Magnetotactic Bacteria And Exobiology

The chemical and microscopic analyses of Martian meteorite ALH 84001 caused a great surprise to the scientific community because it contains magnetite crystals whose origins, either biotic or abiotic, remain an open question (Barber and Scott, 2002; Buseck et al., 2001; Taylor et al., 2001; Thomas-Keprta et al., 2002; McKay et al., 2003; Weiss et al., 2004). McKay et al. (2003) also suggested that more studies of ALH84001, extensive laboratory simulations of non-biological magnetite formation, as well as additional studies of MTB on Earth are required to further address this question.

3. Potential Role of Magnetotactic Bacteria for Terraformation

Terraforming as defined by Fogg (1995) is "a process of planetary engineering, specifically directed at enhancing the capacity of an extra-terrestrial planetary environment to support life". The initial stage in terraformation is ecopoiesis, "the fabrication of an uncontained anaerobic biosphere on the surface of a sterile planet" (Fogg, 1995) while the ultimate stage for terraformation would be to create an uncontained planetary biosphere emulating all the functions of the biosphere of the Earth (Fogg, 1995, 1998).

For now, Mars is considered one of the best candidates for terraformation. However, at present, the Martian surface environment is effectively sterilizing for all forms of terrestrial organisms (Hiscox, 2000b), being very different from the picture concerning the ultimate goal of terraformation.

Mars is a telluric planet, smaller than Earth which receives 43% of the sunlight that reaches Earth. The Martian day is almost equal to a terrestrial one and the gravity is 38% of the terrestrial one. The present-day magnetic field is about 1/800th of Earth's and it is not uniform distributed. The atmosphere has a very low pressure (0.8% of the terrestrial one) and consists mostly of carbon dioxide (95%). Temperatures vary between -75°C and +25°C, with an average of -60°C. Chemical analyses of Martian rocks and of Martian meteorites show that they have almost the same composition as the terrestrial ones, with one exception: iron is much more abundant on Mars than on Earth. The primary rocks contain ferrous iron (Fe2+) whereas the regolith (the upper layer) contains ferric iron (Fe3+) in the form of hematite, jarosite and goethite.

The properties of the nowadays Martian environment: low atmospheric pressure and temperature, extremely low molecular oxygen concentration and thus the absence of ozone layer, the instability of liquid water, strong UV irradiation, and the absence of organic matter (Hiscox, 2000a, b; McKay et al., 1991; Haynes and McKay, 1992; Birch, 1992; Zubrin and Wagner, 1996; McKay and Marinova, 2001) would preclude the survival and growth of terrestrial organisms. In order to allow ecopoiesis to occur (Fogg, 1995; Nussinov et al., 1994; Zubrin and Wagner, 1996; Zubrin and McKay, 1997; Gerstell et al., 2001; Badescu, 2005) the following modifications are needed (Fogg, 1998):

1. Mean global surface temperature must be increased by 60 K.

2. The mass of the atmosphere must be increased and atmospheric composition must be altered to increase its molecular oxygen and nitrogen fractions.

3. Liquid water must be made available.

4. The surface UV and cosmic ray flux must be substantially reduced.

These tasks could be achieved by planetary engineering, the application of technology for the purpose of influencing the global properties of a planet (Fogg, 1995). According to the most important papers in the field, Mars' planetary engineering would be achieved by releasing a certain quantity of gaseous carbon dioxide from the planet's polar caps, permafrost or regolith, in order to trigger a runaway greenhouse effect that would dramatically change the Martian environment. This planetary engineering can be done by various means: the use of orbital mirrors or nuclear explosions and/or by decreasing the planetary albedo etc. (Fogg, 1995; Nussinov et al., 1994; Zubrin and Wagner, 1996; Zubrin and McKay, 1997; Gerstell et al., 2001; Badescu, 2005). After successful planetary engineering, Mars would have a dense CO2 atmosphere, acceptable temperatures, an ozone layer and a hydrographic network with rivers, lakes, seas, and even a boreal ocean. During the first phases of terraformation, the Martian biosphere would be dominated by photoautotrophic microorganisms (Haynes and McKay,

1992) which are able to use solar light to synthesize organic compounds from carbon dioxide. There are already proposed cyanobacteria such us Chroococcidiopsis which is capable of surviving in a large variety of extreme conditions: exceptional aridity, salinity, high and low temperature (Friedmann and Ocampo-Friedmann, 1995). On Earth, Chroococcidiopsis is particularly common in regions with desert pavement morphology, living beneath translucent pebbles which act both as a moisture trap and UV shield. According to the same group (Friedmann et al.,

1993) another candidate is Matteia, the desiccation-resistant cyanobacterium that can dissolve carbonate rock. Furthermore, Matteia has the ability to fix atmospheric nitrogen when nitrogen compounds are not available from the surrounding environment. It has been suggested that this organism might be used to liberate carbon dioxide on Mars as part of a biogeochemical carbon cycle.

In the following, we shortly review the metabolic versatility of MTB and we put forward the idea that MTB could participate to the process of terraformation together with photosynthetic and/or extremophillic microorganisms, which have been already proposed for this task (Nienow et al., 1988; Friedmann et al., 1993; Nussinov et al., 1994; Haynes and McKay, 1992; Hiscox, 2000a, b).

3.1. CARBON METABOLISM

The majority of known MTB use organic acids (succinate, pyruvate, lactate etc.) as carbon source (Blakemore, 1982; Mann et al., 1990; Heyen and Schüler, 2003; Moisescu et al., 2005; Smith et al., 2006) being able to grow on complex or rather simple media (Blakemore, 1982; Mann et al., 1990; Smith et al., 2006).

The ability of some MTB to use carbon dioxide as the sole carbon source for the synthesis of cell components and multiplication could be important for terra-formation. The recent discoveries concerning the biochemical and genetic details of their autotrophy show that MV-1 and MV-2 are chemolitoautotrophs that use the Calvin-Benson-Bassham pathway (Bazylinski et al., 2004) whereas the strain MC-1 seems to fix carbon dioxide via a reverse Krebs cycle (Williams et al., 2006). The growth of MV-1 and MV-2 strains with S2O32- and radiolabeled 14C bicarbonate ion (HCO3-) showed that cell carbon was derived from HCO3-/CO2. Apart from the importance of these findings in understanding carbon metabolism in MTB, they could also reveal a possible role for MTB in the process of terraformation. Chemolitoautotrophy enables MTB to fix carbon dioxide in the dark using the energy released through the oxidation of inorganic chemicals such as thiosulfate, making them interesting candidates even for the early stages of terraformation.

3.2. OXYGEN METABOLISM

MTB behave as typical microaerophilic organisms, preferring to grow at low oxygen levels, but their respiratory metabolism is one of the less studied metabolic processes. Bazylinski and Blakemore (1983) found that M. magnetotacticum, the first MTB isolated and grown in pure culture, can grow either in aerobic or micro-aerobic conditions, but not under strict anaerobic conditions, with nitrate as final respiratory electron acceptor. Tamegai et al. (1993) found a membrane bound novel ccb-type cytochrome c oxidase that seems to function as the terminal oxidase in microaerobic respiratory chain. In addition, the same group (Yamazaki et al., 1995) found a cytochrome cd1-type nitrate reductase which has also Fe(II) nitrite oxidoreductase activity. It was proposed that it could function as an Fe(II) oxidizing enzyme for magnetite synthesis under anaerobic conditions (Fukumori et al., 1997). Short and Blakemore (1986) showed that magnetotactic cells of M. magne-totacticum strain MS1 extruded protons when ferric iron is added to an anaerobic suspension of cells. The same treatment applied to nonmagnetic cells (M. magne-totacticum strain ANM-1A) does not result in the extrusion of protons. The authors concluded that the results are consistent with iron reduction as a terminal site in the electron transport chain (Short and Blakemore, 1986). Guerin and Blakemore (1992) further showed that iron respiration in this strain can sustain bacterial growth and multiplication as well. Thus is seems appropriate to think that this strain, as well as other MTB, could use three terminal respiratory electron acceptors: oxygen, nitrate and iron which could be an advantage in real life on other planets as it is on Earth.

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