[1] Scientists vigorously debate the degree to which rock varnish is formed through the actions of microorganisms. To investigate this enigma, we utilized a three-pronged approach that combined (1) culture-independent molecular methods to characterize bacterial communities associated with varnish that coats the rhyolitic volcanic rocks of Black Canyon, New Mexico, and rocks with no visible varnish; (2) culturing of varnish in media supplemented with reduced forms of manganese and/or iron and no or low amounts of carbon to isolate bacteria capable of precipitating iron and/or manganese oxides; and (3) scanning electron microscopy (SEM) of varnish and nearby rock that lacks macroscopically visible varnish. Our culture-independent studies revealed significant differences between varnish and nonvarnish communities. Chloroflexi and Ktedobacteria dominated one varnish site, while the other varnish site was dominated by Cyanobacteria. The nonvarnish sites were dominated by Actinobacteria and, to a lesser extent, Cyanobacteria and were the only samples to contain Deinococcus-Thermus sequences. Approximately 65% of varnish cultures produced visible manganese precipitates. Most culture isolates were not closely related to known manganese oxidizers, with the exception of Bacillus spp. SEM revealed microbial morphologies and two types of varnish morphologies: (1) relatively smooth layers and (2) patches of botryoidal pinnacles, which were often associated with increased manganese concentrations. "Bare" rock showed evidence of incipient varnish. These results have important implications for the detection of life on extraterrestrial planets such as Mars, where putative varnish coatings have been observed, and represent some of the first culture-independent characterizations of varnish communities.
We hypothesize that a reduced capacity to withstand or repair cellular damage from ultraviolet radiation may be present in caveadapted microorganisms that never experience such conditions. However, a small number of previous studies have shown that some subsurface bacteria do not show greater sensitivity to ultraviolet radiation (UVR) than surface bacteria. To estimate UVR sensitivity in cave bacteria, bacterial isolates were collected from Carlsbad Cavern, New Mexico, U.S.A., and percent survival following exposure to various UVC and UVA radiation doses was determined. Cave bacteria from Left Hand Tunnel in Carlsbad Cavern and surface bacteria from soil and rocks above Carlsbad Cavern were grown on low and high nutrient media then exposed to 0, 10,000 and 20,000 μWs/ cm 2 of UVR in a laboratory biological safety cabinet. Incubations were conducted at 15°C or 37ºC, in accordance with the isolates' natural temperature environments. In addition, DNA repair capacity was estimated by exposing the organisms to various doses of UVC radiation and measuring survivability. Gram status and pigmentation also were determined. Results showed that most of the cave isolates were more sensitive to UVR than the surface isolates, but survivability data suggest that cave microbes retain some of their capacity to repair UV-induced DNA damage. Selection appears to have favored bacteria that can survive in this low nutrient environment, while not maintaining (or paying the cost of maintaining) unneeded traits such as UVR resistance. Cave bacteria appear to have maintained DNA repair capacity, most likely because of the need to repair damage to their DNA from other environmental stressors found in caves.Keywords: Ultraviolet radiation sensitivity, cave-adaptation, bacteria, subsurface, caves unable to show a correlation between natural levels of radiation exposure and species resistance (Nasim & James, 1978;Arrage et al., 1993a).UV radiation (UVR) at wavelengths shorter than 400 nm is absorbed by DNA and can cause cyclobutane dimer formation, interstrand crosslinking, and other direct and indirect damage to DNA (Nasim & James, 1978;Miller et al., 1999). In addition to enzymatic DNA repair mechanisms, microbes have evolved to survive UV exposure through other molecular and structural protection and avoidance methods. However, whether these additional methods actually protect the bacterial DNA is widely debated (Mathews & Sistrom, 1959; Dworkin & Stanley, 2006;Gascon et al., 1995; Lewis et al., 1973; Singer & Ames, 1970; Nasium & James, 1978;Cockell, 1998;Arrage et al., 1993a). Iron compounds, sand, desert crust, rock, water, sulfur and sodium chloride have all been found to be natural UVR shields for bacteria (Cockell, 1998;Rothschild & Giver, 2003). A well-documented predominance of pigmentation in UVR-resistant species of bacteria suggests the use of carotenoids, yellow, orange and red cellular pigments, as UVR screening agents (Mathews & Sistrom, 1959; Dworkin & Stanley, 2006). UVR screening agents also provide protection from ox...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.