Microbial mediation in the formation and alteration of minerals in shallow marine environments (Southern Calabria, Italy)

Abstract

This work wants to investigate the role that microorganisms have in the alteration and precipitation of minerals, in particular Ca-carbonates, on natural and artefact rocks of archaeological interest in shallow marine environments. This finds numerous applications in the recognizing of the abiotic vs. biotic nature of neoformed minerals, such as in the reconstruction of depositional palaeoenvironment, in astrobiology and in medicine. In addition, nanostructured microbially induced minerals have several technical application varying from bioconservation, bioremediation, biogrouting to biomedic. Furthermore, the knowledge of such complex interaction between bacterial communities and hard substrates is essential for the preservation and the valorization in situ of underwater cultural heritage, which represents a particularly important element in the history of peoples. For this purpose, several key-samples of natural and artifact rocks, have been collected in Santa Maria di Ricadi bay and in the archaeological submerged park of Monasterace, respectively sited on the Tyrrhenian and Ionian coast of Southern Calabria. Optical and scanning electron microscopy (SEM) have been used for micro- and nano-scale investigation of the role of microorganisms in the precipitation and alteration of minerals. Observations showed that biofilms are characterized by: (1) Skeletal elements, such as red algae, bryozoan, polychaete, bivalves and diatoms; (2) bacteria such as cocci, bacilli, spirochaete, filamentous bacteria, often associated with organic matter remains which result sometimes mineralized; (3) Neo-formed Cacarbonate and pyrite minerals; (4) bioerosion elements, such as grooves and boreholes Neo-formed Ca-carbonate minerals occur in Monasterace biofilms as sheet deposits, triads or dumbbell- to spherical-shaped crystals; whereas in Santa Maria di Ricadi as aphanitic and peloidal deposits. All these deposits result made up by an assemblage of nanospheres, varying in diameter from 50 to 200 nm, tightly clotted together. The co-existence of degraded EPS and bacteria, strictly associated with Cacarbonate nanospheres, implies that the organic matter and the microbial metabolism played a fundamental role in the precipitation of these minerals. Moreover, in Monasterace biofilms, framboidal pyrite occur both in rock fracture up to 2 mm below rock surface and within cavities of encrusting red algae. The presence of framboidal pyrite and dumbbell- to spherical- shaped crystals imply they formed in anoxic environment by sulphate reducers bacteria. As regards bioerosion products, these results formed by epilithic, endolithic and euendolithic organisms. In particular, in Ca-carbonate deposits forming part of Santa Maria di Ricadi rock samples, an intricate net of grooves is produced by cyanobacterium M. testarum. All these elements suggest that biofilms are complex systems formed by different biotic processes occurred both in oxic and anoxic conditions. Epilithic and endolithic microorganisms resulted capable to modify chemical and physical microconditions leading to the precipitation and alteration of diverse rocky materials, in particular Calcium carbonate, in shallow marine environments.