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Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere.

ABSTRACT: Life on Earth extends to several kilometres below the land surface and seafloor. This deep biosphere is second only to plants in its total biomass, is metabolically active and diverse, and is likely to have played critical roles over geological time in the evolution of microbial diversity, diagenetic processes and biogeochemical cycles. However, these roles are obscured by a paucity of fossil and geochemical evidence. Here we apply the recently developed uranium-isotope proxy for biological uranium reduction to reduction spheroids in continental rocks (red beds). Although these common palaeo-redox features have previously been suggested to reflect deep bacterial activity, unequivocal evidence for biogenicity has been lacking. Our analyses reveal that the uranium present in reduction spheroids is isotopically heavy, which is most parsimoniously explained as a signal of ancient bacterial uranium reduction, revealing a compelling record of Earth's deep biosphere.

SUBMITTER: McMahon S

PROVIDER: S-EPMC6206012 | biostudies-literature | 2018 Oct

REPOSITORIES: biostudies-literature

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Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere.

McMahon Sean S Hood Ashleigh V S AVS Parnell John J Bowden Stephen S

Nature communications 20181029 1

Life on Earth extends to several kilometres below the land surface and seafloor. This deep biosphere is second only to plants in its total biomass, is metabolically active and diverse, and is likely to have played critical roles over geological time in the evolution of microbial diversity, diagenetic processes and biogeochemical cycles. However, these roles are obscured by a paucity of fossil and geochemical evidence. Here we apply the recently developed uranium-isotope proxy for biological uran ...[more]

PMID: 30374101

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Project description:Life in water-filled bedrock fissures in the continental deep biosphere is broadly constrained by energy and nutrient availability. Although these communities are alive, robust studies comparing active populations and metabolic processes across deep aquifers are lacking. This study analyzed three oligotrophic Fennoscandian Shield groundwaters, two "modern marine" waters that are replenished with organic carbon from the Baltic Sea and are likely less than 20 years old (171.3 and 415.4 m below sea level) and an extremely oligotrophic "thoroughly mixed" water (448.8 m below sea level) of unknown age that is composed of very old saline and marine waters. Cells were captured either using a sampling device that rapidly fixed RNA under in situ conditions or by filtering flowing groundwater over an extended period before fixation. Comparison of metatranscriptomes between the methods showed statistically similar transcript profiles for the respective water types, and they were analyzed as biological replicates. Study of the small subunit (SSU) rRNA confirmed active populations from all three domains of life, with many potentially novel unclassified populations present. Statistically supported differences between communities included heterotrophic sulfate-reducing bacteria in the modern marine water at 171.3 m below sea level that has a higher organic carbon content than do largely autotrophic populations in the H2- and CO2-fed thoroughly mixed water. While this modern marine water had signatures of methanogenesis, syntrophic populations were predominantly in the thoroughly mixed water. The study provides a first statistical evaluation of differences in the active microbial communities in groundwaters differentially fed by organic carbon or "geogases."IMPORTANCE Despite being separated from the photosynthesis-driven surface by both distance and time, the deep biosphere is an important driver for the earth's carbon and energy cycles. However, due to the difficulties in gaining access and low cell numbers, robust statistical omics studies have not been carried out, and this limits the conclusions that can be drawn. This study benchmarks the use of two separate sampling systems and demonstrates that they provide statistically similar RNA transcript profiles, importantly validating several previously published studies. The generated data are analyzed to identify statistically valid differences in active microbial community members and metabolic processes. The results highlight contrasting taxa and growth strategies in the modern marine waters that are influenced by recent infiltration of Baltic Sea water versus the hydrogen- and carbon dioxide-fed, extremely oligotrophic, thoroughly mixed water.

Dataset Information

Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere.

Publications

Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere.

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