2013 Annual Science Report
Pennsylvania State University Reporting | SEP 2012 – AUG 2013
At the core of Astrobiology and at the forefront of NASA goals is the construction of a fundamental scientific knowledge base that enables the recognition of signatures of life on the early Earth, in extreme environments, and in extraterrestrial settings. PSARC has continued to pursue an interdisciplinary investigation of biosignatures at all scales, from individual cells to the composition of planetary atmospheres. Our pale blue dot provides the only known example of an inhabited planet. Its record of life extends billions of years into the past, and thus presents a rich variety of indicators of life. Over the past year, our research has included advances in the following four directions.
Developing New Biosignatures
Schopf and his collaborators continued to develop Raman imaging of microfossils as a tool for Astrobiology. In particular, they are working on anaerobic microbial sulfuretums of various ages including the ~775 Ma chert-permineralized Bambui and the ~1,800 Ma chert-permineralized Duck Creek microbiota.
PSARC is also developing Secondary Ion Mass Spectrometry (SIMS) as an approach for investigating microfossils. This year, we published 32 in situ carbon isotopic analyses of 15 individual Farrel Quartzite (FQ) specimens. The ca. 3 Ga FQ contains possible organic microfossils of unusual spindle-like morphology that are surprisingly large and complex, preserved along with spheroids. The spheroids and the spindle-like forms have a weighted mean δ13C value of –37‰, an isotopic composition that is quite consistent with a biogenic origin. Both the spheroids and the spindle-like structures are isotopically distinct from the background organic matter in the same thin section (weighted mean δ13C value of –33‰), which shows that the preserved microstructures are not pseudofossils formed from physical reprocessing of the bulk sedimentary organic material. When considered along with published morphological and chemical studies, these results indicate that the FQ microstructures are bona fide microfossils, and support the interpretation that the spindles were planktonic.
We also published an article reporting experiments conducted to determine (i) cellular trace metal concentrations of the hyperthermophilic Archaea Methanococcus jannaschii and Pyrococcus furiosus, and (ii) a first estimate of the metallome for these hyperthermophilic species via ICP-MS. The metal contents of these cells were compared to parallel experiments using the mesophilic bacterium Escherichia coli grown under aerobic and anaerobic conditions. Fe and Zn were typically the most abundant metals in cells. Metal concentrations for E. coli grown aerobically decreased in the order Fe > Zn > Cu > Mo > Ni > W > Co. In contrast, M. jannaschii and P. furiosus show almost the reverse pattern with elevated Ni, Co, and W concentrations. Of the three organisms, a biosignature is potentially demonstrated for the methanogen M. jannaschii that may, in part, be related to the metallome requirements of methanogenesis. The bioavailability of trace metals more than likely has varied through time. If hyperthermophiles are very ancient, then the trace metal patterns observed here may begin to provide some insights regarding Earth’s earliest cells and in turn, early Earth chemistry.
Finally, we are exploring the extent of DNA preservation in soil from various environments. We have collected soil sample for genetic analysis from sand, lake and sedimentary sites from three locations in the Arctic: the Pribilof Islands, Old Crow Flats, Yukon Territory, and the Klondike gold fields, Yukon Territory. The sites differ in sediment composition and in age. Pribilof is highly sandy and samples were taken from Holocene-age sites (less than 10ky old). Old Crow sites date to the Late Pleistocene and Holocene. Klondike sites span the Pleistocene, with the oldest sites ca 720,000 years old. We find that temperature is the best predictor of DNA survival; we are able to recover DNA from 700,000 year old sediment and from an associated horse metapodial. This is, to date, the oldest recovered DNA sequences produced. However, the fragments are very short and show a variety of chemical and physical damages, as expected for very old DNA. As a result of this work, we predict that DNA will not survive as a biomarker for longer than 1My, even in the most ideal environments.
Biosignatures in Relevant Microbial Ecosystems
PSARC investigates microbial life in some of Earth’s most NASA mission-relevant ecosystems. These environments have geochemistry, energy availability, or fossilization potential similar to that found on other bodies in our solar system. Combining our expertise in molecular biology, geochemistry, microbiology, and metagenomics, we propose to decipher the microbiology, fossilization processes, and recoverable biosignatures from these mission-relevant environments.
This year, the Orphan group continued work on methane derived authigenic carbonates by documenting abundant methanotrophic archaea/ SRB consortia within the interiors of authigenic carbonates from a variety of deep-sea habitats (at the seabed within active seeps and inactive areas as well as sediment hosted carbonate nodules). Stable isotope incubation experiments with 15NH4 and CH4 coupled with FISH-nanoSIMS analysis were also used to demonstrate active growth of methanotrophic consortia within the interiors of carbonate nodules. This research has been submitted for publication. PSARC Ph.D. (now postdoctoral researcher at Caltech) Katherine Dawson published a new paper documenting the anaerobic biodegradation of organic biosignature compounds pristane and phytane. PSARC Ph.D. Daniel Jones (now postdoctoral researcher at U. Minnesota) published a new paper that uses metagenomic data to show how sulfur oxidation in the deep subsurface environments may contribute to the formation of caves and the maintenence of deep subsurface microbial ecosystems. PSARC Ph.D. student Khadouja Harouaka published a new paper that represents some of the first available information about possible Ca isotope biosignatures. Lastly, the Macalady group published a paper showing how ecological models based on available energy resources can be used to predict the distribution of microbial populations in space and time.
Biosignatures in Ancient Rocks
Modern and ancient Earth environments are relatively accessible for detailed study compared with habitable exoplanet environments, and thus provide a crucial link between diverse planetary systems themselves and the interpretation of potential biosignatures that may be present on new habitable worlds. A major focus of our work is making new links between ancient and modern microbial ecosystems, therefore gaining a clearer picture of how life influenced planetary evolution on Earth and opening new insights into both the generation of biosignatures and the limits to life.
New modeling by Jim Kasting’s research group suggests that the no-cloud-feedback inner edge of the liquid water habitable zone around the Sun has is significantly further (around 0.99 astronomical units) out than previously estimated. We also have now showed that increases in atmospheric CO2 cannot cause a runaway greenhouse on Earth, no matter how much CO2 is added. Finally, through similar modeling efforts, we have found that early Mars could have been warmed above the freezing point of water by the greenhouse effect of a CO2-H2 atmosphere.
PSARC student Rybacki, working with advisor Lee Kump and collaborator Victor Melezhik (Norwegian Geological Survey) have sampled the FAR-DEEP core at high resolution through a stack of lava flows that were erupted during the Lomagundi carbon isotope anomaly, the largest in Earth history. These lavas are unusual in that they are highly oxidized, and we have evidence that the oxidation occurred contemporaneously with eruption.
Biosignatures in Extraterrestrial Settings
Mahadevan and collaborators in The Center for Exoplanets and Habitable Worlds continued work on spectrograph development, calibration and exoplanet spectroscopic surveys. Specifically, They woredk on the development of the APOGEE project, and the Habitable Planet Finder. Also, graduate students Matthew Route and Sarah Gettel working with Prof. Wolszczan completed their PhD projects on sub-stellar companions and massive planets around giant stars, respectively. Prof. Wright and collaborators in the Center for Exoplanets and Habitable Worlds continued ongoing observations of exoplanets and looking for new exoplanets, using radial velocity monitoring, and started a new project on early Solar System formation. Prof. Wright continued work on the Exoplanets catalog http://exoplanets.org/. Finally, Prof. Eric Ford joined the Center for Exoplanets and Habitable Worlds and PSARC in July 2013, bringing with him a research group of 3 graduate students from the University of Florida.