Photosystem 2 (PS2) that catalyses light driven water splitting in photosynthesis was wired to electrode surfaces via osmium-containing redox polymers based on poly(vinyl)imidazol. The redox polymer hydrogel worked as both immobilization matrix and electron acceptor for the enzyme. Upon illumination, the enzymatic reaction could be switched on and a catalytic current was observed at the electrode. The catalytic current is directly dependent on the intensity of light used for the excitation of PS2. A typical current density of 45 mA cm À2 at a light intensity of 2.65 mW cm À2 could be demonstrated with a significantly improved operational stability.

To establish a semiartificial device for (bio-)hydrogen production utilizing photosynthetic water oxidation, we report on the immobilization of a Photosystem 2 on electrode surfaces. For this purpose, an isolated Photosystem 2 with a genetically introduced His tag from the cyanobacterium Thermosynechococcus elongatus was attached onto gold electrodes modified with thiolates bearing terminal Ni(II)-nitrilotriacetic acid groups. Surface enhanced infrared absorption spectroscopy showed the binding kinetics of Photosystem 2, whereas surface plasmon resonance measurements allowed the amount of protein adsorbed to be quantified. On the basis of these data, the surface coverage was calculated to be 0.29 pmol protein cm(-2), which is in agreement with the formation of a monomolecular film on the electrode surface. Upon illumination, the generation of a photocurrent was observed with current densities of up to 14 microA cm(-2) . This photocurrent is clearly dependent on light quality showing an action spectrum similar to an isolated Photosystem 2. The achieved current densities are equivalent to the highest reported oxygen evolution activities in solution under comparable conditions.

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The molecular basis of potassium uptake in cyanobacteria has not been elucidated. However, genes known from other bacteria to encode potassium transporters can be identi¢ed in the genome of Synechocystis sp. strain PCC 6803. Mutants defective in kdpA and ntpJ were generated and characterized to address the role of the Kdp and KtrAB systems in this strain. KtrAB is crucial for K + uptake, as the v vntpJ mutant shows slowed growth, slowed potassium uptake kinetics, and increased salt sensitivity. The v vkdpA mutant has the same phenotype as the wild type even at limiting potassium, but a v vkdpAv vntpJ double mutant is not viable, indicating a role of Kdp for potassium uptake when the Ktr system is not functioning. ß