Biogeochemistry of the lichen-rock interface on different silica-rich rock types in the Vredefort Dome, South Africa

Abstract

Land colonization by plants and their fungal and bacterial symbionts was fundamental to the evolution of terrestrial ecosystems, but how their communities influenced mineral weathering and soil development in the southern hemisphere and African environments remains largely unknown. This study considers lichen colonization on the weathering crust on silica-rich rock varieties (K-feldspar rich granite, alkali granite, sub-labile arenite, ferruginous wacke and metaconglomerate) from the ~2023 ± 4 Ma Vredefort impact crater, central South Africa, with an integrated approach using optical microscopy, X-ray micro-computed tomography (μCT) and scanning electron microscopy (SEM). The interaction of four recurrent saxicolous lichen genera (Acarospora sp.pl., Caloplaca s. lat. sp.pl., Buellia sp.pl. and Xanthoparmelia sp.pl.) which targeted silica-rich rock varieties were explored, thus contributing to a deeper understanding of the multifaceted lichen-induced bioweathering processes that are active within impacted regions. In particular, the potential efficacy of lichen colonization on silica-rich rock degradation were investigated. The study characterized and compared the specific biomechanical and biochemical actions of each genus and documented their expected epilithic and endolithic behavior. It was investigated whether different lichen genera could explain variations in mechanisms, patterns and degree of weathering at each site. Granitic rocks with initially low porosity became more porous as a result of impact bulking, whilst metasedimentary rocks showed a well-defined decrease in particle size and permeability, presumably related to pore collapse or impact-induced annealing. However, the heterogeneous distribution of collapsed pores, melt phases, and subsequent recrystallization, resulted in heterogeneous lichen colonization patterns. Cavities and vesicles formed during melting yielded new habitats for both cryptoendoliths and chasmoendoliths, manifested in the natural cryptoendolithic and chasmoendolithic colonization of meteor impact-shocked silica-rich rocks. The pore-fracture network serves for element transfer in the subsurface part of solid rocks and releases important nutrients that otherwise would remain bound. Moreover, the substrate zone where the saxicolous lichens are attached is most affected by weathering reactions and shows the highest co-occurrence of lithobiontic microorganisms. Because of their crustose and foliose morphology, the saxicolous lichens examined here are mainly involved in combined biophysical and biochemical action, mainly on nutrient-rich, weatherable minerals. Generally, fungal hyphae were localised to the feldspar (mostly

plagioclase), mica (mostly biotite and muscovite) and amphibole (mostly Fe-hornblende) component of silica-rich rock varieties compared to quartz and iron oxides. Nevertheless, quartz and iron oxide were also weathered quite significantly. The importance of variable microtextural features, such as twinning and cleavage planes, physical micro-flaws, etch pitting and dissolution lines, seems to be prominent, inducing macro-scale effects which lead to the progressive disintegration of the substrate, with detachment and progressive incorporation of their fragments into the lichen thallus. Major products of rock transformation in situ are the neoformed phyllosilicate clays (and possible amorphous silica), iron release and oxide staining along cracks, oxalate, mycogenic minerals, organic matter accumulation and abundant biofilms that are formed within the porous space in the subsurface part of solid rocks. Such secondary weathering products are mainly attributed to degradative activities, metabolite excretion and/or sorption phenomena. It is thus likely that the physical and chemical properties of the substrate, along with lichen and microorganism activity, determine weathering rates in different microenvironments and microhabitats.