Traces of microbioerosion are recorded on microgastropods shells that occur in association with recent stromatolites from lagoa Salgada, Rio de Janeiro State. The high destruction levels produced by microborers on the bioclasts were directly observed through optical and scanning electron microscroscopy. This study goals the identification and description of the bioerosion patterns on microgastropods of the species Heleobia australis (D'Orbigny). The microborings consist of sinuous ramified microborings galeries that were related to the ichnogenera Fascichnus, as well as groups of circular and irregular perforations that remained unidentified. Energy Dispersive X-Ray Spectrometry (EDS) analysis of the dark shells surfaces detected the elements sulfur and iron, both closely tied to the microbial metabolism.

The lagoa Salgada is situated on the north coast of the State of Rio de Janeiro, between the districts of Campos dos Goytacazes and São João da Barra, at 41º00'30'' W and 21º54'10''S. In the margins of the lagoon the presence of recent stromatolitic constructions was verified. The stromatolite may be defined as lithified biosedimentary structure, growing through the increase of sediment blade trapping by the carbonate precipitation as result of microbian organism activity. The objective of this study was to describe and to determine the internal morphology of domal type stromatolite of the lagoa Salgada. Through cutting, three different stages were observed: the first (base) is present as a thrombolitic stage with spread blade; the second (intermediary) is a thrombolitic-stromatolitic stage with spread and continuous blade; the third (top) is stromatolitic and the most recent stage of the structure, presenting continuous blade and with interruptions rare. Besides the presence of cyanobacteria and mollusk shells, were observed also internee palinomorphs inside the domal stromatolite. It was possible to observe that filament cyanobacteria contributed to the formation of blade of domal stromatolite and the microgastropod contributed for the destruction of the blade.

Lagoa Salgada is situated in the north coast of State of Rio de Janeiro, between the districts of Campos dos Goitacazes and São João da Barra, some 41º00'30" W and 21º54'10"S. In the margins of the lagoon the presence of recent stromatolitic constructions was verified. The stromatolite may be defined as lithifying biosedimentary structure, growing through of sediment blade trapping by the carbonate precipitation as result of microbian organism activity. The aim of this study was to characterize the cyanobacteria assemblage in stratiform stromatolites found on the floor of lagoa Salgada. Within the stratiform stromatolites 21 species of cyanobacteria were found, Microcoleus chthonoplastes (Thuret) Gomont 1892 and Lyngbya aestuarii (Liebman) Gomont 1892 are frequent in these stromatolites. The presence of calcite was observed in the stratiform stromatolites. The filamentous cyanobacteria are responsible for the union and imprisonment of sediment to form the layers in the stromatolites. The skeleton remains of mollusks, foraminifers and ostracod found in the area, work as a source of calcium carbonate and of sediment to structure the stromatolites

The Pernambuco Lagoon is a hypersaline water body located in the Massambaba barrier-island, coastal plain of Rio de Janeiro State, southeastern Brazil. Its marginal environments are characterized by the development of microbial mats resulting from the organic activity produced by sediment trapping and binding in the matrix. The aim of this study is to characterize the cyanobacterial and sedimentary composition of these mats, mainly those with polygonal morphology, where a porous compact structure limits and forms polygonal fi ssures and cracks. The MEV and SED analysis show a calcitic and sodium chlorate composition and dominance of fi lamentous cyanobacteria Microcoleus chthonoplastes (Thuret) Gomont 1892 in the polygonal microbial mats. The disposition of fi lamentous forms on the surface level and the spherical ones in the deeper layers results in a stratifi cation of the mats. These observations are useful in the identifi cation of similar organic generated deposits in the fossil record.