|dc.contributor.author||Nixon, Sophie Louise||
|dc.description.abstract||The search for life beyond Earth is the driving force behind several future missions
to Mars. An essential task in the lead-up to these missions is a critical assessment of the
habitability for, and feasibility of, life. However, little research has been conducted on this
issue, and our understanding of the plausibility for life on Mars remains unconstrained.
Owing to the anoxic and iron-rich nature of Mars, microbial iron reduction (MIR) represents
a compelling candidate metabolism to operate in the Martian subsurface, past and present.
The objectives of this thesis are to address the feasibility of MIR on Mars by i) better
defining the habitability of MIR on Earth, and ii) assessing the range and availability of
organic electron donors in the subsurface of Earth and Mars.
Samples collected from Mars-relevant environments on Earth were used to initiate
MIR enrichment cultures at 4°C, 15°C and 30°C. Results indicate MIR is widespread in
riverbed and subglacial sediments but not sediments from desert or recent volcanic plains.
The iron-reducing microorganisms in subglacial enrichments are at least psychrotolerant and
in some cases psychrophilc. Culture-independent methods highlighted the changes in
diversity between temperature conditions for subglacial sediments, and indicated that
members of the prolific MIR Geobacteraceae family are common. The genera Geobacter
and Desulfosporosinus are responsible for MIR in the majority of enrichments. Long-term
anoxia and the availability of redox constituents are the major factors controlling MIR in
A MIR enrichment culture was unable to use shales and kerogens as the sole source
of electron donors for MIR, despite the presence of known electron donors. Furthermore,
MIR was inhibited by the presence of certain kerogens. The causes of inhibition are
unknown, and are likely to be a combination of chemical and physical factors.
Experiments were conducted to assess the ability of three pure strains and a MIR
enrichment to use non-proteinogenic amino acids common to carbonaceous meteorites as
electron donors for MIR. Results demonstrate that γ-aminobutyric acid served as an electron
donor for the enrichment culture, but no other amino acids supported MIR by this or other
iron-reducing cultures. The D-form of chiral amino acids was found to exert a strong
inhibitory effect, which decreased in line with concentration. Theoretical calculations using
published meteoritic accretion rates onto the surface of Mars indicate that the build up
inhibitory amino acids may place important constrains on habitability over geologic time
Contamination of a pure strain of Geobacter metallireducens with a strain of
Clostridium revealed a syntrophic relationship between these microorganisms. Anaerobic
heterotrophs are likely to play an important role in maintaining an available supply of
electron donors for MIR and similar chemoorganic metabolisms operating in the subsurface.
This research indicates that MIR remains a feasible metabolism to operate on Mars providing
a readily available redox couple is present. However, given the observed inhibition in the
presence of bulk carbonaceous material and certain amino acids found in meteorites, the use
of extraterrestrial carbonaceous material in the Martian subsurface for microbial iron
reduction is questionable, and should be the focus of future research||en
|dc.contributor.sponsor||Science and Technology Facilities Council (STFC)||en
|dc.publisher||The University of Edinburgh||en
|dc.relation.hasversion||Nixon, S.L., Cockell, C.S. and Tranter, M., 2012. Limitations to a microbial iron cycle on Mars. Planetary and Space Sciences, 72, 116-128. http://dx.doi.org/10.1016/j.pss.2012.04.003||en
|dc.relation.hasversion||Nixon, S.L., Cousins, C.R. and Cockell, C.S., 2013. Plausible microbial metabolisms on Mars. Astronomy and Geophysics, 54(1), 1.13-1.16. http://dx.doi.org/10.1093/astrogeo/ats034||en
|dc.subject||microbial iron reduction||en
|dc.subject||life in extremes||en
|dc.title||Microbial iron reduction on Earth and Mars||en
|dc.type||Thesis or Dissertation||en
|dc.type.qualificationname||PhD Doctor of Philosophy||en