Microbially mediated carbonate precipitation is globally significant and can provide a mechanism for understanding and potentially manipulating carbon sequestration. Arid-land caves, as semi-closed systems stripped of the influence of surface weathering, provide a particularly valuable window into the world of carbonate-precipitating microorganisms. Carbon moves from the surface to the subsurface, where it is preserved in carbonate precipitates. These precipitates, many of which are biogenic, record microbial influences, surface climate, and ecosystem changes. Our investigations will identify a suite of biosignatures useful in the search for these signals and will aid speleothem data interpretation in paleoclimate studies.
Prior work on speleothems by our team has shown a number of significant points. A variety of carbonate biomorphs exist in paleo- and active pools in Guadalupe Mountain caves in New Mexico (Melim et al. 2004; Queen and Melim, 2006). Fossil pool fingers from Hidden Cave revealed alternating microbial laminations with micritic textures and abiotic, clear spar layers (Melim et al. 2001). This distinct pattern has led us to hypothesize an episodic input of fixed carbon and other nutrients derived from the surface. Recent observations of forest fire-derived carbon visibly incorporated in nearby cave speleothems suggest that surface carbon may be incorporated into pool precipitates and provides a testable model of nutrient linkages between the surface and subsurface that fuel the geomicrobiology. We believe that the banding observed in these pool precipitates and episodic forest-fire-derived carbon mobilization are linked and more obvious in shallow caves more closely coupled to the surface than in deeper caves.
As an established multidisciplinary team, we will investigate these linkages and microbial and abiotic processes that produce these cave pool precipitates. To further the understanding of biogenic carbonate precipitation processes we propose to: 1) Quantify and characterize the movement of carbon and other possible energy sources that fuel geomicrobial metabolism from the surface to the subsurface as recorded in biogenic carbonate pool precipitates. 2) Characterize the pulsed input of carbon as recorded in layered cave pool precipitates as a reflection of surface ecosystem dynamics. 3) Characterize carbonate pool precipitates and their geologic setting to identify resulting biosignatures along a gradient of biogenic to abiologic, correlated with depth below the surface or hydrologic flow path. 4) Characterize the aqueous/gas geochemistry and microbial communities of living pools with and without pool carbonate precipitates and their corresponding drip waters. 5) Determine whether cultured microorganisms that form carbonate are reproducing textural, geochemical, and isotopic biosignatures found in natural cave settings.
Broader scientific impact of this work focuses on the determination of the degree of biogenicity of cave pool carbonate, which bears on interpretation of the paleoclimate, hydrology, and geochemistry of the vadose zone. Results will allow us to better understand carbon flow from the surface to subsurface and will have implications for global carbon storage.
Careful multidisciplinary work is essential to establish an active microbial role in cave carbonate formation and serves as an excellent platform for education efforts. Team members are involved in training a large number of students (graduate, undergraduate, and high school) including minority students through a number of UNM and NMT minority programs. This proposed work marks an important symbiosis with interdisciplinary education initiatives (EPSCOR and AGEP collaborations among UNM, NMT and NMSU). Importantly, WIU students have a summer research opportunity at UNM or NMT to encourage their pursuit of graduate education. Our research has a high broadcast and print media visibility helping to attract students to our work. We interact extensively with the general public via talks and other avenues.
: 9/1/07 - 8/31/11 Principal Investigator: L.A. Melim, Geology Department, WIU; Co-Principal Investigators: D.E Northup, M.N. Spilde, L.J. Crossey, University of New Mexico; P.J. Boston NM Tech The goal of this multidisciplinary project was to investigate whether microorganisms help form structures of carbonate that resemble fingers (i.e. pool fingers), which form underwater in pools in caves, far below the surface where no sunlight and little carbon penetrates. Specifically we: (1) characterized the kinds of carbonates found in cave pools, (2) identified isotopic and other chemical biosignatures in cave pool carbonate, (3) characterized the aqueous and gaseous geochemistry of cave pools to determine saturation and identify potential nutrients for microbes living in cave pools, (4) characterized the microbial communities living in cave pools using microbiological culturing and culture-independent (i.e. molecular) methods and (5) determined if cultured microorganisms that form carbonate in the laboratory are reproducing textural, geochemical, and isotopic biosignatures found in cave settings. Key Findings: Pendant biothems (pool fingers, drippies, u-loops, webulite; Queen and Melim 2006) are found in 60% of the 228 cave pools described. They occur in a wide range of pools with only one clear requirement: the pool must have something for them to hang from such as shelf stone or an overhanging cave wall. We find that cave carbonate biothems commonly preserve fossil microbes. This is in contrast to their general absence in fossil travertines and stromatolites. Interestingly, fossil microbes are found in methane seep carbonates. The common thread between methane seeps and caves is lack of sunlight, so perhaps solar degradation is important in surface environments. If this could be shown, it would have important implications for the preservation of fossils in the subsurface of Mars and other extraterrestrial bodies. The carbonate biothems may contain micritic layers, which usually appear darker in petrographic thin section, and contain clay particles that have been brought from the surface. These layers also contain trace levels of phosphate, sulfate and nitrogen, also likely derived from the surface, which are nutrients that can be utilized by the microbial community. Pool fingers from Hidden and Cottonwood Cave show a 3-4‰ isotopic depletion relative to the pool spar, which is a strong piece of evidence supporting active microbial participation in mineralization. Our molecular results reveal the presence of a diverse bacterial community in the cave pool water that includes members related to the Gamma-, Delta-, Beta- and Alphaproteobacteria, Verrucomicrobia, Chloroflexi, Nitrospirae, Acidobacteria, Actinobacteria, Cyanobacteria, Gemmatimonadetes, Chloroflexi, Bacteroidetes, Planctomycetes, and Firmicutes. These results overlap significantly with those of the only published cave pool biodiversity study, which was done in Europe. The large number of clones related to Gammaproteobacteria and, in particular, some putative Nitrosococcus, and Nitrospira, suggests that ammonia oxidation and nitrite oxidation may occur. Nitrite oxidation is also suggested by the thermodynamic modeling of the water chemistry. Phylogenetic analyses of cultured organisms and environmental samples from the pools and a flowing calcite-depositing underground river, show that several of the bacteria are related to known calcite precipitating bacteria (Bacillus and Pseudomonas) and that differences exist between communities associated with standing pool water and flowing underground river habitats. A comparison of communities in pools without fingers to pools with fingers shows little overlap between the two. However, a further analysis suggests that some of this difference may be due to human impact, suggesting that it is essential to extend our studies to pristine caves. Our studies are shedding light on the nature of these communities and represent some of the first culture-independent studies of cave pool waters, and their possible roles in precipitating calcium carbonate. The growth rates of natural rock habitat organisms are often exceedingly slow and even further patience is required for these organisms to produce clear mineralization. We have learned much from this slow process including: 1) the intrinsically apparent growth rate of organisms from environments in which they are not being grazed by invertebrates or other predators, 2) a strong or apparently obligatory association of multi-strain consortia often involving filamentous, pleiomorphic, and other bacteria, and often with fungal partners, 3) inhibition of strains by other organism communities, and 4) that even stripped down derivative communities can produce the mineralization of interest over time. Outreach: Thirteen undergraduates (six minorities; six women), and six graduate students (one minority, four women) have been involved with this project. There have been 27 professional presentations, 14 by students; nine published papers and one Master’s thesis have resulted. Outreach includes 10-20 public presentations, classroom presentations (K-12 and college), media interviews, filming for TV specials on caves, or short courses per year. One female high school teacher from Carlsbad High School was involved with the research through the SEIS Program funded by Sandia National Laboratories and UNM.
