The Confluence of Heavy Metal Biooxidation and Heavy Metal Resistance: Implications for Bioleaching by Extreme Thermoacidophiles

Abstract

Extreme thermoacidophiles (Topt > 65 °C, pHopt < 3.5) inhabit unique environments fraught with challenges, including extremely high temperatures, low pH, as well as high levels of soluble metal species. In fact, certain members of this group thrive by metabolizing heavy metals, creating a dynamic equilibrium between biooxidation to meet bioenergetic needs and mechanisms for tolerating and resisting the toxic effects of solubilized metals. Extremely thermoacidophilic archaea dominate bioleaching operations at elevated temperatures and have been considered for processing certain mineral types (e.g., chalcopyrite), some of which are recalcitrant to their mesophilic counterparts. A key issue to consider, in addition to temperature and pH, is the extent to which solid phase heavy metals are solubilized and the concomitant impact of these mobilized metals on the microorganism's growth physiology. Here, extreme thermoacidophiles are examined from the perspectives of biodiversity, heavy metal biooxidation, metal resistance mechanisms, microbe-solid interactions, and application of these archaea in biomining operations.

Metabolic diversity and adaptive mechanisms of iron‐ and/or sulfur‐oxidizing autotrophic acidophiles in extremely acidic environments

“…2) (Vald es et al, 2008b;Mi et al, 2011;C ardenas et al, 2014;Jiang et al, 2015). The most pervasive system involved in arsenic resistance employs an ars operon (Wheaton et al, 2015), including ArsR (an As 31 responsive transcriptional repressor) (Xu et al, 1998), ArsC (an arsenate reductase, which is responsible for the reduction of As(V) to As(III)) (Mukhopadhyay and Rosen, 2002), and ArsB (an antiporter governing the exchange of As(OH) 3 for protons) (Meng et al, 2004). The ars operons are widely dispersed in acidophilic bacteria such as A. ferrooxidans (Butcher et al, 2000), A. thiooxidans (Jiang et al, 2015), A. caldus (Kotze et al, 2006), and Leptospirillum ferriphilum (Li et al, 2010;Mi et al, 2011).…”

Reaction kinetic analysis of the 3-hydroxypropionate/4-hydroxybutyrate CO2 fixation cycle in extremely thermoacidophilic archaea

“…An alternative strategy would be to engineer a related organism with available genetic tools, such as Sulfolobus acidocaldarius . This presents certain difficulties, however, because while S. acidocaldarius was initially reported as having the capability for sulfur chemolithoautotrophy, currently used strains with genetic systems are obligate heterotrophs (Wheaton et al, 2015). This suggests that the 3HP/4HB cycle and/or sulfur lithotrophy would need to be repaired prior to using this organism as a host for autotrophic chemical production.…”

VapC toxins drive cellular dormancy under uranium stress for the extreme thermoacidophile Metallosphaera prunae

“…The biooxidation of metals leads to their mobilization into a soluble form. This gives rise to an interesting dynamic in certain thermoacidophilic archaea, based on the balancing of their energy demands against the resultant toxic effects of increased local metal concentrations (Wheaton et al, 2015). On a related note, the dissolution of ores potentially leads to stressors related to transient levels of solutes affecting osmotic and chaotropic stress in these types of environments, as well as stress associated with decreased water activity (Grant, 2004;Stevenson et al, 2015).…”