Mn-Fe-Enhancing Budding Bacteria in Century-Old Rock Varnish, Erie Barge Canal, New York

Abstract

Fossil remnants of bacteria involved in the enhancement of manganese and iron rarely occur within the microstratigraphy of rock varnishes collected from warm desert environments, because varnish formation processes ultimately destroy these microfossils through remobilization of Mn-Fe and reprecipitation in a clay-mineral matrix. In contrast, Mn-Fe encrustations on budding bacteria commonly occur within varnishes that formed within just a century along the Erie Barge Canal, New York. Nanoscale imagery and elemental analyses reveal that these budding bacterial forms greatly enhance Mn, Fe, or both in encrustations surrounding hyphae and cells. The Mn and Fe precipitates have a granular texture on the scale of !1 nm to ∼10 nm. The precipitates also have a stringy texture, where strings are typically only a few nanometers wide. These in situ observations are consistent with expectations from studies of budding-bacteria cultures and with the polygenetic model of varnish formation. Given that the Erie Canal site presents the fastest known rate of varnishing, with typical thicknesses around 15 mm formed in a century, only one or two budding bacteria encrusting Mn-Fe oxides each year would be sufficient to generate the observed Erie Canal varnish. This contrasts with one bacterium growing every ∼400 y being needed to generate observed rates of varnishing in typical warm desert settings.

Nanoscale Observations Support the Importance of Chemical Processes in Rock Decay and Rock Coating Development in Cold Climates

“…The Mn-rich casts observed in association with fossilized budding bacteria display a fibrous texture of fine spines and long bundles of hair-like manganese oxides very similar to what has been seen on the cell wall of natural microbial communities [124], culturing studies [125], and fossilized budding bacteria in varnishes non-cold-climate settings [126]. These fibrous forms could be the result of different processes, such as Mn(II) absorption [127], bacterial oxidation [126], bacterial use of Mn (IV) as extracellular respiratory electron acceptors [128], or some combination of processes.…”

Rock varnish on petroglyphs from the Hima region, southwestern Saudi Arabia: Chemical composition, growth rates, and tentative ages

“…Some authors have suggested that abiotic processes are sufficient to explain the Mn oxidation and deposition (Collins and Buol, 1970;Elvidge and Iverson, 1983;Goldsmith et al, 2014;Perry et al, 2005;Thiagarajan and Lee, 2004). On the contrary, there is a large body of evidence that suggests an involvement of microorganisms (bacteria or fungi) in the oxidation and precipitation of Mn (Dorn et al, 2013;Gadd, 2017;Jones, 1991;Krinsley et al, 2012Krinsley et al, , 2017Krumbein and Jens, 1981;Kuhlman et al, 2006Kuhlman et al, , 2008Laudermilk, 1931;Marnocha and Dixon, 2013;Wang et al, 2011). The lack of microfossils (Macholdt et al, 2015(Macholdt et al, , 2017b and of evidence for the presence of activated Mnoxidizing enzymes in varnish (Lang-Yona et al, 2018) does not rule out a bacterial role in Mn precipitation, considering the length of time involved in its formation (Dorn and Krinsley, 2011;Krinsley et al, 2013).…”

Necrogeomorphology and the life expectancy of desert bedrock landforms

“…Using objective data in warm deserts, the vertical rate of varnish accumulation is in the order of micrometers per thousand years (Dorn, 1998; Liu and Broecker, 2000). In more mesic microenvironments, even in a desert, rates can accelerate to micrometers per century (Spilde et al, 2013), with varnishing in humid settings reaching micrometers per decade (Krinsley et al, 2017). All of these studies, however, focus on varnish that ‘started from scratch’ on a bedrock surface and not on a pre-existing rock coating.…”