CO2/HCO3- Accelerates Iron Reduction through Phenolic Compounds

Iron is a vital mineral for almost all living organisms and has a pivotal role in central metabolism. Despite its great abundance on earth, the accessibility for microorganisms is often limited, because poorly soluble ferric iron (Fe-3(+)) is the predominant oxidation state in an aerobic environment. Hence, the reduction of Fe-3(+) is of essential importance to meet the cellular demand of ferrous iron (Fe2+) but might become detrimental as excessive amounts of intracellular Fe2+ tend to undergo the cytotoxic Fenton reaction in the presence of hydrogen peroxide. We demonstrate that the complex formation rate of Fe-3(+) and phenolic compounds like protocatechuic acid was increased by 46% in the presence of HCO3- and thus accelerated the subsequent redox reaction, yielding reduced Fe2+. Consequently, elevated CO2/HCO3- levels increased the intracellular Fe2+ availability, which resulted in at least 50% higher biomass-specific fluorescence of a DtxR-based Corynebacterium glutamicum reporter strain, and stimulated growth. Since the increased Fe2+ availability was attributed to the interaction of HCO3- and chemical iron reduction, the abiotic effect postulated in this study is of general relevance in geochemical and biological environments. IMPORTANCE In an oxygenic environment, poorly soluble Fe-3(+) must be reduced to meet the cellular Fe2+ demand. This study demonstrates that elevated CO2/HCO3- levels accelerate chemical Fe-3(+) reduction through phenolic compounds, thus increasing intracellular Fe2+ availability. A number of biological environments are characterized by the presence of phenolic compounds and elevated HCO3- levels and include soil habitats and the human body. Fe2+ availability is of particular interest in the latter, as it controls the infectiousness of pathogens. Since the effect postulated here is abiotic, it generally affects the Fe2+ distribution in nature.