As I emphasized in my introduction, the rigors of the Moon, or .other space environments (asteroids, Mars, Phobos, Deimos, a station orbiting the Earth) are inimical to terrestrial life forms, including, of course, microorganisms. While it would be a far simpler matter to provide, on the Moon for example, conditions conducive to microbial existence than those conducive to human life, no ore could be beneficiated by bacteria there without the provision of a gas-tight container affording certain minimal conditions. No doubt the partial pressures of 02, N2, and CO2 could be held to levels substantially lower than those found on the Earth - how much lower would have to be determined experimentally. The water supply would need to be adequate and continuous. Full radiation protection would be necessary, and temperature fluctuations would need to be minimized. It might well be best to select thermophilic bacteria for this endeavor.
Once a biobeneficiation reactor was constructed and all support systems were activated, it would be inoculated with appropriate strains of bacteria. More than likely, lyophilized (freeze-dried) cultures, probably of genetically engineered strains, would be reconstituted onsite in an aqueous solution, containing a mixture of nutrients brought from Earth as a dehydrated powder. The culture would be added to the moistened ore bed once the operator was satisfied that the cells were growing within the culture vessel. Initially, a leaching solution of dilute sulfuric acid would be added. The enc,losure would have to be tight enough to retain all water vapors as well as the atmospheric gases.
As the size of the operation increased by expanding the size of the incubator, more water would need to be added. Unless sufficient sulfur or reduced sulfur in the ore was available for biological oxidation, additional sulfuric acid would be required. As the bacteria became established (as measured by the growth of a subculture or by microscopic examination of samples from the reactor or by chemical determination of the ratio of the concentrations of Fe + + + and Fe + +), further additions of the culture would become unnecessary. Before the biological operation was established, chemical reductive processing of oxide ores (e.g., ilmenite) would need to be functioning well to provide the necessary water and oxygen.
One problem with the foregoing scenario is that some of the minerals or elements in the ore might be toxic to the bacteria. Studies conducted more than 15 years ago revealed that lunar fines or their extracts inhibited as well as stimulated or proved innocuous to a variety of microorganisms (Silverman, Munoz, and Oyama 1971; Taylor et al. 1970; Taylor et al. 1971; Taylor and Wooley 1973). Silverman and colleagues found that, while the lunar substrate stimulated pigment production by the bacterium Pseudomonas aeruginosa, it inhibited pigment production by the bacterium Serratia marcescens. Others (Walkinshaw et al. 1970, Walkinshaw and Johnson 1971) found that lunar soil enhanced chlorophyll production and early development of ferns, bryophytes, and a number of seed plants. In keeping with these pioneering studies of the interplay between lunar materials and living organisms, I undertook to make a preliminary study (Johansson 1984) of the effects of leachates of lunar fines on the growth of the common colon bacillus, Escherichia coli, in a glucose medium with minimal mineral salts. I found that, depending on its concentration, the leachate either enhanced or inhibited the growth of the bacteria. The specific elements or minerals responsible for the various biological responses to the lunar materials have not been identified.
Prior terrestrial research would be necessary to provide guidance on dealing with a toxicity problem, if it could be identified in advance by appropriate tests of lunar materials. Means of dealing with such a problem could include the addition of hydrogen sulfide, chelators, or certain mineral salts known to block specific toxic ions. There is a vast literature on handling toxicity, but research specifically applied to lunar or asteroidal materials is needed.
Another biological approach to the problem of recovery of certain metals from lunar ores might be the application of metal-binding agents produced, at least initially, on Earth. Candidate substances include ionophores and metallothionein.
The introduction of the micro- organisms needed for the bioprocessing of lunar ores would probably not be an early event in the establishment of an outpost on the Moon. If a human habitation with recycling (of water, oxygen, carbon dioxide, nitrogen, sulfur, and other essential nutrients) was established, the cultures needed in the mining operation would be useful as part of the microflora of that ecosystem. The thiobacilli and other bacteria involved in the leaching of ores are important in the cycles of sulfur, nitrogen (some fix N2), carbon, and oxygen on Earth (see fig. 32).
One concern is the availability in the lunar soil of the sulfur needed for the ore-beneficiating bacteria to gain energy and by the same process to produce the sulfate for the acid leaching process. In this regard, carbonaceous chondritic asteroids might yield material more suitable for biological ore beneficiation than would the Moon. Of course, sulfur from the Earth could be added to the beneficiation enclosure on the Moon. A fringe benefit of using asteroidal materia! would be its content of organic matter, which desirable heterotrophic bacteria might possibly utilize for their carbon needs.
Another question that needs to be answered is whether the low level of nitrogen in the lunar regolith (Gibson 1975) is sufficient to enable significant bacterial colonization. It may be necessary to provide, at the onset, nitrogen in the form of nitrate. (Ammonium nitrogen would work but some of it would be oxidized to nitrate, thus imposing a demand on atmospheric oxygen.) The amount of phosphorus may also not be adequate for bacterial life on the Moon. These bacterial nutritional problems would also apply to the development of an ecosystem supporting human life.
In summary, bioprocessing using bacteria in closed reactors may be a viable option for the recovery of metals from the lunar regolith. Obviously, considerable research must be done to define the process, specify the appropriate bacteria, determine the necessary conditions and limitations, and evaluate the overall feasibility.
Table of Contents
WebWork: Al Globus, Bryan Yager, and Tugrul Sezen