Susan A. Welch

EFFECT OF MICROORGANISMS AND MICROBIAL METABOLITES ON APATITE DISSOLUTION

The dissolution rate of apatite was determined in batch reactors in organic acid solutions and in microbial cultures. Inoculum for the cultures was from biotite plus apatite crystals from a granite weathering profile in South Eastern Australia. In both the biotic and the abiotic experiments, etching of the apatite surface leads to the formation of elongated spires parallel to the c axis. Apatite dissolution rates in the inorganic, acetate, and oxalate solutions increase as pH decreases from approximately 10 -11 mol/m -2 · s -1 at initial pH 5.5 to 10 -7 mol/m -2 · s -1 at initial pH 2. Under mildly acidic to near neutral pH conditions, both oxalate and acetate increased apatite dissolution by up to an order of magnitude compared to the inorganic conditions. Acetate catalyzed the reaction by forming complexes with Ca, either in solution or at the mineral surfaces. Oxalate forms complexes with Ca as well, and can also affect reaction rates and stoichiometry by forming Ca-oxalate precipitates, thus affecting solution saturation states. In all abiotic experiments, net phosphate release to solution approaches zero even when solutions are apparently undersaturated by several orders of magnitude with respect to the solubility of an ideal fluoroapatite mineral. In the microbial experiments, two enrichment cultures increased both apatite and biotite dissolution by producing organic acids, primarily pyruvate, fermentation products, and oxalate, and by lowering bulk solution pH to between 3 and 5. However, the microorganisms were also able to increase phosphate release from apatite (by two orders of magnitude) without lowering bulk solution pH by producing pyruvate and other compounds.

Laboratory evidence for microbially mediated silicate mineral dissolution in nature

Abstract Bacteria may potentially enhance or inhibit silicate mineral dissolution in nature by a variety of mechanisms. In the laboratory, some microbial metabolites enhanced dissolution rates by a factor of ten above the expected proton-promoted rate by an additional ligand-promoted mechanism focussed principally at Al sites at the mineral surface. In investigations with bacteria, it was found that organic acids are produced in organic-rich/nutrient-poor cultures, resulting in increased mineral dissolution rates compared to abiotic controls. Alginate and poly-aspartate inhibited dissolution rates either by a reduction in surface reactivity or reactive surface area (or both). Bacteria may also influence dissolution rates by creating and maintaining microenvironments where metabolite concentrations are higher than in the bulk solution.

Microbial extracellular polysaccharides and plagioclase dissolution

Abstract Bytownite feldspar was dissolved in batch reactors in solutions of starch (glucose polymer), gum xanthan (glucose, mannose, glucuronic acid), pectin (poly-galacturonic acid), and four alginates (mannuronic and guluronic acid) with a range of molecular weights (low, medium, high and uncharacterized) to evaluate the effect of extracellular microbial polymers on mineral dissolution rates. Solutions were analyzed for dissolved Si and Al as an indicator of feldspar dissolution. At neutral pH, feldspar dissolution was inhibited by five of the acid polysaccharides, gum xanthan, pectin, alginate low, alginate medium, alginate high, compared to an organic-free control. An uncharacterized alginate substantially enhanced both Si and Al release from the feldspar. Starch, a neutral polysaccharide, had no apparent effect. Under mildly acidic conditions, initial pH ≈ 4, all of the polymers enhanced feldspar dissolution compared to the inorganic controls. Si release from feldspar in starch solution exceeded the control by a factor of three. Pectin and gum xanthan increased feldspar dissolution by a factor of 10, and the alginates enhanced feldspar dissolution by a factor of 50 to 100. Si and Al concentrations increased with time, even though solutions were supersaturated with respect to several possible secondary phases. Under acidic conditions, initial pH ≈ 3, below the pKa of the carboxylic acid groups, dissolution rates increased, but the relative increase due to the polysaccharides is lower, approximately a factor of two to ten. Microbial extracellular polymers play a complex role in mineral weathering. Polymers appear to inhibit dissolution under some conditions, possibly by irreversibly binding to the mineral surfaces. The extracellular polysaccharides can also enhance dissolution by providing protons and complexing with ions in solution.

FELDSPAR DISSOLUTION IN ACIDIC AND ORGANIC SOLUTIONS : COMPOSITIONAL AND PH DEPENDENCE OF DISSOLUTION RATE

The steady-state dissolution rates of plagioclase feldspars into inorganic acid solutions in a flow-through reactor increased with Al content of the mineral from 1.4 · 10−11 mol Si/m2/s for albite to 5.6 · 10−9 mol Si/m2/s for bytownite. A similar trend was observed for minerals dissolved in neutral solutions although the rates were lower. The results of these experiments are used to develop a simple empirical equation to describe the dissolution of tectosilicates (quartz + feldspars): RH=kHaH+nH where RH is the dissolution rate of tectosilicates in acid solution, aH+ is the activity of H+ ion, and kH and nH are dependent on the aluminum fraction in the tectosilicate framework [AlAl+Si]:logkH=−11.24+25.98*[AlAl+Si]2andnH=−0.052+4.23*[AlAl+Si]2. This model, with its strong dependence on Al fraction, suggests that tectosilicate dissolution in acid solution results primarily from attack at Al sites at the mineral surface. In acidic oxalate solutions the steady-state dissolution rates were, in some cases, up to a factor of 10 higher than dissolution rates in inorganic solutions at the same pH and appeared to have a similar dependence on pH and mineral composition, at least away from the extremes in aluminum fraction (quartz and bytownite). On the basis of the results of the experiments with acidic oxalate and previous experiments showing a linear dependence of feldspar dissolution rate on organic ligand concentration, an empirical expression for the ligand-promoted component of tectosilicate dissolution rates as measured by silica release (RL) is proposed: RL = (κHL [L] − kH)aH+n + ℝHL(Si) where the first term describes the effect of competitive proton and ligand attack at Al sites at the mineral surface leading to silica release to solution and RHL(Si) reflects the smaller rate of attack at Si sites (κHL is a factor depending on the ligand, [L] is the ligand concentration, kH and aH+ are as given above, and n describes the pH dependence of ligand- and proton-promoted dissolution and is taken to be equal to nH away from the extremes of aluminum fraction). The strong dependence of dissolution rate in acidic organic solutions on aluminum fraction indicates that both protons and ligands attack the mineral surface at the same, presumably Al, sites.

Enhanced dissolution of silicate minerals by bacteria at near-neutral pH

Previous studies have shown that various microorganisms can enhance the dissolution of silicate minerals at low (<5) or high (>8) pH. However, it was not known if they can have an effect at near-neutral pH. Almost half of 17 isolates examined in this study stimulated bytownite dissolution at near-neutral pH while in a resting state in buffered glucose. Most of the isolates found to stimulate dissolution also oxidized glucose to gluconic acid. More detailed analysis with one of these isolates suggested that this partial oxidation was the predominant, if not sole, mechanism of enhanced dissolution. Enhanced dissolution did not require direct contact between the dissolving mineral and the bacteria. Gluconate-promoted dissolution was also observed with other silicate minerals such as albite, quartz, and kaolinite.

Effect of Microbial and Other Naturally Occurring Polymers on Mineral Dissolution

Several naturally occurring polymers were tested for their effect on mineral dissolution. Polymers composed primarily of neutral sugars had no effect on dissolution, even at concentrations 1000 times greater than average dissolved organic carbon concentration in groundwater. In contrast, alginate, a polysaccharide composed of two uronic acids, inhibited dissolution by 80% at the highest concentration. A high‐molecular‐weight (26 kD) polyaspartate also inhibited dissolution, though a lower molecular weight (6 kD) polyaspartate had no effect. Solutions of fresh microbial extracellular polysaccharides (EPS) extracted from subsurface microbes increased the dissolution rate of feldspars, probably by forming complexes with framework ions in solution. However, EPS inhibited dissolution in experiments with both high‐ and low‐molecular‐weight microbial metabolites by irreversibly binding to mineral surfaces.

Microbial controls on phosphate and lanthanide distributions during granite weathering and soil formation

Abstract Both microbial and geochemical factors control the form, distribution, and abundance of lanthanides during weathering and soil formation in a profile in the Bemboka Granodiorite from southern New South Wales, Australia. During the initial stages of weathering, hydrous Ce-bearing lanthanide and lanthanide-aluminum phosphates (including rhabdophane and florencite) crystallize on etched apatite surfaces throughout the profile. However, their fate depends greatly on their location within the profile. In rocks weathered 5–6 m below the soil zone (the lower profile), secondary lanthanide phosphates persist long after all apatite has been dissolved. However, in rocks weathered within 2 m of the soil zone (the upper profile), secondary phosphates are dissolved and lanthanides other than Ce are removed. Bacteria and fungal hyphae are localized on secondary phosphate surfaces, suggesting that rhabdophane and florencite are solubilized in the upper profile due to organic complexation of dissolved lanthanides and/or uptake of phosphate by cells. In the soils (defined by loss of granitic texture), secondary phosphates are replaced by Ce-oxides. Lanthanides (other than Ce) removed in solution from the upper profile precipitate as Ce-poor phosphates in the lower profile when the solubility product is exceeded due to high dissolved phosphate concentrations in proximity to apatite. Thus, the upper profile is the source of lanthanides added to the lower profile. In both the lower and upper weathered granite, the degree to which weight-based Ce abundances increase with increasing weathering is consistent with Ce immobility on the centimeter scale. We attribute the very high weight-based Ce abundances (up to 12× concentrations in fresh rock) to extensive leaching and compaction during transformation of weathered rock to soil.

The effect of Fe-oxidizing bacteria on Fe-silicate mineral dissolution

Abstract Acidithiobacillus ferrooxidans are commonly present in acid mine drainage (AMD). A. ferrooxidans derive metabolic energy from oxidation of Fe2+ present in natural acid solutions and also may be able to utilize Fe2+ released by dissolution of silicate minerals during acid neutralization reactions. Natural and synthetic fayalites were reacted in solutions with initial pH values of 2.0, 3.0 and 4.0 in the presence of A. ferrooxidans and in abiotic solutions in order to determine whether these chemolithotrophic bacteria can be sustained by acid-promoted fayalite dissolution and to measure the impact of their metabolism on acid neutralization rates. The production of almost the maximum Fe3+ from the available Fe in solution in microbial experiments (compared to no production of Fe3+ in abiotic controls) confirms A. ferrooxidans metabolism. Furthermore, cell division was detected and the total cell numbers increased over the duration of experiments. Thus, over the pH range 2–4, fayalite dissolution can sustain growth of A. ferrooxidans. However, ferric iron released by A. ferrooxidans metabolism dramatically inhibited dissolution rates by 50–98% compared to the abiotic controls. Two sets of abiotic experiments were conducted to determine why microbial iron oxidation suppressed fayalite dissolution. Firstly, fayalite was dissolved at pH 2 in fully oxygenated and anoxic solutions. No significant difference was observed between rates in these experiments, as expected, due to extremely slow inorganic ferrous iron oxidation rates at pH 2. Experiments were also carried out to determine the effects of the concentrations of Fe2+, Mg2+ and Fe3+ on fayalite dissolution. Neither Fe2+ nor Mg2+ had an effect on the dissolution reaction. However, Fe3+, in the solution, inhibited both silica and iron release in the control, very similar to the biologically mediated fayalite dissolution reaction. Because ferric iron produced in microbial experiments was partitioned into nanocrystalline goethite (with very low Si) that was loosely associated with fayalite surfaces or coated the A. ferrooxidans cells, the decreased rates of accumulation of Fe and Si in solution cannot be attributed to diffusion inhibition by goethite or to precipitation of Fe–Si-rich minerals. The magnitude of the effect of Fe3+ addition (or enzymatic iron oxidation) on fayalite dissolution rates, especially at low extents of fayalite reaction, is most consistent with suppression of dissolution by interaction between Fe3+ and surface sites. These results suggest that microorganisms can significantly reduce the rate at which silicate hydrolysis reactions can neutralize acidic solutions in the environment.

Mineralogical biosignatures and the search for life on Mars.

If life ever existed, or still exists, on Mars, its record is likely to be found in minerals formed by, or in association with, microorganisms. An important concept regarding interpretation of the mineralogical record for evidence of life is that, broadly defined, life perturbs disequilibria that arise due to kinetic barriers and can impart unexpected structure to an abiotic system. Many features of minerals and mineral assemblages may serve as biosignatures even if life does not have a familiar terrestrial chemical basis. Biological impacts on minerals and mineral assemblages may be direct or indirect. Crystalline or amorphous biominerals, an important category of mineralogical biosignatures, precipitate under direct cellular control as part of the life cycle of the organism (shells, tests, phytoliths) or indirectly when cell surface layers provide sites for heterogeneous nucleation. Biominerals also form indirectly as by-products of metabolism due to changing mineral solubility. Mineralogical biosignatures include distinctive mineral surface structures or chemistry that arise when dissolution and/or crystal growth kinetics are influenced by metabolic by-products. Mineral assemblages themselves may be diagnostic of the prior activity of organisms where barriers to precipitation or dissolution of specific phases have been overcome. Critical to resolving the question of whether life exists, or existed, on Mars is knowing how to distinguish biologically induced structure and organization patterns from inorganic phenomena and inorganic self-organization. This task assumes special significance when it is acknowledged that the majority of, and perhaps the only, material to be returned from Mars will be mineralogical.

The effect of microbial glucose metabolism on bytownite feldspar dissolution rates between 5° and 35°C

Abstract The rate of Si release from dissolving bytownite feldspar in abiotic batch reactors increased as temperatures increased from 5° to 35°C. Metabolically inert subsurface bacteria (bacteria in solution with no organic substrate) had no apparent effect on dissolution rates over this temperature range. When glucose was added to the microbial cultures, the bacteria responded by producing gluconic acid, which catalyzed the dissolution reaction by both proton- and ligand-promoted mechanisms. The metabolic production, excretion, and consumption of gluconic acid in the course of glucose oxidation, and therefore, the degree of microbial enhancement of mineral dissolution, depend on temperature. There was little accumulation of gluconic acid and therefore, no significant enhancement of mineral dissolution rates at 35°C compared to the abiotic controls. At 20°C, gluconate accumulated in the experimental solutions only at the beginning of the experiment and led to a twofold increase in dissolved Si release compared to the controls, primarily by the ligand-promoted dissolution mechanism. There was significant accumulation of gluconic acid in the 5°C experiment, which is reflected in a significant reduction in pH, leading to 20-fold increase in Si release, primarily attributable to the proton-promoted dissolution mechanism. These results indicate that bacteria and microbial metabolism can affect mineral dissolution rates in organic-rich, nutrient-poor environments; the impact of microbial metabolism on aluminum silicate dissolution rates may be greater at lower rather than at higher temperatures due to the metabolic accumulation of dissolution-enhancing protons and ligands in solution.