Stephen A. Bowden

Biomarker evidence for green and purple sulphur bacteria in a stratified Palaeoproterozoic sea

The disappearance of iron formations from the geological record ∼1.8 billion years (Gyr) ago was the consequence of rising oxygen levels in the atmosphere starting 2.45–2.32 Gyr ago. It marks the end of a 2.5-Gyr period dominated by anoxic and iron-rich deep oceans. However, despite rising oxygen levels and a concomitant increase in marine sulphate concentration, related to enhanced sulphide oxidation during continental weathering, the chemistry of the oceans in the following mid-Proterozoic interval (∼1.8–0.8 Gyr ago) probably did not yet resemble our oxygen-rich modern oceans. Recent data indicate that marine oxygen and sulphate concentrations may have remained well below current levels during this period, with one model indicating that anoxic and sulphidic marine basins were widespread, and perhaps even globally distributed. Here we present hydrocarbon biomarkers (molecular fossils) from a 1.64-Gyr-old basin in northern Australia, revealing the ecological structure of mid-Proterozoic marine communities. The biomarkers signify a marine basin with anoxic, sulphidic, sulphate-poor and permanently stratified deep waters, hostile to eukaryotic algae. Phototrophic purple sulphur bacteria (Chromatiaceae) were detected in the geological record based on the new carotenoid biomarker okenane, and they seem to have co-existed with communities of green sulphur bacteria (Chlorobiaceae). Collectively, the biomarkers support mounting evidence for a long-lasting Proterozoic world in which oxygen levels remained well below modern levels.

Exploring deep microbial life in coal-bearing sediment down to ~2.5 km below the ocean floor

A deep sleep in coal beds Deep below the ocean floor, microorganisms from forest soils continue to thrive. Inagaki et al. analyzed the microbial communities in several drill cores off the coast of Japan, some sampling more than 2 km below the seafloor (see the Perspective by Huber). Although cell counts decreased with depth, deep coal beds harbored active communities of methanogenic bacteria. These communities were more similar to those found in forest soils than in other deep marine sediments. Science, this issue p. 420; see also p. 376 Coal beds more than 2 kilometers below the seafloor host methanogenic bacteria related to those found in forest soils. [Also see Perspective by Huber] Microbial life inhabits deeply buried marine sediments, but the extent of this vast ecosystem remains poorly constrained. Here we provide evidence for the existence of microbial communities in ~40° to 60°C sediment associated with lignite coal beds at ~1.5 to 2.5 km below the seafloor in the Pacific Ocean off Japan. Microbial methanogenesis was indicated by the isotopic compositions of methane and carbon dioxide, biomarkers, cultivation data, and gas compositions. Concentrations of indigenous microbial cells below 1.5 km ranged from <10 to ~104 cells cm−3. Peak concentrations occurred in lignite layers, where communities differed markedly from shallower subseafloor communities and instead resembled organotrophic communities in forest soils. This suggests that terrigenous sediments retain indigenous community members tens of millions of years after burial in the seabed.

Signal Enhancement of Surface Enhanced Raman Scattering and Surface Enhanced Resonance Raman Scattering Using in Situ Colloidal Synthesis in Microfluidics

We demonstrate the enhanced analytical sensitivity of both surface enhanced Raman scattering (SERS) and surface enhanced resonance Raman scattering (SERRS) responses, resulting from the in situ synthesis of silver colloid in a microfluidic flow structure, where both mixing and optical interrogation were integrated on-chip. The chip-based sensor was characterized with a model Raman active label, rhodamine-6G (R6G), and had a limit of detection (LOD) of ca. 50 fM (equivalent to single molecule detection). The device was also used for the determination of the natural pigment, scytonemin, from cyanobacteria (as an analogue for extraterrestrial life existing in extreme environments). The observed LOD of approximately 10 pM (ca. <400 molecules) demonstrated the analytical advantages of working with freshly synthesized colloid in such a flow system. In both cases, sensitivities were between 1 and 2 orders of magnitude greater in the microfluidic system than those measured using the same experimental parameters, with colloid synthesized off-chip, under quiescent conditions.

Early oxygenation of the terrestrial environment during the Mesoproterozoic

Geochemical data from ancient sedimentary successions provide evidence for the progressive evolution of Earth’s atmosphere and oceans. Key stages in increasing oxygenation are postulated for the Palaeoproterozoic era (∼2.3 billion years ago, Gyr ago) and the late Proterozoic eon (about 0.8 Gyr ago), with the latter implicated in the subsequent metazoan evolutionary expansion. In support of this rise in oxygen concentrations, a large database shows a marked change in the bacterially mediated fractionation of seawater sulphate to sulphide of Δ34S < 25‰ before 1 Gyr to ≥50‰ after 0.64 Gyr. This change in Δ34S has been interpreted to represent the evolution from single-step bacterial sulphate reduction to a combination of bacterial sulphate reduction and sulphide oxidation, largely bacterially mediated. This evolution is seen as marking the rise in atmospheric oxygen concentrations and the evolution of non-photosynthetic sulphide-oxidizing bacteria. Here we report Δ34S values exceeding 50‰ from a terrestrial Mesoproterozoic (1.18 Gyr old) succession in Scotland, a time period that is at present poorly characterized. This level of fractionation implies disproportionation in the sulphur cycle, probably involving sulphide-oxidizing bacteria, that is not evident from Δ34S data in the marine record. Disproportionation in both red beds and lacustrine black shales at our study site suggests that the Mesoproterozoic terrestrial environment was sufficiently oxygenated to support a biota that was adapted to an oxygen-rich atmosphere, but had also penetrated into subsurface sediment.

Heavy metal, sex and granites: Crustal differentiation and bioavailability in the mid-Proterozoic

Biogeochemical and genomic studies have suggested that the availability of trace metals has been essential to the progressive evolution of life on Earth. In particular, the evolution of eukaryotes to diverse complex multicellular life has been related to the availability of trace metals. The radiation of eukaryotes, and the evolution of sex, is timed as Mesoproterozoic, but at that time the metals may have been preferentially sequestered in a sulfidic deep ocean. However, the addition of a huge volume of new crust to form a supercontinent ca. 1.9 Ga, including an unprecedented episode of within-plate anorogenic magmatism, led to an extraordinary new flux of metals to the upper crust in the late Paleoproterozoic–Mesoproterozoic. The appearance of sulfate evaporites from ca. 1.7–1.6 Ga indicates extensive weathering of metallic sulfides. Erosion of the supercontinent into Mesoproterozoic sediments introduced key metals to near-surface reservoirs, providing an enhanced biogeochemical environment conducive to the expansion of an evolving biota.

Sulfur isotope signatures for rapid colonization of an impact crater by thermophilic microbes

In the 23-km-diameter Haughton impact structure, Canadian High Arctic, in sulfate-rich bedrock, widespread hydrothermal sulfide mineralization occurred in breccias formed during the impact. The sulfides exhibit extreme sulfur isotopic fractionation relative to the original sulfate, requiring microbial sulfate reduction by thermophiles throughout the crater. This evidence of widespread microbial activity demonstrates that colonization could occur within the lifetime of a moderately sized, impact-induced hydrothermal system. The pyrite was subsequently oxidized to jarosite, which may also have been microbially mediated. The successful detection of evidence for microbial life suggests that it would be a valuable technique to deploy in sulfate-rich impact terrain on Mars.

Reduction spots in the Mesoproterozoic age: implications for life in the early terrestrial record

Reduction spots are common within continental red beds in the geological record. The method of formation of reduction spots is a subject of debate, but they are thought to be the result of the reducing nature of microbial life present in the sediment during burial, which caused localized reduction in sediment that was otherwise oxidized during diagenesis. Reduction spots often have dark concretionary cores commonly enriched in elements such as vanadium and uranium. This enrichment is also believed to be associated with the microbial reduction of the sediment. Isotopic data from sulphides present in the cores of analogue Triassic reduction spots are consistent with a potential microbial formation mechanism. Here we report the presence of reduction spots with vanadium-rich mica (roscoelite) - enriched cores within a terrestrial red bed sequence of the Mesoproterozoic age. These findings may be a possible indicator of life within the terrestrial geological record during the Mesoproterozoic age, a time when such evidence is otherwise very rare. These findings suggest that life had not only colonized terrestrial environments during the Mesoproterozoic age, but had established a deep biosphere in the sediment. Received 7 May 2001, accepted 20 July 2010, first published online 19 August 2010

Elevated flux of cosmic spherules (micrometeorites) in Ordovician rocks of the Durness Group, NW Scotland

Synopsis Limestone samples from the Cambro-Ordovician Durness Group were crushed, acid-digested and searched for evidence of micrometeorites. Eleven melted micrometeorites were extracted from the magnetic fraction of samples from the Balnakeil and Croisaphuill formations near the top of the group. Other formations in the Durness Group did not yield micrometeorites. Only melted spherules with a distinctive dendroidal crystalline structure (I-type cosmic spherules) were accepted as definite micrometeorites. They represent a flux of micrometeorites one to two orders of magnitude greater than at present. The micrometeorite-bearing formations are of Arenig age, coincident with the onset of an enhanced flux of extraterrestrial material identified by the occurrence of fossil meteorites in Sweden.

Survival of organic compounds in ejecta from hypervelocity impacts on ice

Hypervelocity impacts (HVIs) where organic-bearing ice constitutes the target material are important in several aspects of planetary and space science: (1) sampling of planetary surfaces using a hypervelocity projectile to impact the surface and eject surface materials for measurement or collection by a spacecraft; (2) the transfer of organic material between planetary bodies; and (3) providing energy for chemical processes involving surface materials. While small organic molecules (~6 carbon atoms), if present in surface materials, will likely be present in HVI-ejecta, uncertainty remains for larger organic molecules. It is the larger molecular weight compounds which could constitute direct evidence of life, and thus their survival within an HVI-ejecta plume is of key importance when evaluating strategies for life detection on icy bodies. It is not currently known what large organic molecules, and in what concentrations, may be present on icy bodies in the Solar System, but it is highly likely some will be more chemically stable during a HVI than others. Accordingly, in this study we examined a range of chemicals (?,? carotene, stearic acid and anthracene) with molecular weights between 178 and 536 daltons, and three different types of chemical structure. The compounds were solvated in a dimethylsulfoxide/water mixture and frozen. The frozen targets were impacted with steel spheres 1 and 1.5 mm in diameter at velocities of about 4.9 km s?1. Ice ejected during the impact was collected and underwent chemical analysis. The most labile compound (?,? carotene) was only detected (in small amounts) in the ejecta (and only that emitted at the lowest angles of ejection), although the other compounds were present in larger quantities and at a range of ejection angles. A concentration gradient was observed within the ejecta as a function of angle of ejection. This was not the same for both stearic acid and anthracene: the greatest concentrations of stearic acid were found at shallow angles of ejection whereas anthracene was most abundant at both intermediate and large angles of ejection, implying an inverted concentration gradient. These observations may indicate that organic compounds are variably altered and destroyed during a HVI with ice and that the ejecta plume does not sample the original materials equally at all angles of ejection. Future work is planned and will evaluate fractional survival for a greater range of compound types, impact materials and velocities.

Recreating mineralogical petrographic heterogeneity within microfluidic chips: assembly, examples, and applications

To date, the visualisation of flow through porous media assembled in microfluidic chips was confined to mineralogically homogenous systems. Here we present a key evolution in the method that permits the investigation of mineralogically realistic rock analogues.