Two-Component Systems in the Regulation of Sulfur and Ferrous Iron Oxidation in Acidophilic Bacteria

The two-component system (TCS) is a regulatory system composed of a sensor histidine kinase (HK) and a cytoplasmic response regulator (RR), which participates in the bacterial adaptation to external stimuli. Sulfur oxidation and ferrous iron oxidation are basic energy metabolism systems for chemoautotrophic acidophilic bacteria in acid mine environments. Understanding how these bacteria perceive and respond to complex environmental stimuli offers insights into oxidization mechanisms and the potential for improved applications. In this chapter, we summarized the TCSs involved in the regulation of sulfur and ferrous iron metabolic pathways in these acidophilic bacteria. In particular, we examined the role and molecular mechanism of these TCSs in the regulation of iron and sulfur oxidation in Acidithiobacillus spp.. Moreover, research perspectives on TCSs in acidophilic bacteria are discussed in this section.

The Effect of Metal Ions on the Growth and Ferrous Iron Oxidation by Leptospirillum ferriphilum CC Isolated from Armenia Mine Sites

The aim of this study is to investigate the potential of newly isolated strain Leptospirillum (L.) ferriphilum CC for bioleaching of pyrite and chalcopyrite in pure or mixed culture with other iron- and/or sulfur-oxidizing bacteria. In this paper, kinetics of ferrous iron (Fe2+) oxidation by newly isolated strain Leptospirillum (L.) ferriphilum CC was studied. The effect of initial Fe2+ in the concentration range of 50–400 mM on bacterial growth and iron oxidation was studied. It was shown that microbial Fe2+ oxidation was competitively inhibited by Fe3+. The influence of copper, zinc, nickel and cobalt ions on the oxidation of Fe2+ by L. ferriphilum CC was also studied. Minimal inhibitory concentrations (MIC) for each metal ion were determined. The toxicity of the ions was found to be as follows: Co > Zn > Ni > Cu. The comparison of iron oxidation kinetic parameters of L. ferriphilum CC with other strains of L. ferriphilum indicates the high potential of strain L. ferriphilum CC for biogenic regeneration of concentrated ferric iron (Fe3+) in bioleaching processes of ores and ore concentrates. Bioleaching tests indicated that the newly isolated L. ferriphilum CC can be a prospective strain for the bioleaching of sulfide minerals in pure culture or in association with other iron- and/or sulfur-oxidizing bacteria.

Dispersion of sulfur enables sulfur oxidation before iron oxidation in Acidithiobacillus ferrooxidans: A valuable formulation for the genetic engineering toolbox

Acidithiobacillus ferrooxidans are acidophilic chemolithoautotrophs that are commonly reported to exhibit diauxic population growth behavior where ferrous iron is oxidized before elemental sulfur when both are available, despite the higher energy content of sulfur. We have discovered sulfur dispersion formulations that enables sulfur oxidation before ferrous iron oxidation. The oxidation of dispersed sulfur can lower the culture pH within days below the range where aerobic ferrous iron oxidation can occur so that ferric iron reduction occurs which had previously been reported over extended incubation periods with untreated sulfur. Therefore, we demonstrate that this substrate utilization pattern is strongly dependent on the cell loading in relation to sulfur concentration, sulfur surface hydrophobicity, and the pH of the culture. Our dispersed sulfur formulation, lig-sulfur, can be used to support the rapid antibiotic selection of plasmid-transformed cells, which is not possible in liquid cultures where ferrous iron is the main source of energy for these acidophiles. Furthermore, we find that media containing lig-sulfur supports higher production of green fluorescent protein (GFP) compared to media containing ferrous iron. The use of dispersed sulfur is a valuable new tool for the development of engineered A. ferrooxidans strains and it provides a new method to control iron and sulfur oxidation behaviors.

Kinetics of different bioreactor systems with Acidithiobacillus ferrooxidans for ferrous iron oxidation

The relative performance of two biofilm-based airlift reactors using different kinds of packing materials and one fixed bed biofilm reactor with a homemade packing material of high specific area (~ 1000 m2/m3) was addressed. The bioreactors operated under ferrous iron loading rates in the range of 8–120 mol Fe(II)/m3 h. Acidithiobacillus ferrooxidans cells immobilized in the three bioreactors afforded the reactions for an extended period of 120 days of continuous operation at the dilution rates of 0.2, 0.4, 0.7, 1 and 1.2 h−1. The maximum ferrous iron oxidation rates achieved in this study at a hydraulic residence time of 1.2 h were about 91, 68 and 51 mol Fe(II)/m3 h for the fixed bed, airlift1, and airlft2 bioreactors. The performance data from the fixed-bed bioreactor offered a higher potential for ferrous iron oxidation because of fast biofilm development, the formation of a thick biofilm, and lower sensitivity to shear, which enhanced the startup time of the bioreactor and the higher reactor productivity. Proper kinetic models were also presented for both the startup period and the steady-state process.

Desulfurization of Biogas from a Closed Landfill under Acidic Conditions Deploying an Iron-Redox Biological Process

Desulfurization processes play an important role in the use of biogas in the emerging market of renewable energy. In this study, an iron-redox biological process was evaluated at bench scale and pilot scale to remove hydrogen sulfide (H2S) from biogas. The pilot scale system performance was assessed with real biogas emitted from a closed landfill to determine the desulfurization capacity under outdoor conditions. The system consisted of an Absorption Bubble Column (ABC) and a Biotrickling Filter (BTF) with useful volumes of 3 L and 47 L, respectively. An acidophilic mineral-oxidizing bacterial consortium immobilized in polyurethane foam was utilized to regenerate Fe(III) ion, which in turn accomplished the continuous H2S removal from inlet biogas. The H2S removal efficiencies were higher than 99.5% when H2S inlet concentrations were 120–250 ppmv, yielding a treated biogas with H2S < 2 ppmv. The ferrous iron oxidation rate (0.31 g·L−1·h−1) attained when the system was operating in natural air convection mode showed that the BTF can operate without pumping air. A brief analysis of the system and the economic aspects are briefly analyzed.