Weathering of rocks by lichens: fragmentation, dissolution and precipitation of minerals in a microbial microcosm

Despite the abundant evidence that rock-encrusting lichens can weather their substrates, it is currently unclear whether rates of lichen-mediated weathering are faster or slower than rates of abiotic weathering of otherwise identical rock surfaces (i.e. are lichens biodestructive or bioprotective?). This question is of considerable academic and commercial importance. Lichens weather rocks by a combination of biophysical and biochemical mechanisms. Fungal hyphae can penetrate into rocks at less than or equal to similar to0.1 mm y(-1) and sandstones and limestones are especially vulnerable. Biophysical weathering leads to the fragmentation of minerals, exposing grain interiors. These fresh surfaces may be attacked by a variety of compounds including extracellular polysaccharides, lichen acids and oxalic acid. Evidence for the effectiveness of these biochemical agents includes etched and leached mineral grains and reaction products such as oxalate salts, clay minerals and Fe-hydroxides. Few studies have quantified rates of lichen-mediated weathering and fewer still have compared these data with the weathering rate of unencrusted rock surfaces. The conclusion of this work is that lichens enhance the weathering rate of rock surfaces relative to identical but abiotic substrates. As weathered mineral grains and weathering products are bound within the lichen, these materials will not be eroded until the lichen dies after similar to 10(1)-10(3) y. Thus, despite being active agents of weathering, lichens should stabilize and protect rock surfaces over the short term. Studies of dated surfaces of a variety of rock types colonized by diverse lichen populations are essential before the impact of lichen colonization on rates of rock weathering can be accurately quantified and predicted.