Multi-technique Characterization Of Iron Reduction By An Antarctic Shewanella: An Analog System For Putative Martian Biosignature Identification
Microbes from terrestrial extreme environments enable testing of biosignature production in conditions relevant to astrobiological targets.
Mars, which was likely more conducive to life during early warmer and wetter epochs, has inspired missions that search for signs of early life in the surficial rock record, including mineral or organic biosignatures. Microbial iron reduction is a common and ancient metabolism that may have also operated on other rocky celestial bodies.
To investigate biosignature production during iron reduction, a Shewanella sp. (strain BF02_Schw) isolated from a subglacial discharge known as Blood Falls, Antarctica, was incubated with the electron acceptor ferrihydrite (Fh).
Identification of mineralogy of Live FLT treatments following 2 weeks (a–c) and 8 weeks of incubation (d–f) by TEM and SAED. (a) Two-week incubations feature peaks consistent with both magnetite and Fh, with minor peaks associated with goethite, as seen in both (b) TEM images and (c) SAED diffraction patterns. (d) Eight-week incubations show an increase in features consistent with goethite, in addition to the presence of magnetite and the appearance of peaks consistent with halite formation, which are consistent with both (e) TEM images and (f) SAED patterns. — Appl Environ Microbiol via PubMed
Biosignatures associated with Fh reduction were identified using a suite of techniques currently utilized or proposed for Mars missions, including X-ray diffraction and infrared, Mössbauer, and Raman spectroscopy. The biotic origin of features was validated by transcriptional changes observed between treatments with and without Fh and comparison to killed controls. In live treatments, Fh was reduced to magnetite and goethite, both detected in Martian lacustrine basins.
Several soluble and volatile metabolites were also detected, including riboflavin and dimethyl sulfide (DMS), which could be astrobiological indicators of active microbial processes. While none of the identified biosignatures individually would serve as definitive proof of life (past or present), detecting concomitant features associated with known terrestrial biotic processes would provide compelling rationale for more targeted life detection missions.
Terrestrial extremophiles can support the exploration of astrobiologically relevant microbial processes, validation of life detection instrumentation, and potentially the discovery of new biomarkers.
IMPORTANCE
Culture-based experiments with terrestrial extremophiles can elucidate biosignatures that may be analogous to those produced under extraterrestrial conditions, and thus inform sampling and technology strategies for future missions.
Here, we demonstrate the production of several biosignatures under iron-reducing conditions by Shewanella sp. BF02_Schw, originally isolated from an Antarctic analog feature. These biosignatures could be detectable using flight-ready instrumentation. Growth experiments with terrestrial extremophiles can identify biosignatures measurable by current methodologies and inform the development and optimization of techniques for detecting extant or extinct life on other worlds.
Multi-technique characterization of iron reduction by an Antarctic Shewanella: an analog system for putative Martian biosignature identification, Appl Environ Microbiol via PubMed (open access)
Astrobiology, Antarctica,
