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Executive Summary

The University of Rhode Island (URI) team of the NASA Astrobiology Institute (NAI) works to gain a fundamental understanding of Earth’s subsurface life. Our principal research objectives are to understand (1) the subsurface microbial ecosystems of marine sediments, (2) their role in Earth’s biogeochemical cycles, and (3) their relevance to the search for life on other planets. Our investigators are based at the University of Rhode Island (URI), the University of North Carolina at Chapel Hill (UNC), and Woods Hole Oceanographic Institution (WHOI). To effectively accomplish our objectives, URI team researchers collaborate with each other and with scientists at other institutions throughout the world (e.g., D’Hondt et al., 2004). Our current collaborators include members of the Marine Biological Laboratory (MBL), Pennsylvania State University (PSU) and Indiana/Princeton/Tennessee (IPTAI) NAI teams. Other collaborators include scientists at the University of Bremen (Germany), the Max Planck Institute for Marine Microbiology (Germany), the Japan Marine Science and Technology Center, the Massachusetts Institute of Technology, Roger Williams University, and a number of other institutions. To gain access to subsurface environments in Year 7, we participated in field expeditions to many regions, including the Arctic Ocean, the northeastern Pacific, the northeastern Atlantic (the Ireland Margin and the Lost City Hydrothermal Field), and subarctic Canada (Figure 1).

Figure 1. Arial view of the three ice-breakers used in the Integrated Ocean Drilling Program’s Arctic Coring Expedition. The drill-ship is at the bottom. URI Team Member David C. Smith sailed on the expedition as the shipboard microbiologist.

Members of our team at UNC Chapel Hill focus on molecular studies of deep subsurface communities. In year 7, we published the first molecular community analyses (16S rRNA genes) of organic-poor deep marine subsurface sediments. In collaboration with MBL team members, we also undertook what may be the first rRNA analyses of deep subsurface communities. The latter analyses allow us to identify the active members of deep subsurface communities (in contrast to DNA-based assays that combine signatures of active, inactive and dead prokaryotes).

With our 16S rRNA gene studies, we finished and published the molecular community analysis for archaea in organic-poor deep subseafloor sediments of the Peru Basin (Sørensen et al. 2004) and the equatorial Pacific (Lauer et al. 2004). These environments are model systems for abyssal basins with very low sedimentation of organic matter. Such organic-poor sediments had not been studied by 16S rDNA sequencing before. Sequencing results for the equatorial Pacific site indicated an archaeal community that was similar to most other cold sediment communities. The Peru Basin site is marked by much lower activity and lower biomass than previously studied sites. Its archaeal community exhibits considerable overlap with hydrothermal vent communities, suggesting transport and dispersal of vent archaea either through surface or subsurface environments (Sørensen et al. 2004).

We are presently extracting, reverse-transcribing and PCR-amplifying rRNA from deep sediments at several Peru Margin sites (ODP Sites 1227, 1228, 1229, 1230). We are cloning and sequencing these samples in collaboration with members of the MBL team. Our goal is to identify the active microbial components of these deep subsurface sediments, and to differentiate them from potentially inactive or dead cells that harbor DNA, but no rRNA (Lever and Teske, 2005).

By 16S rDNA and Rubisco gene sequencing, w e also analyzed formation fluids from a sealed borehole (ODP Site 896A) (Harris and Teske, 2005). Our data indicate that the inorganic electron donors in the formation fluid provide energy for a diverse, partially chemolithotrophic bacterial community.

Team members at WHOI principally focus on organic biogeochemical and isotopic signatures of life in subsurface environments. During year 7, our WHOI investigators continued their research on (1) compound-specific analyses of intact polar lipids (IPLs) in deeply buried sediments of the Peru Margin, (2) the construction of a taxonomic database of IPL distributions in prokaryotes that encompasses now more than 100 cultures, (3) the study of biological formation of ethane and propane in subsurface sediments, (4) and peridotite-based hydrothermal ecosystems with abundant chemolithoautotrophic prokaryotes.

For our taxonomic database, we have systematically studied more than 100 cultures of biogeochemically and astrobiologically relevant prokaryotes from all major phyla of Archaea and Bacteria. Major distinctive features examined are structure and distribution of polar headgroups, bond type between alkyl chains and glycerol, and the lengths of and number of rings and/or bonds in alkyl chains. In combination, these structural features are distinctive on the mono-specific level and yield taxonomically valuable chemical fingerprints. Because environmental communities are typically complex, their lipid signatures represent ecosystem fingerprints, which are relevant to both environmental and life sciences. As a result of these studies, analysis of intact membrane lipids is now taxonomically far more specific than previously used lipid techniques.

From the distribution and isotopic systematics of methane, ethane and propane in marine sediments we have identified evidence that ethane and propane are produced biologically at low temperatures (Hinrichs et al., in review). To undertake these studies, we decreased the detection limit for isotopic analysis of acetate in marine pore waters by almost two orders of magnitude. To test our hypothesis of low-temperature microbial ethanogenesis and propanogenesis, we are currently searching for potential abiotic pathways of hydrocarbon formation that could be catalyzed by sedimentary components such as minerals. Ongoing experiments, in which we treat sediments with D₂O and NaOH, have so far not yielded D-labeled hydrocarbons. This is consistent with our proposed biological mechanism.

Members of our team at URI principally focus on studies of deep subsurface activities, ecosystem structure and biogeochemical fluxes. To further these studies, we continue to develop and apply techniques for quantification of deep subsurface activities and ecosystem structure.

A recent technical advance of our team is our development and utilization of an extremely sensitive tritium-based method for the detection of life based on the measurement of hydrogenase activity. This work has been co-funded by the NAI and the US National Science Foundation (NSF). We have submitted the protocol for publication (Soffientino et al., in review). We are currently refining this technique and applying it to samples from deep subseafloor sediments and (in collaboration with IPTAI team members) deep gold mines (Figure 2).

Figure 2. Hydrogenase activity of samples from 1130-m deep boreholes at Lupin gold mine (Canada). Microbes from sample 1130-64 exhibited significant activity (Error bars are 95% confidence intervals; asterisk denotes significant difference from killed control). The samples were taken by URI team member Bruno Soffientino on an expedition led by T.C. Onstott of the IPTAI team. Data and figure courtesy of B. Soffientino.

Another recent technical advance is a new method for determining in situ concentrations of hydrocarbons (e.g., methane, ethane, propane) from samples that depressurize during their recovery from great depth (Spivack et al., in review). Our method is based on direct gas chromatographic analysis of the methane/argon/nitrogen ratios of gas voids. It does not involve the expensive and time-consuming technology of existing methods. Another current technological focus is development of techniques for isolating intact cells from sediments. Future application of the latter techniques will greatly improve enumeration of subsurface cells, molecular studies of populational composition, and direct study of individual organisms.

In the last year, we also showed that a deep subseafloor ecosystem is a thermodynamic homeostat where sulfate reduction, iron reduction, methanogenesis and possibly methanotrophy are mutually dependent and produce biologically useful energy throughout a sediment column deposited over millions of years (Wang et al., 2005, in preparation).

In all settings, we are combining our microbiological and biogeochemical studies for a comprehensive picture of the microbial community and its interaction with its environment. For example, biogeochemical data are used to define ecological niches for each microbial community defined by our DNA and RNA results. The synthesis of these different lines of research is nicely illustrated by our studies of archaeal communities in deeply buried sulfate-methane transition zones, which URI team members at WHOI and UNC are undertaking with PSU team member C. House. Extractable archaeal 16S rRNA indicates that these communities are dominated by two novel lineages that are distinct from archaeal methanotrophs in surface sediments. We have reconstructed the carbon flow in the ecosystem from isotopic compositions of various sedimentary carbon pools, including live archaeal biomass (as whole cells and intact lipids). Our data indicate that the archaea are heterotrophic and, unlike near-surface methanotrophs, do not assimilate carbon from methane. Slow decomposition of fossil organic carbon on geological time scales is likely providing the organic compounds assimilated by the archaeal community. In short, sulfate-methane transition zones in deeply buried marine sediments are uniformly dominated by heterotrophic archaea that appear to ferment and assimilate fossil organic carbon. This collaborative work is currently being prepared for submission to a scientific journal with an appropriately broad readership (Biddle, Lipp et al., in preparation).

To help strengthen the infrastructure of the astrobiology community, members of the URI team sought and received significant support from the NSF and the University of Rhode Island for a containerized Subsurface Life field laboratory. We are now building the laboratory. It will be fully outfitted for: (1) microbiological and biogeochemical sampling of diverse subsurface environments, (2) on-site analyses of biologically significant transient properties, and (3) on-site analyses of chemical and physical properties that will be used to guide microbiological and biogeochemical sampling strategies. The proposed laboratory will be made internationally accessible to support studies of subsurface life by scientists from institutions throughout the world.

Graduate, undergraduate and post-doctoral education and research are integral parts of the URI team’s mission. During the Year-7 report interval, our active research team included five post-doctoral scholars (Jens Kallmeyer, Antje Lauer, Ketil Sørensen, Bruno Soffientino and Sturt), four graduate students (Lever, Lloyd, Guizhi Wang and Carly Blair) and nine undergraduate students. To introduce astrobiology to young scientists with particularly strong potential, each year the URI team provides internationally competed summer research fellowships for upper-class (junior or senior) students to work with URI investigators on projects of astrobiological significance. All of these individuals played vital roles in our ongoing program.

To build a deeper institutional base of astrobiology awareness among undergraduate and graduate scientists and engineers, our team members continue to expand the academic programs of our home institutions by integrating astrobiological themes into existing courses, and by offering new astrobiology courses. In Year 7, we offered a Freshman Honors seminar at URI titled “Life in the Universe” (Spring, 2005). This course will be a regular course offering in future years. To introduce our work and the field of astrobiology to the broader public, URI team investigators gave public presentations and lectures at a variety of universities and other venues (including a lecture on subsurface life and planetary exploration by Steven D’Hondt at the Annual Meeting of the New England Association of Chemistry Teachers). To disseminate our work and its relevance more broadly, in the Year-7 interval the team continues to participate in collaborative outreach with Rhode Island Space Grant and with the Rhode Island GEMS-NET program. We also continue to maintain our website on URI astrobiology efforts in research and education (http://www.gso.uri.edu/astrobiology).

In short, research, education and outreach by the NAI URI Team and its collaborators continue to steadily advance knowledge and awareness of life deep beneath Earth’s surface, its role in Earth’s surface processes, and its relevance to the search for life on other planets.