Biologically versus inorganically mediated weathering reactions: relationships between minerals and extracellular microbial polymers in lithobiontic communities

Abstract

Biophysical and biogeochemical weathering of amphibole syenite associated with the Stettin complex near Wausau, Wisconsin, has been examined by HRTEM, WDS, LM, and XRD. The rock consists of microperthitic feldspar, ferriannite, quartz, and ferrohastingsite. Crustose saxicolous lichens, Rhizocarpon grande and Porpidea albocaerulescens, penetrate the rock surface to a depth of 10 mm. Within the intact rock, amphibole surfaces along hyphae-filled cracks are highly corroded. Fungal hyphae exploit grain boundaries, cleavages, and cracks to gain access to mineral surfaces, resulting in accumulations of cleavage-bound mineral fragments as small as 5 μm within the lower thallus. Bacterial microcolonies are common and all mineral surfaces are completely coated in extracellular acidic mucopolysaccharides from fungal and bacterial sources. In the cases of amphibole, quartz, and feldspar, dissolution does not appear to involve pervasive leaching, for even the smallest mineral fragments retain their chemical and structural identity. Biotite directly in contact with the lichen thallus is intimately interpenetrated by fungal hyphae growing along (001) cleavages and is partially converted to vermiculite. No siliceous relics have been identified. Biologically mediated weathering involves a complex dissolution/ selective transport/recrystallization mechanism occurring within the acidic extracellular gels coating all mineral surfaces. A specialized weathering microenvironment around each mineral grain initially produces minute phyllosilicate crystallites. A rind of clay minerals forms around the dissolving parent phase, eventually culminating in abundant 5–10 μm diameter polymer-bound aggregates of face-to-face oriented clay minerals of homogeneous composition. Physiochemical weathering of ferrohastingsite produces topotactically oriented smectite and goethite. The cleavage-controlled reaction is neither isochemical nor isovolumetric.