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We report a novel weathering mechanism in South African sandstone formations, where cryptoendolithic cyanobacteria induce weathering by substrate alkalization during photosynthesis. As a result, the upper rock part is loosened and then eroded away by physical forces such as wind, water, trampling. This special type of‘exfoliation’ is widely distributed and affects the geomorphology of whole sandstone mountain ranges and outcrops across several biomes. We show, that this weathering type is initiated by bioalkalization because of the photosynthesis of cryptoendolithic (i.e. those organisms living in small tight open spaces between the sand grains) cyanobacteria causing pH values high enough to enhance silica solution in the cryptoendolithic zone. As modern cyanobacteria are the initial photoautotrophic colonizers of bare rocks in arid and semiarid landscapes, it is possible that they may also have played a significant role in shaping sandstone landscapes in the geological past. [ABSTRACT FROM AUTHOR]

The Atacama Desert is the driest non-polar desert on Earth, presenting precarious conditions for biological activity. In the arid coastal belt, life is restricted to areas with fog events that cause almost daily wet-dry cycles. In such an area, we discovered a hitherto unknown and unique ground covering biocenosis dominated by lichens, fungi, and algae attached to grit-sized (~6 mm) quartz and granitoid stones. Comparable biocenosis forming a kind of a layer on top of soil and rock surfaces in general is summarized as cryptogamic ground covers (CGC) in literature. In contrast to known CGC from arid environments to which frequent cyclic wetting events are lethal, in the Atacama Desert every fog event is answered by photosynthetic activity of the soil community and thus considered as the desert's breath. Photosynthesis of the new CGC type is activated by the lowest amount of water known for such a community worldwide thus enabling the unique biocenosis to fulfill a variety of ecosystem services. In a considerable portion of the coastal Atacama Desert, it protects the soil from sporadically occurring splash erosion and contributes to the accumulation of soil carbon and nitrogen as well as soil formation through bio-weathering. The structure and function of the new CGC type are discussed, and we suggest the name grit-crust. We conclude that this type of CGC can be expected in all non-polar fog deserts of the world and may resemble the cryptogam communities that shaped ancient Earth. It may thus represent a relevant player in current and ancient biogeochemical cycling., (© 2019 The Authors. Geobiology published by John Wiley & Sons Ltd.)

The two endolithic lichen species Hymenelia prevostii and Hymenelia coerulea were investigated with regard to their thallus morphology and their effects on the surrounding substrate. The physiological processes responsible for the observed alterations of the rock were identified. Whereas the thallus surface of H. coerulea was level, H. prevostii formed small depressions that were deepest in the thallus center. In a cross-section, both species revealed an algal zone consisting of algal cavities parallel to the substrate surface and a fungal zone below. However, H. prevostii revealed significantly larger cavities with more than twice the cell number and a denser pattern of cavities than H. coerulea, resulting in a biomass per surface area being more than twice as large. Below H. prevostii the layer of macroscopically visibly altered rock material was about twice as deep and within this layer, the depletion of calcium and manganese was considerably higher. In simultaneous measurements of the oxygen uptake/oxygen release and pH shift, the isolated algal strains of both lichens revealed respiration-induced acidification of the medium in the dark. At higher light intensities, H. coerulea and to a lesser extent also H. prevostii alkalized the medium which may lessen the acidification effect somewhat under natural conditions. In a long-term growth experiment, the isolated algal strains of both lichens revealed acidification of the medium to a similar extent. Neither acidic lichen substances nor oxalic acid was identified. The significant differences between the weathering patterns of both species are based on the same respiration-induced acidification mechanism, with H. prevostii having a greater effect due to its higher biomass per area., (© 2010 Blackwell Publishing Ltd.)