POLYCHAETE WORM TUBES AGGLUTINATE MICROBIAL PELOIDS IN PRESENT-DAY MARINE BIOSTALACTITES

Biostalactites built by frameworks of serpulid worm tubes occur in sea-caves in Sicily. These decimetric bioconstructions are lithified by calcareous crusts dominated by clotted peloidal micrite. The crusts contain bacterial lipid biomarkers, including abundant compounds derived from sulfate-reducing bacteria. These biomarker signatures are remarkably similar to those reported from late Quaternary cryptic microbialites in tropical reefs. In the Sicilian biostalactites, in addition to forming the crusts, peloids occur agglutinated within the calcified tubes of small sabellariid and terebellid worms. We suggest that calcification, promoted by sulfate reducing bacteria that utilize organic matter excreted by the worms, provides material used by the agglutinating polychaetes to build their tubes. We infer that the bacteria obtain nutrients for growth and that the worms obtain support, stability and protection from bacterial calcification. This unexpected style of biocalcification and its significance for commensal relationships between invertebrates and bacteria requires further study. It appears to be the first report of a mutualistic consortium in which invertebrates and sulfate-reducing bacteria create skeletal deposits. Peloidal microbial carbonates are well-known to be associated with reef cavities and to be linked to BSR processes. Our examples from Sicily now suggest a direct link between BSR and biocalcification in some reefal polychaetes.