Microbialites: What on Earth?

Earth and Planetary …, 2000

We present a study of the textural signature of terrestrial weathering and related biological activity in the Tatahouine meteorite. Scanning and transmission electron microscopy images obtained on the weathered samples of the Tatahouine meteorite and surrounding soil show two types of bacteria-like forms lying on mineral surfaces: (1) rod-shaped forms (RSF) about 70^80 nm wide and ranging from 100 nm to 600 nm in length; (2) ovoid forms (OVF) with diameters between 70 and 300 nm. They look like single cells surrounded by a cell wall. Only Na, K, C, O and N with traces of P and S are observed in the bulk of these objects. The chemical analyses and electron diffraction patterns confirm that the RSF and OVF cannot be magnetite or other iron oxides, iron hydroxides, silicates or carbonates. The sizes of the RSF and OVF are below those commonly observed for bacteria but are very similar to some bacteria-like forms described in the Martian meteorite ALH84001. All the previous observations strongly suggest that they are bacteria or their remnants. This conclusion is further supported by microbiological experiments in which pleomorphic bacteria with morphology similar to the OVF and RSF objects are obtained from biological culture of the soil surrounding the meteorite pieces. The present results show that bacteriomorphs of diameter less than 100 nm may in fact represent real bacteria or their remnants. ß

Geology, 2008

Microsedimentary structures referred to as nanobacteria-like particles were described from modern carbonate environments, where they form in close spatial association with sulfate-reducing bacteria (SRB). However, the exact mechanism of their formation, as well as their paleontological signifi cance, remains controversial. Here we report on an investigation of microbe-mineral interactions in experimentally produced carbonate globules. The experiments were carried out under anoxic conditions at 30 °C with Desulfovibrio brasiliensis, a SRB known to mediate dolomite formation. We observed that extracellular polymeric substances (EPS) secreted by the microbial community play a key role in the mineralization process. Nanobacteria-like particles represent the early stage of carbonate nucleation within the EPS, which progressively evolve to larger globules displaying a grainy texture. We excluded the possibilities that these structures are fossils of nanobacteria, dissolution surfaces, or artifacts created during sample preparation. D. brasiliensis cells are predominantly located outside of the EPS aggregates where mineral growth takes place. As a result, they remain mobile and are rarely entombed within the mineral. This self-preservation behavior may not be limited to D. brasiliensis. Other microbes may produce, or may have produced during the geological past, biogenic minerals through a similar process. Mineralization within EPS explains why microbial relics are not necessarily present in biogenic carbonates.

Geology, 1998

To explore the formation and preservation of biogenic features in igneous rocks, we have examined the organisms in experimental basaltic microcosms using scanning and transmission electron microscopy. Four types of microorganisms were recognized on the basis of size, morphology, and chemical composition. Some of the organisms mineralized rapidly, whereas others show no evidence of mineralization. Many mineralized cells are hollow and do not contain evidence of microstructure. Filaments, either attached or no longer attached to organisms, are common. Unattached filaments are mineralized and are most likely bacterial appendages (e.g., prosthecae). Features similar in size and morphology to unattached, mineralized filaments are recognized in martian meteorite ALH84001. We examined more than 30 surfaces of rock chips from the inoculated basaltic microcosms. Each chip displayed forms that we interpret as organisms and related biological features. A detailed SEM study of five chips of uninoculated basalt revealed no cell-like forms or appendages. We observed microorganisms, or apparent microorganisms, with four dominant morphological types: Type 1 is a long spiral-coiled organism (250 nm wide and ~750 nm long). Type 2 is an oval organism with smooth surface texture that ranges from ~500 to 2500 nm in length (Fig. ). These

Frontiers in Microbiology, 2015

Functional Ecology, 1992

The geologic record suggests the presence of microbes on Earth as early as the Precambrian (Hall-Stoodley et al., 2004). Microbes are involved in practically every aspect of earth evolution. The term microbe is a general descriptor for tiny organisms that individually are too small to be seen with the unaided eye. Microbes may include bacteria, archaea, fungi, and protists. Viruses are also included as a major type of microbe, although there is some debate whether viruses can be classified as living organisms. The role microbes play in altering environmental systems is well documented in many biogeochemical studies. Notable is the role of microbes in water-rock interactions (Chapelle and Bradley, 1997). Field observations and laboratory experiments suggest that bacteria can accelerate silicate weathering either by direct contact with minerals or by producing organic and inorganic acids that enhance the dissolution of silicates (Heibert and Bennett, 1992). Thus, microbes are able to directly alter mineral surface chemistry and pore water chemistry over short to geologic time scales. Microbial induced changes in water-rock-regolith environments over variable time scales cause changes in the physical properties of these environments that may be detected and measured using geophysical methodologies.

Goldschmidt2021 abstracts, 2021

Atacama's microbialites, able to live in such extreme environment, are possible candidates as models for searching life in other planets or moons. At present, little is known about their microstructure and composition. This study analyzes, mainly, the terrestrial microstructures in a dimensional field longer than 0.1 mm, through an original approach using photographs and macro pictures, appropriately magnified and consequently little defocused, in comparison with analogous images shot by NASA Rovers on Martian outcroppings. A method able to permit comparison of structures and textures of terrestrial microbialites to the microscopic photo images (MI) shot by the cameras mounted on the NASA rovers that since more ten years are present on the Red Planet (Opportunity, Spirit, Curiosity). The study highlights occurrence of widespread structures likes microspherules (or clots), often organized into some higher order settings, such are donuts, polispherules, filaments and, above all, intertwined filaments of 1 microspherules, all showing features of an imperfect geometrical repetitiveness. The structural analysis has been connected with a textural study by a multifractal analysis, that is able to distinguish terrestrial biogenic stromatolites from abiogenic pseudo-stromatolites, and giving us a tool that might be applied for astrobiological purposes.