Application of Mössbauer spectroscopy for studying chemical effects of environmental factors on microbial signalling: Redox processes involving iron(III) and some microbial autoinducer molecules

Diverse processes of microbial remote intercellular communication as well as the exchange of molecular signals between microbial cells and their host macroorganisms, involving specific low-molecular-mass diffusible substances (used as a 'chemical language'), are at the peak of current research in biosciences. This fundamental interest is due to the unique possibility of controlling the microbial behaviour and metabolism by influencing merely their signalling pathways. On the other hand, abiotic impact of the environment (medium) on extracellular molecular signals is also of great importance, as any of their chemical interactions (e.g., complexation or oxidation) represent direct interferences in the process of 'signal delivery' through the medium. In this work, chemical interactions of microbial extracellular molecular signals (alkylresorcinols (AR), homoserine lactone (HL) - chemical analogues of microbial autoregulatory substances) with iron(III) were monitored using freeze-quench 57Fe Mössbauer spectroscopy in moderately acidic aqueous solutions as well as in the dried solids obtained thereof. The conditions applied were designed to simulate possible processes occurring in soils, where ferric iron is commonly ubiquitous. Gradual reduction of iron(III) by AR was observed, coupled to oxidative degradation of the organics, in solution, while iron(II) also remained dominant upon drying, whereas for HL, some iron(II) was detected in the dried solid only. The iron(III) reduction rate in solution for AR with a longer alkyl chain (4-n-C6) was found to be much higher than that for the methyl (5-C1)-substituted derivative, pointing to the importance of the structure (i.e., the position and/or the nature of the alkyl substituent) of the alkylresorcinol molecule for the redox process rate. The results obtained indicate that ARs can be readily oxidised abiotically by soil iron(III) in moderately acidic media, thus being excluded from signalling pathways, which is equal to “message undelivery”, directly affecting microbial autoregulation.