Geochemical Niches of Iron-Oxidizing Acidophiles in Acidic Coal Mine Drainage

A legacy of coal mining in the Appalachians has provided a unique opportunity to study the ecological niches of iron-oxidizing microorganisms. Mine-impacted, anoxic groundwater with high dissolved-metal concentrations emerges at springs and seeps associated with iron oxide mounds and deposits. These deposits are colonized by iron-oxidizing microorganisms that in some cases efficiently remove most of the dissolved iron at low pH, making subsequent treatment of the polluted stream water less expensive. We used full-cycle rRNA methods to describe the composition of sediment communities at two geochemically similar acidic discharges, Upper and Lower Red Eyes in Somerset County, PA, USA. The dominant microorganisms at both discharges were acidophilic Gallionella-like organisms, “Ferrovum” spp., and Acidithiobacillus spp. Archaea and Leptospirillum spp. accounted for less than 2% of cells. The distribution of microorganisms at the two sites could be best explained by a combination of iron(II) concentration and pH. Populations of the Gallionella-like organisms were restricted to locations with pH>3 and iron(II) concentration of >4 mM, while Acidithiobacillus spp. were restricted to pH<3 and iron(II) concentration of <4 mM. Ferrovum spp. were present at low levels in most samples but dominated sediment communities at pH<3 and iron(II) concentration of >4 mM. Our findings offer a predictive framework that could prove useful for describing the distribution of microorganisms in acid mine drainage, based on readily accessible geochemical parameters

Thermodynamic Controls on the Kinetics of Microbial Low-pH Fe(II) Oxidation

Acid mine drainage (AMD) is a major worldwide environmental threat to surface and groundwater quality. Microbial low-pH Fe­(II) oxidation could be exploited for cost-effective AMD treatment; however, its use is limited because of uncertainties associated with its rate and ability to remove Fe from solution. We developed a thermodynamic-based framework to evaluate the kinetics of low-pH Fe­(II) oxidation. We measured the kinetics of low-pH Fe­(II) oxidation at five sites in the Appalachian Coal Basin in the US and three sites in the Iberian Pyrite Belt in Spain and found that the fastest rates of Fe­(II) oxidation occurred at the sites with the lowest pH values. Thermodynamic calculations showed that the Gibbs free energy of Fe­(II) oxidation (Δ<i>G</i>_oxidation) was also most negative at the sites with the lowest pH values. We then conducted two series of microbial Fe­(II) oxidation experiments in laboratory-scale chemostatic bioreactors operated through a series of pH values (2.1–4.2) and found the same relationships between Fe­(II) oxidation kinetics, Δ<i>G</i>_oxidation, and pH. Conditions that favored the fastest rates of Fe­(II) oxidation coincided with higher Fe­(III) solubility. The solubility of Fe­(III) minerals, thus plays an important role on Fe­(II) oxidation kinetics. Methods to incorporate microbial low-pH Fe­(II) oxidation into active and passive AMD treatment systems are discussed in the context of these findings. This study presents a simplified model that describes the relationship between free energy and microbial kinetics and should be broadly applicable to many biogeochemical systems

Photometric survey of binary near-Earth asteroids

International audiencePhotometric data on 17 binary near-Earth asteroids (15 of them are certain detections, two are probables) were analysed and characteristic properties of the near-Earth asteroid (NEA) binary population were inferred. We have found that binary systems with a secondary-to-primary mean diameter ratio D/D=>0.18 concentrate among NEAs smaller than 2 km in diameter; the abundance of such binaries decreases significantly among larger NEAs. Secondaries show an upper size limit of D=0.5-1 km. Systems with D/D20 h. The specific total angular momentum of most of the binary systems is similar to within ±20% and close to the angular momentum of a sphere with the same total mass and density, rotating at the disruption limit; this suggests that the binaries were created by mechanism(s) related to rotation near the critical limit and that they neither gained nor lost significant amounts of angular momentum during or since formation. A comparison with six small asynchronous binaries detected in the main belt of asteroids suggests that the population extends beyond the region of terrestrial planets, but with characteristics shifted to larger sizes and longer periods. The estimated mean proportion of binaries with D/D=>0.18 among NEAs larger than 0.3 km is 15±4%. Among fastest rotating NEAs larger than 0.3 km with periods between 2.2 and 2.8 h, the mean proportion of such binaries is (66 10-12)%