3.5 Ga of glass bioalteration: Inferring microbial function from trace fossil morphology

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Staudigel, Hubert۱; Furnes, Harald۲; McLoughlin, Nicola۲; Banerjee, Neil۳; Connell, Laurie۴; Templeton, Alexis۵
Holding Date 24 September 2008

The interaction of microbes with volcanic glass may have a significant impact on a range of first-order earth science issues, including the earliest life on earth, geochemical fluxes between seawater and the oceanic crust, and a substantive contribution to global biomass. Glass bioalteration has been recognized on the basis of their distinct textures, and supported by geochemical fingerprinting and characteristic DNA accumulations associated with biotextures in altered glass.
Textural arguments rely on the observation of two distinct types of textures associated with volcanic glass alteration: (1) annular bands that propagate from the exterior into the glass towards its fresh core indicating abiotic/diffusive hydration and chemical exchange, and (2) micron-sized tunnels or agglomeration of spherical cavities in glass surfaces that are likely to be caused by microbial, cavity-forming, congruent dissolution. It has been proposed that these textures are formed by colonizing prokaryotic microbes that locally dissolve glass by changing the pH in their contact area. These bioalteration textures are common in submarine lavas throughout the world’s ocean, and the dominant glass alteration mode in the upper 300 m of the oceanic crust. They can be found in nearly all well-preserved ophiolites and greenstone belts dating back to 3.5 Ga. Chemical erosion by colonizing microbes, however, is an unlikely mechanism leading to tubular features that are 100µm deep and less than 1 µm wide tunnels. A microbe deep inside this tunnel has to acquiring the organic carbon and energy necessary to promote dissolution and it has to remove the byproducts of its activity. Furthermore, individual microbes are unlikely to have the sense of "directionality" needed to make straight tubes, or regularly spiraling tubes. To address these issues, we propose that the distinct morphology of tubular bioalteration implies a different mechanism of microbial dissolution. Granular glass alteration is well explained by colonizing microbes that selectively dissolve the glass in their contact area, forming a sponge-like interconnected network of micron sized cavities along glass surfaces exposed to water. Tubular alteration, however, is more likely to be caused by processes similar to fungal tunneling as observed in soil feldspars and marine carbonates. Fungal hyphae clearly have the dimensions and can take on shapes that could generate the diversity of tunneling morphologies observed. Fungal hyphae have been shown to excrete oxalic acid at their tips and to recover nutrients that are brought back into the fungal host cell. Hyphae like organelle are not unique to fungi, however, whereby some bacterial groups (e.g. Actinomyces) may display a similar function. While any of these processes remain poorly studied, we suggest that tubular textures are caused by such cell extensions offering us some important new clues in the search for identifying the microbes involved in this process.