Response of Rhizobium to Cd exposure: A volatile perspective

The volatile metabolome of Rhizobium sp. strain E20-8 exposed to three concentrations of cadmium (2.5, 5.0 and 7.5 μM) was screened using comprehensive two-dimensional gas chromatography coupled to time of flight mass spectrometry (GC × GC-ToFMS), combined with headspace solid phase microextraction (HS-SPME). Cd exposure induced a global increase in the concentration of volatile organic compounds (VOCs) both intra and extracellularly. Peak areas of several linear alkanes, ketones, aldehydes, alcohols, terpenic and volatile sulfur compounds, and one ester (ethyl acetate), were especially increased when compared with the control condition (no Cd). These compounds might originate from the metabolization of toxic membrane peroxidation products, the proteolysis of oxidized proteins or the alteration of metabolic pathways, resulting from the oxidative stress imposed by Cd. Several VOCs are related to oxidative damage, but the production of VOCs involved in antioxidant response (menthol, α-pinene, dimethyl sulfide, disulfide and trisulfide, 1-butanol and 2-butanone) and in cell aggregation (2,3-butanedione, 3-methyl-1-butanol and 2-butanone) is also observed. These results bring new information that highlights the role of VOCs on bacteria response to Cd stress, identify a novel set of biomarkers related with metal stress and provide information to be applied in biotechnological and remediation contexts.