Sulfur Metabolism

MTB would also take part in the sulfur cycle. In nature, sulfur is mainly found under three forms: compounds of the sulfate ion (SO42-), compounds of the sulfide ion (S2-) and elemental sulfur (S0). Sulfates are very abundant on Mars and some MTB can reduce the sulfate ion to sulfide. This biochemical process would form sulfydric acid and sulfides. Other bacterial species, such as strain MC-1, can oxidize sulfides and thiosulfates and producing elemental sulfur. The sulfide ion can be used by anoxygenic photosynthetic bacteria, including facultative anoxygenic cyanobacteria, as the electron source for cell metabolism enabling them to reduce carbon dioxide during the synthesis of organic compounds.

4. Conclusions and Future Prospects

In the new environment created by ecopoiesis, allowing autotrophic microorganisms and/or extremophilic ones to growth and multiply, MTB could play roles in carbon, oxygen, nitrogen, iron and sulfur cycles on Mars or on other planets (Fig. 1). MTB have several particularities that argue for their potential in the terraforming process, the following being the most important:

1. The ability of some MTB to fix carbon dioxide in the dark using the energy released through the oxidation of inorganic chemicals such as thiosulfate;

trophic chains trophic chains

Figure 1. The importance of magnetotactic bacteria for terraformation (for details see text).

trophic chains trophic chains

Figure 1. The importance of magnetotactic bacteria for terraformation (for details see text).

these chemolitoautotrophic MTB could be a primary source of organic carbon once molecular oxygen, even in limited concentrations, is available.

2. The ability to carry out aerobic or anaerobic respiration with either nitrate or ferric iron. The use of nitrate as the terminal electron acceptor in anaerobic respiration (nitrate respiration) by some MTB could enable them to work together with other nitrate respiring bacteria during terraformation of Mars and other planets. Nitrate respiration should be very intense during the first phases of terraforma-tion, until the atmospheric oxygen level would increase, inhibiting the process at aerobic sites and forcing nitrate-respiring bacteria to withdraw to anaerobic sites.

3. MTB together with other types of microorganisms could contribute to a nitrogen cycle on Mars, carrying out two important tasks: a constant percentage of N2 in the atmosphere (by denitrification) and the availability of this macro element to ecosystems (by nitrogen fixation).

4. MTB could consume ferric iron which, at high concentrations, is toxic for living organisms. At a neutral pH, the solubility of ferric iron is very low, but for each pH unit less, its solubility increases 1,000 times. During the first stages of terraformation of Mars, due to the CO2 atmosphere, Martian waters would be acid and ferric iron would dissolve causing problems to living cells. MTB could have a contribution in solving these problems by fixing the iron in the form of solid magnetite or greigite.

5. Magneto-aerotaxis, as well as other magnetic assisted taxies, could constitute specific advantages of MTB in their navigation toward optimum growth conditions during the process of terraformation of planets with a global magnetic field similar to that of Earth. MTB could keep this advantage even on Mars, in the regions having a local magnetism of 100 to 600 nT (http://mgs-mager.gsfc.nasa. gov; In those regions containing large iron deposits in the crust, one can develop microcosms where MTB could use magneto-aerotaxis or other magnetic assisted taxies to reach the appropriate concentration of nutrients. The experiments concerning microcosms are important for terraformation as it seems rational that microcosms could be used to start up ecosystems on Mars or others planets, once ecopoiesis is established.

The improvement of our knowledge concerning the biology of MTB, including their relationship with biotic and abiotic factors, is needed for the use of MTB in the terraformation of Mars or other planets. Furthermore, genetic modification of MTB could increase their potential for terraformation by improving their relationship with autotrophic and extremophilic microorganisms as well as making them more robust to face adverse physical and chemical conditions.

5. Acknowledgments

Thanks are due to the both referees whose professional criticism and valuable suggestions helped the authors to improve the manuscript.

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Biodata of Charles H. Lineweaver, author of "Paleontological Tests: Human-Like Intelligence Is Not a Convergent Feature of Evolution"

Dr. Charles H. Lineweaver is currently an Associate Professor at the Planetary Science Institute in the Research School of Astronomy and Astrophysics and the Research School of Earth Sciences at the Australian National University, Canberra, Australia. He obtained his Ph.D. from the University of California at Berkeley in 1994 and continued his studies and research at Strasbourg Observatory and the University of New South Wales. Dr. Lineweaver's scientific interests are in the areas of: planetology, cosmology and astrobiology.

E-mail: [email protected]

Dr. Charles H. Lineweaver
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