Hydrogen sulfide: clandestine microbial messenger?

Although the toxicity of hydrogen sulfide (H(2)S) has been substantiated for almost 230 years, its pivotal roles in both aerobic and anaerobic organisms have only recently become evident. In low oxygen environments with millimolar concentrations of H(2)S, it functions as an electron donor and as an energy source in some systems. At micromolar levels, intracellular H(2)S in aerobic organisms has a vital role in redox balancing. At even lower concentrations, H(2)S provides essential signals in yeast, in the brain and in smooth and cardiac muscles. Here, other possible coordinating roles within and between microorganisms are suggested, including the possibility that H(2)S functions as a signalling mediator in prokaryotes. It is expected that future research will uncover a host of novel functions, not only in eukaryotes but also in prokaryotic species.