LEAVING THEIR MARK: BIOLOGICALLY-INDUCED MINERAL WEATHERING AND SOIL DEVELOPMENT IN ANCIENT LAND-PLANT ECOSYSTEMS

Colonization of the land by plants during the Early Paleozoic was fundamental to the evolution of terrestrial ecosystems, biologically-mediated weathering, and the development of soils. However, the method by which communities of primitive land plants and their fungal and bacterial symbionts microscopically weathered their substrates to produce clays and influence soil development remains largely unknown. We investigated Cryptogamic Ground Covers (CGCs) from volcanic and fluvial settings in Iceland to identify modern analogous communities and to characterize soil structure and associated biologically-mediated weathering features. Using a novel application of X-ray Micro Computed Tomography, we show that CGCs dominated by lichens and bryophytes that form symbioses with fungi and cyanobacteria are the best modern analogues of early soil communities. CGC soils are aggregated and stabilized by cyanobacteria, mycorrhizal and lichenized fungi, plant rhizoids and their associated exudates. Smectite has developed in CGCs dominated by basal thalloid liverworts. Microscopic biologically-mediated dissolution features on soil minerals are diverse and attributable to different organisms (e.g., bacteria, fungi) and interaction (e.g., symbiosis). We postulate that these grain features provide a novel indirect means of inferring the nature of the biotic component of fossil soils (paleosols). Furthermore, approaches to estimate weathering rates and clay formation in early terrestrial ecosystems – currently based on inferences drawn from experiments on model organisms – needs to consider a broad diversity of microorganisms and their interactions.