The fact that varnish is rich in organics and exposed to light-conditions that promote manganese photoreduction-and yet the manganese is maintained largely as Mn 4+ oxides supports the view that dynamic redox cycling occurs within varnish. The sulfur in varnish displayed complex speciation sulfur is present in organic forms observed in biological material (variable mixtures of thiols with disulfides, sulfoxides, and sulfonates) as well as sulfate salts ( SI Appendix, Fig. To confirm that this sulfur reflects organics rather than just sulfur-bearing mineral phases, we used XANES spectroscopy to assess electronic structure at the sulfur K-edge. 32S appeared throughout varnish, notably concentrated in micrometer-scale particles that sit along varnish laminations and are distinct from detrital grains observed in the 28Si and 57Fe mass channels ( SI Appendix, Fig. To visualize organic matter in varnish, we used NanoSIMS imaging of sulfur. This heterogeneity is consistent with the view that manganese redox cycling occurs within varnish, and the distribution of these domains indicates that such cycling is not restricted to surficial processes contributing to the ongoing accretion of varnish, but rather also characterizes the ecosystem that exists within well-developed varnish. Although predominantly Mn 4+, it contains common discrete microscale domains with variable and considerable mixtures of Mn 3+ ( Fig. Using synchrotron X-ray absorption near-edge structure (XANES) spectroscopy and multiple-energy “redox” mapping at the manganese K-edge, we found that varnish birnessite is not homogenous in its redox properties.
The manganese oxide mineral phase in varnish has been described as poorly crystalline birnessite ( 2)-a phase composed of manganese octahedra organized in layers ( 35) it is formally Mn 4+O 2, but can incorporate a substantial fraction of Mn 3+ instead of Mn 4+ when accompanied by charge balance with heteroatoms (Na +, K +, Ba 2+, etc.) between the layers ( 36). In varnish, these microtextures supported the relationship with sunlight that has been documented with macroscale field observations and strengthened the evidence for a role for light in varnish genesis. On a topographically irregular surface, relative highs receive more light while relative lows get shaded thus, the highs grow higher forming columnar features. In certain stromatolites, these textures have been interpreted in terms of light-dependent growth, models of which come from studies of coral growth ( 34). In all varnish samples examined, we observed laminae with crinkly to columnar or domal textures that mark an emergent topography similar to that of stromatolites-macroscopic sedimentary structures commonly understood as mineralized residue of ancient microbial mats ( 33). Varnish cross-sections revealed micrometer-scale, subhorizontal laminations that reflect its accretionary mode of growth ( Fig. To understand better the physical processes controlling varnish development, we examined depositional textures in petrographic thin sections using backscatter scanning electron microscopy (SEM). Taken together, these results indicated that the manganese enrichment in varnish is related to its specific uptake and use by likely founding members of varnish microbial communities. The speciation of this manganese determined by advanced paramagnetic resonance techniques suggested that the Cyanobacteria use it as a catalytic antioxidant-a valuable adaptation for coping with the substantial oxidative stress present in this environment. We then showed that diverse Cyanobacteria, including the relevant Chroococcidiopsis taxon, accumulate extraordinary amounts of intracellular manganese-over two orders of magnitude higher manganese content than other cells. Our analyses described a material governed by sunlight, water, and manganese redox cycling that hosts an unusually aerobic microbial ecosystem characterized by a remarkable abundance of photosynthetic Cyanobacteria in the genus Chroococcidiopsis as the major autotrophic constituent. We collected varnish samples from diverse sites across the western United States, examined them in petrographic thin section using microscale chemical imaging techniques, and investigated the associated microbial communities using 16S amplicon and shotgun metagenomic DNA sequencing. It is highly and selectively enriched in manganese, the mechanism for which has been a long-standing geological mystery. Desert varnish is a dark rock coating that forms in arid environments worldwide.
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