Commensal symbiosis between agglutinated polychaetes and sulfate reducing bacteria

Pendant bioconstructions occur within submerged caves in the Plemmirio Marine Protected Area in SESicily, Italy. These rigid structures, here termed biostalactites, were synsedimentarily lithified by clotted-peloidalmicrobial carbonate that has a high bacterial lipid biomarker content with abundant compoundsderived from sulfate-reducing bacteria. The main framework builders are polychaete serpulid worms,mainly Protula with subordinate Semivermilia and Josephella. These polychaetes have lamellar and/orfibrillar wall structure. In contrast, small agglutinated terebellid tubes, which are a minor component ofthe biostalactites, are discontinuous and irregular with a peloidal micritic microfabric. The peloids, formedby bacterial sulfate reduction, appear to have been utilized by terebellids to construct tubes in an environmentwhere other particulate sediment is scarce. We suggest that the bacteria obtained food from theworms in the form of fecal material and/or from the decaying tissue of surrounding organisms and thatthe worms obtained peloidal micrite with which to construct their tubes, either as grains and/or as tubeencompassing biofilm. Peloidal worm tubes have rarely been reported in the recent but closely resembleexamples in the geological record that extend back at least to the early Carboniferous. This suggests along-lived commensal relationship between some polychaete worms and heterotrophic, especially sulfatereducing,bacteria.