Bioconstructions in cryptic cave environments: a consortium between bacteria and polychaeta

Pendant bioconstructions occur within submerged caves of the Plemmirio Marine Protected Area in SE Sicily, Italy. These rigid structures, termed biostalactites, were synsedimentarily lithified by clotted-peloidal micrite that has a high bacterial lipid biomarker content with abundant compounds derived from sulfate-reducing bacteria. The main framework builders are polychaete serpulid worms, mainly Protula with subordinate Semivermilia and Josephella (Guido et al., 2013, 2014). These polychaetes have lamellar and/or fibrillar wall structure. In contrast, small agglutinated terebellid tubes, which are a minor component of the biostalactites, are discontinuous and irregular with a peloidal micritic microfabric. This distinctive association has been reported in several ancient deposits, such as Oxfordian and Upper Jurassic reefs, and in Late Jurassic to Early Cretaceous platform margin facies. The same composition of the clotted peloidal micrite and agglutinated polychaete tubes suggests that calcification, promoted by bacterial sulfate reduction (BSR), is utilized by the terebellids to help develop their tubes. The bacteria obtain nutrients for growth from decaying metazoan organic matter, and the worms utilize the microbial induced peloids to form their skeletons. This commensal symbiosis may not be restricted to grain-poor environments but, as suggested by the widespread occurrence of Terebella-like tubes in the geological record, might be a favored mechanism of tube construction, considering the close physical and ecological relationship of worms and sulfate-reducing bacteria in these bioconstructions.