A new type of metal centre was detected in the membranes of the thermoacidophilic archaeon Sulfolobus metallicus. This centre has an S = 1/2 ground state in the oxidised form, yielding an axial EPR signal with g values at 2.035 (g(parallel)) and 1.97 (g(perpendicular)), optimally detected at 4.6-10 K; in the reduced form it is EPR silent (even spin). These magnetic properties point to a spin-coupled iron cluster, with a minimum of two iron ions. The centre has a high reduction potential of +350 mV, at pH 6.5. The physiological role of this novel centre was probed through a general characterisation of S. metallicus respiratory chain: this archaeon contains NADH and succinate dehydrogenase activities, and cytochromes b562, a586 and a600 on the oxygen reductase system. Since it is reduced in the presence of succinate, and taking into account its high reduction potential, this centre is proposed to be a functional analogue of the Rieske centres.