Rare earth elements (REEs, ‘lanthanides’) constitute a vital commodity for technological applications. Although these elements occur at trace levels in many minerals, they can comprise major constituents of low abundance phosphate, carbonate, silicate and oxide minerals, some of which form during granite weathering. REE-phosphate phases can be a source of phosphorus for essential biomolecules and certain REEs are required by some bacterial enzymes involved in the oxidation of methanol, an important compound in the global biogeochemical carbon cycle. The mechanisms that promote the dissolution of lanthanide phosphate minerals are largely unknown, but probably vary with the lanthanide phosphate mineralogy of weathered rock and soil. Here, we studied weathering of five I-type, three S-type and one A-type granite to determine the extent of weathering of primary REE- and/or P-bearing minerals apatite, allanite and monazite, and the formation of secondary REE/P-bearing minerals. We found evidence for greater mobilisation of REE and P in weathered I-type and A-type granites than in S-types, reflecting the higher solubility of apatite and allanite relative to monazite. Although monazite persisted in highly weathered S-type granites, some alteration was detected. Secondary REE/P-bearing minerals were not detected in two S-type profiles, while spherical secondary REE/P-bearing mineral aggregates were abundant throughout the third S-type profile. Secondary euhedral REE/P-bearing crystals were abundant even in the slightly weathered I-type and A-type granite material, yet they were not detected in the highly weathered material, indicating that these minerals had dissolved. Our findings indicate that mineralogy constrains substantially, but does not control completely, lanthanide availability as a function of degree of weathering. These results have implications for predicting REE and phosphate bioavailability in soils derived from granitic rock types and suggest that highly weathered I-type granites may provide inocula for bioleaching experiments.

In this paper, we describe the development of an International Space Station experiment, BioRock. The purpose of this experiment is to investigate biofilm formation and microbe–mineral interactions in space. The latter research has application in areas as diverse as regolith amelioration and extraterrestrial mining. We describe the design of a prototype biomining reactor for use in space experimentation and investigations on in situ Resource Use and we describe the results of pre-flight tests.

Bioleaching has been suggested as an alternative to traditional mining techniques in extraterrestrial environments because it does not require extensive infrastructure and bulky hardware. In situ bioleaching of silicate minerals, such as those found on the moon or Mars, has been proposed as a feasible alternative to traditional extraction techniques that require either extreme heat and/or substantial chemical treatment. In this study, we investigated the biotic and abiotic leaching of basaltic rocks (analogues to those found on the moon and Mars) and the mineral ilmenite (FeTiO3) in aqueous environments under acidic (pH ∼ 2.5) and circumneutral pH conditions. The biological leaching experiments were conducted using Acidithiobacillus ferrooxidans, an iron (Fe)-oxidizing bacteria, and Pseudomonas mendocina, an Fe-scavenging bacteria. We found that both strains were able to grow using the Fe(II) derived from the tested basaltic rocks and ilmenite. Although silica leaching rates were the same or slightly less in the bacterial systems with A. ferrooxidans than in the abiotic control systems, the extent of Fe, Al and Ti released (and re-precipitated in new solid phases) was actually greater in the biotic systems. This is likely because the Fe(II) leached from the basalt was immediately oxidized by A. ferrooxidans, and precipitated into Fe(III) phases which causes a change in the equilibrium of the system, i.e. Le Chatelier's principle. Iron(II) in the abiotic experiment was allowed to build up in solution which led to a decrease in its overall release rate. For example, the percentage of Fe, Al and Ti leached (dissolved + reactive mineral precipitates) from the Mars simulant in the A. ferrooxidans experimental system was 34, 41 and 13% of the total Fe, Al and Ti in the basalt, respectively, while the abiotic experimental system released totals of only 11, 25 and 2%. There was, however, no measurable difference in the amounts of Fe and Ti released from ilmenite in the experiments with A. ferrooxidans versus the abiotic controls. P. mendocina scavenged some Fe from the rock/mineral substrates, but the overall amount of leaching was small (<2% of total Fe in rocks) when compared with the acidophilic systems. Although the mineralogy of the tested basaltic rocks was roughly similar, the surface areas of the lunar and Mars simulants varied greatly and thus were possible factors in the overall amount of metals released. Overall, our results indicate that the presence of bacteria does not increase the overall silica leaching rates of basaltic rocks; however, the presence of A. ferrooxidans does lead to enhanced release of Fe, Al and Ti and subsequent sequestration of Fe (and other metals) in Fe(III)-precipitates.

Synthetic geomicrobiology is a potentially new branch of synthetic biology that seeks to achieve improvements in microbe–mineral interactions for practical applications. In this paper, laboratory and field data are provided on three geomicrobiology challenges in space: (1) soil formation from extraterrestrial regolith by biological rock weathering and/or the use of regolith as life support system feedstock, (2) biological extraction of economically important elements from rocks (biomining) and (3) biological solidification of surfaces and dust control on other planetary surfaces. The use of synthetic or engineered organisms in these three applications is discussed. These three examples are used to extract general common principles that might be applied to the design of organisms used in synthetic geomicrobiology.