing the deposition of large amount of autochthonous micrite. These environments can be considered a natural laboratory to study the relationships between metazoans and bacteria in confined environments and can help in clarifying the role of microbialite vs cryptic metazoans in the fossil record.