Geomicrobiology of a Weathering Crust from an Impact Crater and a Hypothesis for its Formation

Understanding the role of microbe-mineral interactions in rock weathering is vital to an understanding of nutrient availability to the biosphere and, in so far as weathering influences carbon dioxide drawdown, climate control. We studied a weathering crust on a resurge tsunami deposit (Loftarstone) from the ～ 455 Ma old Lockne impact crater, central Sweden with an integrated approach using XRD, electron microprobe analysis, SEM-EDS and GCMS analysis of organics. The lichens and fungal hyphae network preferentially weather the chlorite in the Loftarstone compared to feldspars and quartz. We demonstrate, using a fungal isolate (identified by ITS sequencing), that biologically induced dissolution of the calcite component produces cavities which increase the surface area of interaction between the biota and the rock substrate. The weathering crust exfoliates from the rock surface in sheets, which we attribute to the dissolution of the calcite matrix. We present a hypothesis for the crust development. As well as providing insights into weathering on substrates derived from a diversity of high-energy geological disturbances, such as impact events and tsunamis, the weathering crust provides a model system to understand weathering processes in other common lithologies with mixed mineralogies at small spatial scales, including many sedimentary rocks. This work reveals how each different clast plays a unique part in the weathering process, leading to a well-defined weathering sequence.     

The role of fungi in biogenic weathering in boreal forest soils

In this article we discuss the possible significance of biological processes, and of fungi in particular, in weathering of minerals. We consider biological activity to be a significant driver of mineral weathering in forest ecosystems. In these environments fungi play key roles in organic matter decomposition, uptake, transfer and cycling of organic and inorganic nutrients, biogenic mineral formation, as well as transformation and accumulation of metals. The ability of lichens, mutualistic symbioses between fungi and photobionts such as algae or cyanobacteria, to weather minerals is well documented. The role of mycorrhizal fungi forming symbioses with forest trees is less well understood, but the mineral horizons of boreal forests are intensively colonised by mycorrhizal mycelia which transfer protons and organic metabolites derived from plant photosynthates to mineral surfaces, resulting in mineral dissolution and mobilisation and redistribution of anionic nutrients and metal cations. The mycorrhizal mycelia, in turn provide efficient systems for the uptake and direct transport of mobilised essential nutrients to their host plants which are large sinks. Since almost all (99.99 %) non-suberised lateral plant roots involved in nutrient uptake are covered by ectomycorrhizal fungi, most of this exchange of metabolites must take place through the plant–fungus interface. This idea is still consistent with a linear relationship between soil mineral surface area and weathering rate since the mycelia that emanate from the tree roots will have a larger area of contact with minerals if the mineral surface area is higher. Although empirical models based on bulk soil solution chemistry may fit field data, we argue that biological processes make an important contribution to mineral weathering and that a more detailed mechanistic understanding of these must be developed in order to predict responses to environmental changes and anthropogenic impact.     

Ecological aspects of mycorrhizal symbiosis: with special emphasis on the functional diversity of interactions involving the extraradical mycelium

Different symbiotic mycorrhizal associations between plantsand fungi occur, almost ubiquitously, in a wide range of terrestrialecosystems. Historically, these have mainly been consideredwithin the rather narrow perspective of their effects on theuptake of dissolved mineral nutrients by individual plants.More recent research has placed emphasis on a wider, multifunctionalperspective, including the effects of mycorrhizal symbiosison plant and microbial communities, and on ecosystem processes.This includes mobilization of N and P from organic polymers,release of nutrients from mineral particles or rock surfacesvia weathering, effects on carbon cycling, interactions withmyco-heterotrophic plants, mediation of plant responses to stressfactors such as drought, soil acidification, toxic metals, andplant pathogens, as well as a range of possible interactionswith groups of other soil micro-organisms. Mycorrhizal fungiconnect their plant hosts to the heterogeneously distributednutrients required for their growth, enabling the flow of energy-richcompounds required for nutrient mobilization whilst simultaneouslyproviding conduits for the translocation of mobilized productsback to their hosts. In addition to increasing the nutrientabsorptive surface area of their host plant root systems, theextraradical mycelium of mycorrhizal fungi provides a directpathway for translocation of photosynthetically derived carbonto microsites in the soil and a large surface area for interactionwith other micro-organisms. The detailed functioning and regulationof these mycorrhizosphere processes is still poorly understoodbut recent progress is reviewed and potential benefits of improvedunderstanding of mycorrhizosphere interactions are discussed. Key words: Arbuscular mycorrhiza, biotic interactions, carbon flow, ectomycorrhiza, ericoid mycorrhiza, mycelium, nutrient uptake, symbiosis, weathering Received 22 January 2008; Revised 7 February 2008 Accepted 7 February 2008     

Geomycology: biogeochemical transformations of rocks, minerals, metals and radionuclides by fungi, bioweathering and bioremediation

The study of the role that fungi have played and are playing in fundamental geological processes can be termed ‘geomycology’ and this article seeks to emphasize the fundamental importance of fungi in several key areas. These include organic and inorganic transformations and element cycling, rock and mineral transformations, bioweathering, mycogenic mineral formation, fungal–clay interactions, metal–fungal interactions, and the significance of such processes in the environment and their relevance to areas of environmental biotechnology such as bioremediation. Fungi are intimately involved in biogeochemical transformations at local and global scales, and although such transformations occur in both aquatic and terrestrial habitats, it is the latter environment where fungi probably have the greatest influence. Within terrestrial aerobic ecosystems, fungi may exert an especially profound influence on biogeochemical processes, particularly when considering soil, rock and mineral surfaces, and the plant root–soil interface. The geochemical transformations that take place can influence plant productivity and the mobility of toxic elements and substances, and are therefore of considerable socio-economic relevance, including human health. Of special significance are the mutualistic symbioses, lichens and mycorrhizas. Some of the fungal transformations discussed have beneficial applications in environmental biotechnology, e.g. in metal leaching, recovery and detoxification, and xenobiotic and organic pollutant degradation. They may also result in adverse effects when these processes are associated with the degradation of foodstuffs, natural products, and building materials, including wood, stone and concrete. It is clear that a multidisciplinary approach is essential to understand fully all the phenomena encompassed within geomycology, and it is hoped that this review will serve to catalyse further research, as well as stimulate interest in an area of mycology of global significance.     

Biogenic Forsterite and Opal as a Product of Biodeterioration and Lichen Stromatolite Formation in Table Mountain Systems (Tepuis) of Venezuela

Biogenic physical weathering and leaching of the quartzite dominated table mountain systems of South America is a slow but strongly biologically influenced process. Observations and analyses on the basis of sample materials collected during an expedition to the protected areas of the most conspicuous tepuis of Venezuela are reported. The rock material consists of more than 98% silica, and the waters collected reflect rainwater quality further deprived of some essential elements. Wear-down of these rocks is recognized as a biogeomorphogenetic process ruled by the microbiota surviving under harsh and nutrient-poor conditions. Poikilotrophic subaerial biofilms of cyanobacteria, fungi, and some lichens perforate quartz grains and idiomorphic quartz crystals, as well as the subcrystalline cement. The typical pattern of biopitting is regarded as proof of biogenic quartz destruction. Within the subaerial biofilms, which form a massive cover of the slowly biocorroded rock surface, lichens were found that mineralize to microstromatolitic structures in situ. The mineralization occurs exclusively in the lichen thalli and not in the associated massive biofilms of free-living cyanobacteria and fungi. The minerals deposited were identified as opal and considerable amounts of forsterite, the pure Mg end-member of the olivine mixing series (fayalite being the pure iron silicate). Forsterite, thus far, has been regarded as an igneous mineral phase typical for mantle-derived rocks, highly metamorphic dolomitic marbles, and as a planetary mineral found in meteorites. The biogenic dissolution/mineralization paragenesis is explained by the slow weathering and bioleaching processes dominant on these table mountains and by the exclusion of all other potential biomineralization products due to the peculiar geochemistry of the interstitial and run-off water on these plateaus, thus leading to the exceptional biogenesis of forsterite under surface conditions.     

The fungal–mineral interface: challenges and considerations of micro-analytical developments

Over recent years, the role of fungi, especially mycorrhizal fungi, in the weathering of rock-forming minerals has been increasingly recognised. Much of our understanding of the effects of fungi on mineral weathering is based on macroscopic studies. However, the ability of fungi to translocate materials, including organic acids and siderophores, to specific areas of a mineral surface leads to significant spatial heterogeneity in the weathering process. Thus, geomycologists are confronted with unique challenges of how to comprehend and quantify such a high degree of diversity and complicated arrays of interactions. Recent advances in experimental and analytical techniques have increased our ability to probe the fungal–mineral interface at the resolution necessary to decouple significant biogeochemical processes. Modern microscopy, spectroscopy, mass spectrometry, wet chemistry, and scattering techniques allow for the selective extraction of physical, chemical, and structural data at the micro- to nano-scale. These techniques offer exciting possibilities to study fungal–mineral interactions at the scale of individual hyphae. In this review, we give an overview of some of these techniques with their characteristics, advantages and limitations, and how they can be used to further our understanding of biotic mineral weathering.     

The Multifunctional Role of Ectomycorrhizal Associations in Forest Ecosystem Processes

Belowground biological interactions that occur among plant roots, microorganisms and animals are dynamic and substantially influence ecosystem processes. Among these interactions, the ectomycorrhizal (ECM) symbiosis is remarkable but unfortunately these associations have mainly been considered within the rather narrow perspective of their effects on the uptake of dissolved mineral nutrients by individual plants. More recent research has placed emphasis on a wider, multifunctional perspective, including the effects of ectomycorrhizal symbiosis on plant and microbial communities, and on ecosystem processes. This includes mobilization of N and P from organic polymers, release of nutrients from mineral particles or rock surfaces via weathering, effects on carbon cycling, interactions with mycoheterotrophic plants, mediation of plant responses to stress factors such as drought, soil acidification, toxic metals, and plant pathogens, rehabilitation and regeneration of degraded forest ecosystems, as well as a range of possible interactions with groups of other soil microorganisms. Ectomycorrhizas are almost invariably characterized by a Hartig net composed of highly branched hyphae which entirely surround the outer root cortical cells. The Hartig net is the place of massive bidirectional exchanges of nutrients between the host and the fungus. Through these branched hyphae ectomycorrhizal fungi connect their plant hosts to the heterogeneously distributed nutrients required for their growth, enabling the flow of energy-rich compounds required for nutrient mobilization whilst simultaneously providing conduits for the translocation of mobilized products back to their hosts. In addition to increasing the nutrient absorptive surface area of their host plant root systems, the extraradical mycelium of ectomycorrhizal fungi provides a direct pathway for translocation of photosynthetically derived carbon from their hosts to microsites in the soil and a large surface area for interaction with other soil micro-organisms. The detailed functioning and regulation of these mycorrhizosphere processes is still poorly understood and needs detailed molecular approach to study these mycorrhizosphere processes but recent progress in ectomycorrhizal associations is reviewed and potential benefits of improved understanding of mycorrhizosphere interactions are discussed.     

Oxidoreductases and cellulases in lichens: Possible roles in lichen biology and soil organic matter turnover

Lichens are symbiotic associations of a fungus (usually an Ascomycete) with green algae and/or a cyanobacterium. They dominate on 8 % of the world's land surface, mainly in Arctic and Antarctic regions, tundra, high mountain elevations and as components of dryland crusts. In many ecosystems, lichens are the pioneers on the bare rock or soil following disturbance, presumably because of their tolerance to desiccation and high temperature. Lichens have long been recognized as agents of mineral weathering and fine-earth stabilization. Being dominant biomass producers in extreme environments they contribute to primary accumulation of soil organic matter. However, biochemical role of lichens in soil processes is unknown. Our recent research has demonstrated that Peltigeralean lichens contain redox enzymes which in free-living fungi participate in lignocellulose degradation and humification. Thus lichen enzymes may catalyse formation and degradation of soil organic matter, particularly in high-stress communities dominated by lower plants. In the present review we synthesize recently published data on lichen phenol oxidases, peroxidases, and cellulases and discuss their possible roles in lichen physiology and soil organic matter transformations.     

Organic acids,siderophores, enzymes and mechanical pressure for black slate bioweathering with the basidiomycete Schizophyllum commune

Although many fungi are known to be able to perform bioweathering of rocks and minerals, little information is available concerning the role of basidiomycetes in this process. The wood-rotting basidiomycete Schizophyllum commune was investigated for its ability to degrade black slate, a rock rich in organic carbon. Mechanical pressure of hyphae and extracellular polymeric substances was investigated for biophysical weathering. A mixed ß1-3/ß1-6 glucan, likely schizophyllan that is well known from S. commune, could be identified on black slate surfaces. Secretion of siderophores and organic acids as biochemical weathering agents was shown. Both may contribute to biochemical weathering in addition to enzymatic functions. Previously, the exoenzyme laccase was believed to attack organic the matter within the black slate, thereby releasing metals from the rock. Here, overexpression of laccase showed enhanced dissolution of quartz phases by etching and pitting. At the same time, the formation of a new secondary mineral phase, whewellite, could be demonstrated. Hence, a more comprehensive understanding of biophysical as well as biochemical weathering by S. commune could be reached and unexpected mechanisms like quartz dissolution linked to shale degradation.     

Biological weathering and the long-term carbon cycle: integrating mycorrhizal evolution and function into the current paradigm

The dramatic decline in atmospheric CO₂ evidenced by proxy data during the Devonian (416.0–359.2 Ma) and the gradual decline from the Cretaceous (145.5–65.5 Ma) onwards have been linked to the spread of deeply rooted trees and the rise of angiosperms, respectively. But this paradigm overlooks the coevolution of roots with the major groups of symbiotic fungal partners that have dominated terrestrial ecosystems throughout Earth history. The colonization of land by plants was coincident with the rise of arbuscular mycorrhizal fungi (AMF), while the Cenozoic (c. 65.5–0 Ma) witnessed the rise of ectomycorrhizal fungi (EMF) that associate with both gymnosperm and angiosperm tree roots. Here, we critically review evidence for the influence of AMF and EMF on mineral weathering processes. We show that the key weathering processes underpinning the current paradigm and ascribed to plants are actually driven by the combined activities of roots and mycorrhizal fungi. Fuelled by substantial amounts of recent photosynthate transported from shoots to roots, these fungi form extensive mycelial networks which extend into soil actively foraging for nutrients by altering minerals through the acidification of the immediate root environment. EMF aggressively weather minerals through the additional mechanism of releasing low molecular weight organic chelators. Rates of biotic weathering might therefore be more usefully conceptualized as being fundamentally controlled by the biomass, surface area of contact, and capacity of roots and their mycorrhizal fungal partners to interact physically and chemically with minerals. All of these activities are ultimately controlled by rates of carbon‐energy supply from photosynthetic organisms. The weathering functions in leading carbon cycle models require experiments and field studies of evolutionary grades of plants with appropriate mycorrhizal associations. Representation of the coevolution of roots and fungi in geochemical carbon cycle models is required to further our understanding of the role of the biota in Earth's CO₂ and climate history.     

Approaches to modelling mineral weathering by fungi

Fungi are agents of geochemical change in the environment and play important roles in the soil, the plant-root zone, and in rock and mineral habitats. Modelling may serve as a tool to quantify fungal weathering under natural conditions. This paper provides a review of existing mycelial growth models and examines how these can be adapted to describe weathering by ectomycorrhizal and other fungi in mineral soil.     

The effect of rock composition on cyanobacterial weathering of crystalline basalt and rhyolite

The weathering of volcanic rocks contributes significantly to the global silicate weathering budget, effecting carbon dioxide drawdown and long‐term climate control. The rate of chemical weathering is influenced by the composition of the rock. Rock‐dwelling micro‐organisms are known to play a role in changing the rate of weathering reactions; however, the influence of rock composition on bio‐weathering is unknown. Cyanobacteria are known to be a ubiquitous surface taxon in volcanic rocks. In this study, we used a selection of fast and slow growing cyanobacterial species to compare microbial‐mediated weathering of bulk crystalline rocks of basaltic and rhyolitic composition, under batch conditions. Cyanobacterial growth caused an increase in the pH of the medium and an acceleration of rock dissolution compared to the abiotic controls. For example, Anabaena cylindrica increased the linear release rate () of Ca, Mg, Si and K from the basalt by more than fivefold (5.21–12.48) and increased the pH of the medium by 1.9 units. Although A. cylindrica enhanced rhyolite weathering, the increase in was less than threefold (2.04–2.97) and the pH increase was only 0.83 units. The values obtained with A. cylindrica were at least ninefold greater with the basalt than the rhyolite, whereas in the abiotic controls, the difference was less than fivefold. Factors accounting for the slower rate of rhyolite weathering and lower biomass achieved are likely to include the higher content of quartz, which has a low rate of weathering and lower concentrations of bio‐essential elements, such as, Ca, Fe and Mg, which are known to be important in controlling cyanobacterial growth. We show that at conditions where weathering is favoured, biota can enhance the difference between low and high Si‐rock weathering. Our data show that cyanobacteria can play a significant role in enhancing rock weathering and likely have done since they evolved on the early Earth.     

Evolution of trees and mycorrhizal fungi intensifies silicate mineral weathering

Forested ecosystems diversified more than 350 Ma to become major engines of continental silicate weathering, regulating the Earth''s atmospheric carbon dioxide concentration by driving calcium export into ocean carbonates. Our field experiments with mature trees demonstrate intensification of this weathering engine as tree lineages diversified in concert with their symbiotic mycorrhizal fungi. Preferential hyphal colonization of the calcium silicate-bearing rock, basalt, progressively increased with advancement from arbuscular mycorrhizal (AM) to later, independently evolved ectomycorrhizal (EM) fungi, and from gymnosperm to angiosperm hosts with both fungal groups. This led to ‘trenching’ of silicate mineral surfaces by AM and EM fungi, with EM gymnosperms and angiosperms releasing calcium from basalt at twice the rate of AM gymnosperms. Our findings indicate mycorrhiza-driven weathering may have originated hundreds of millions of years earlier than previously recognized and subsequently intensified with the evolution of trees and mycorrhizas to affect the Earth''s long-term CO₂ and climate history.     

Temperature and NaCl- tolerance of rock-inhabiting meristematic fungi

Black meristematic fungi together with lichens and cyanobacteria dominate the micro-flora of rock surfaces in arid and semi-arid environments of hot and cold deserts. This study shows that rock inhabiting meristematic fungi are extremely tolerant against high temperatures, desiccation and osmotic stress. Their temperature tolerance increases with increasing dehydration of the fungal thallus. Air dried mycelia of black yeasts stand temperatures up to 120 °C for at least 0.5 hours. As response to high temperatures multilayered cell walls are developed and trehalose is accumulated whereas the intracellular glycerol regulates the osmotic potential under NaCl stress. Strains from rock in moderate climate (North Germany) show the same tolerance than those isolated from the Mediterranean area. Hortaea werneckii – hitherto only described as agent of human Tinea nigra – is shown to be the most tolerant rock inhabiting species tested. Meristematic fungi cannot be pre-adapted to higher growth temperatures by increased incubation temperatures. Considering the results of this study the justification of the term stress is discussed with regard to rock inhabiting fungi and their natural environment. Consequences for conservation treatments of monuments decayed by meristematic fungi are discussed on the basis of the ecophysiological properties of the fungi.     

Soft‐Bodied Fossils Are Not Simply Rotten Carcasses – Toward a Holistic Understanding of Exceptional Fossil Preservation

Exceptionally preserved fossils are the product of complex interplays of biological and geological processes including burial, autolysis and microbial decay, authigenic mineralization, diagenesis, metamorphism, and finally weathering and exhumation. Determining which tissues are preserved and how biases affect their preservation pathways is important for interpreting fossils in phylogenetic, ecological, and evolutionary frameworks. Although laboratory decay experiments reveal important aspects of fossilization, applying the results directly to the interpretation of exceptionally preserved fossils may overlook the impact of other key processes that remove or preserve morphological information. Investigations of fossils preserving non‐biomineralized tissues suggest that certain structures that are decay resistant (e.g., the notochord) are rarely preserved (even where carbonaceous components survive), and decay‐prone structures (e.g., nervous systems) can fossilize, albeit rarely. As we review here, decay resistance is an imperfect indicator of fossilization potential, and a suite of biological and geological processes account for the features preserved in exceptional fossils.     

The diversity,distribution, and pathogenic potential of cultivable fungi present in rocks from the South Shetlands archipelago,Maritime Antarctica

We studied the molecular taxonomy and diversity of cultivable rock fungi from Antarctic islands. From 50 rock samples, 386 fungal isolates were obtained and identified as 33 taxa of 20 genera. The genera Cladophialophora, Cladosporium, Cyphellophora, Eichleriella, Paracladophialophora, and Penicillium displayed the highest densities. Ecological diversity indices showed that the fungal assemblages are diverse and rich with low dominance. The genera Cladophialophora, Cladosporium, and Penicillium showed a broad distribution from rocks of the various islands. One hundred and fifty-nine fungi, grown at 37 °C, were identified as Penicillium chrysogenum, Fusarium sp., and Rhodotorula mucilaginosa. One hundred and three fungi displayed haemolytic activity, 81 produced proteinase, 9 produced phospholipase, and 25 presented dimorphism and a spore diameter ≤ 4 µm. The Antarctic Peninsula region appears to be under the effects of global climate changes, which may expose and accelerate the rock’s weathering processes, and expose and release cryptic fungi and other microbes, especially those with innate pathogenic potential, previously arrested in rocks. Consequently, these rocks and their particles may represent a vehicle for the dispersal of microbial propagules, including those able to spread pathogens, along, across, and out of Antarctica.

     

Fungal Diversity in Rock Beneath a Crustose Lichen as Revealed by Molecular Markers

Lichen-forming fungi have been assumed to be more or less restricted to the surface of the substrate on which they grow, Conclusive identification of hyphae or an assessment of the fungal diversity inside lichen-covered rock has not been possible using methods based on direct observation. We circumvented this problem by using a DNA sequencing approach. Cores were drilled from a Devonian arcosic sandstone rock harboring the crustose lichen Ophioparma ventosa (L.) Norman on the surface. The cores were cut vertically, and DNA was extracted from the pulverized rock slices. A series of polymerase chain reactions using fungal-specific primers as well as Ophioparma ventosa specific primers were employed to amplify the internal transcribed spacer region of the nuclear ribosomal DNA. The results show that hyphae of O. ventosa penetrate approximately 10–12 mm into the rock. Consequently, the hyphal layer formed by the lichen fungus inside the rock could be 7–12 times as thick as the symbiotic thallus at the surface of the rock. In addition, eight non-lichenized fungal taxa and five that could not be identified to species were encountered. One fungal species in the order Helotiales occurs in six of the eight cores. The significance of these results to the colonization and weathering of rock by lichenized fungi is discussed.     

Fungal biogeochemistry: a central role in the environmental fate of lead

Fungi play major roles in biogeochemistry and are responsible for many metal transformations during mineral weathering. A recent finding that fungi transform lead to chloropyromorphite highlights the importance of fungi in biogeochemical processes.     

Mycorrhizal types differ in ecophysiology and alter plant nutrition and soil processes

Mycorrhizal fungi benefit plants by improved mineral nutrition and protection against stress, yet information about fundamental differences among mycorrhizal types in fungi and trees and their relative importance in biogeochemical processes is only beginning to accumulate. We critically review and synthesize the ecophysiological differences in ectomycorrhizal, ericoid mycorrhizal and arbuscular mycorrhizal symbioses and the effect of these mycorrhizal types on soil processes from local to global scales. We demonstrate that guilds of mycorrhizal fungi display substantial differences in genome‐encoded capacity for mineral nutrition, particularly acquisition of nitrogen and phosphorus from organic material. Mycorrhizal associations alter the trade‐off between allocation to roots or mycelium, ecophysiological traits such as root exudation, weathering, enzyme production, plant protection, and community assembly as well as response to climate change. Mycorrhizal types exhibit differential effects on ecosystem carbon and nutrient cycling that affect global elemental fluxes and may mediate biome shifts in response to global change. We also note that most studies performed to date have not been properly replicated and collectively suffer from strong geographical sampling bias towards temperate biomes. We advocate that combining carefully replicated field experiments and controlled laboratory experiments with isotope labelling and ‐omics techniques offers great promise towards understanding differences in ecophysiology and ecosystem services among mycorrhizal types.     

Actinobacteria—An Ancient Phylum Active in Volcanic Rock Weathering

A molecular biological analysis of Icelandic volcanic rocks of different compositions and glassiness revealed the presence of Actinobacteria as an abundant phylum. In outcrops of basaltic glass they were the dominant bacterial phylum. A diversity of Actinobacteria were cultured from the rocks on rock-agar plates showing that they are capable of growing on rock-derived nutrient sources and that many of the taxa identified by molecular methods are viable, potentially active members of the community. Laboratory batch-culture experiments using a Streptomyces isolate showed that it was capable of enhancing the release of major elements from volcanic rocks, including weathered basaltic glass, crystalline basalt and komatiite, when provided with a carbon source. Actinobacteria of a variety of other sub-orders were also capable of enhancing volcanic rock weathering, measured as Si release. However, most strains did not significantly increase the weathering of the silica-rich rock, obsidian. These data show that Actinobacteria can contribute to volcanic rock weathering and, therefore, the carbonate-silicate cycle. Given their ancient lineage, it is likely they have played a role in rock weathering for over two billion years.