Ligand-enhanced abiotic iron oxidation and the effects of chemical versus biological iron cycling in anoxic environments

Abstract

This study introduces a newly isolated, genetically tractable bacterium ( Pseudogulbenkiania sp. strain MAI-1) and explores the extent to which its nitrate-dependent iron-oxidation activity is directly biologically catalyzed. Specifically, we focused on the role of iron chelating ligands in promoting chemical oxidation of Fe(II) by nitrite under anoxic conditions. Strong organic ligands such as nitrilotriacetate and citrate can substantially enhance chemical oxidation of Fe(II) by nitrite at circumneutral pH. We show that strain MAI-1 exhibits unambiguous biological Fe(II) oxidation despite a significant contribution (∼30-35%) from ligand-enhanced chemical oxidation. Our work with the model denitrifying strain Paracoccus denitrificans further shows that ligand-enhanced chemical oxidation of Fe(II) by microbially produced nitrite can be an important general side effect of biological denitrification. Our assessment of reaction rates derived from literature reports of anaerobic Fe(II) oxidation, both chemical and biological, highlights the potential competition and likely co-occurrence of chemical Fe(II) oxidation (mediated by microbial production of nitrite) and truly biological Fe(II) oxidation.

Figure 1

Ligands affect the abiotic oxidation of Fe(II) by NO₂^–. Error bars omitted for clarity (relative standard deviation of Fe(II) and NO₂^– quantitation from all seven experiments estimated at 3% and 2%, respectively).

Figure 2

Fe(II) oxidation by Pseudogulbenkiania sp. strain MAI-1 during anaerobic growth with nitrate. Nitrite accumulation during growth depicted in top panel, concomitant Fe(II) oxidation in middle panel, modeled abiotic Fe(II) oxidation in bottom panel (see Materials and Methods for details on computation). Solid and dashed lines indicate Fe(II) oxidation without/with biological NO consumption, respectively. Dotted line indicates Fe(II) oxidation with 6× higher rate constant and NO consumption. Model range for three biological replicates shaded in gray. Vertical line indicates time point addressed in text. Experiment conducted in biological triplicates (solid markers) and with abiotic control (empty circles, ○). All data are shown.

Figure 3

Fe(II) oxidation in P. denitrificans cultures and filter-sterilized spent medium. Fe(II) concentrations shown as solid lines, NO₂^– concentrations as dashed lines. Samples are drawn from triplicate cultures (Supporting Information Figure S6) after accumulation of ∼5 mM NO₂^– and spiked with Fe(II) ± citrate at 0 h. All data are shown.

Figure 4

Rate constants increase with increasing degree of Fe(II) complexation. Second-order rate constants for oxidation experiments in the presence of citrate (black symbols) and NTA (gray symbols) are plotted against the degree of Fe(II) complexation by citrate/NTA. Rate constants derived from [Fe(II)] depicted as circles (○), constants derived from [NO₂^–] as squares (□). Error bars indicate 95% confidence intervals (Tables 1 and 2). Details on speciation can be found in Supporting Information Table S1. Larger confidence intervals for data reported in Table 2 are a consequence of reduced temporal resolution and greater deviation from the assumption that initial Fe(II) and NO₂^– concentrations are equimolar.