Limitations of microbial iron reduction under extreme conditions

Abstract

Microbial iron reduction is a widespread and ancient metabolism on Earth, and may plausibly support microbial life on Mars and beyond. Yet, the extreme limits of this metabolism are yet to be defined. To investigate this, we surveyed the recorded limits to microbial iron reduction in a wide range of characterized iron-reducing microorganisms (n = 141), with a focus on pH and temperature. We then calculated Gibbs free energy of common microbially mediated iron reduction reactions across the pH-temperature habitability space to identify thermodynamic limits. Comparing predicted and observed limits, we show that microbial iron reduction is generally reported at extremes of pH or temperature alone, but not when these extremes are combined (with the exception of a small number of acidophilic hyperthermophiles). These patterns leave thermodynamically favourable combinations of pH and temperature apparently unoccupied. The empty spaces could be explained by experimental bias, but they could also be explained by energetic and biochemical limits to iron reduction at combined extremes. Our data allow for a review of our current understanding of the limits to microbial iron reduction at extremes and provide a basis to test more general hypotheses about the extent to which biochemistry establishes the limits to life.

Keywords: biochemistry; extremophiles; limits to life; microbial iron reduction; thermodynamics.

Figure 1.

Adaptations and diversity of iron-reducing strains in the databas. (A) Classifications used to categorize strains; (B) abundance of extremophilic and polyextremophilic (grey box) strains; and (C) 16S rRNA gene phylogenetic tree of genera represented in the database, the number of strains per genus (numbered spots) and adaptations of those strains (colours correspond to those in (A). All green (mesophilic/neutrophilic) data points are bordered with black to help distinguish them from red data points.

Figure 2.

Origin and habitat type of characterized iron-reducing strains, classified by adaptation (see Fig. 1(A) for definitions). The inset barchart indicates the number of strains isolated by country/territory of origin. All green (mesophilic/neutrophilic) data points have a dotted pattern to distinguish them from red data points.

Figure 3.

pH–temperature habitability parameter space of iron-reducing strains in the database. Each point represents optimal growth conditions with respect to temperature (T) and pH. Solid error bars represent the range in growth T and pH conditions for each strain, and dashed error bars represent the range of T and pH conditions tested for each strain. All points are colour-coded according to their pH and T adaptation classification (see Fig. 1A). The grey box highlights the ‘Goldilocks zone’ of relatively narrow pH (6–8) and T (20–40°C) growth optima that almost all strains (135 of 141) fall within.

Figure 4.

Electron donor use by all strains in the database (A) and grouped by adaptation (B). Solid colour indicates the electron donor is used for iron reduction, cross-hatched colour indicates tested but not used. Numbers in parentheses indicate the number of strains that can use an electron donor in that group compared with the total number of strains tested. ‘Other’ includes carbon monoxide and elemental sulfur. Green (mesophilic/neutrophilic) bars have a black border to distinguish them from red bars.

Figure 5.

Gibbs free energy of reaction (ΔGr) for iron reduction reactions known to support growth by strains in the database across the pH–temperature habitability space, expressed in kJ per electron transferred for the following iron reduction reactions considered: goethite (FeO(OH)) with acetate (A) and hydrogen (B); hematite (Fe₂O₃) with acetate (C) and hydrogen (D); and ferrihydrite (Fe(OH)₃) with acetate (E) and hydrogen (F). In all cases, the ferric iron phase is assumed to be aqueous (Fe³⁺(aq)) below pH3, regardless of redox couple and temperature. Cells in green are considered favourable for growth by iron reduction; cells in patterned red are considered favourable enough for survival and maintenance only but unfavourable for growth (calculated as −20 kJ per electron transferred for the given reaction); black cells represent positive ΔGr and, therefore, indicate thermodynamically infeasible reactions at the associated conditions. Strains that have been shown to grow from these iron reduction redox couples are overlain on corresponding heatmaps.

Figure 6.

Observed pH and temperature growth conditions plotted with thermodynamic predictions for the most (A) and least (B) favourable redox couples across the parameter space. In both cases, the ferric iron phase is assumed to be aqueous (Fe³⁺(aq)) below pH 3, regardless of redox couple and temperature. Cells in green are considered favourable for growth by iron reduction; cells in patterned red are considered favourable enough for survival and maintenance, but not growth (calculated as −20 kJ per electron transferred for the given reaction); black cells represent positive ΔGr and, therefore, indicate thermodynamically infeasible reactions at the associated conditions.