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Mediated herbicide inhibition in a PET biosensor

Many commonly employed herbicides are active on the photosynthetic electron transport chain. The main components of the reaction center of photosystem II (PSII) are the so-called D1 and D2 proteins, where herbicidal inhibition can occur. The inhibition can be monitored due to the mediated photocurrent, employing mediators that report on the reduction of the native plastoquinone, Q B , at D1. This study employs diaminodurene to mediate the photocurrent in cells from the cyanobacteria Synechoccus sp. A comparison of the inhibition kinetics of atrazine and diuron is reported and the noncompetitive inhibitor, diuron, is shown to give an apparent first-order rate constant for binding that is ∼7 times that of atrazine under the same light and dark cycle. Atrazine was shown to have a stronger binding affinity during the light regime than with the dark unexcited PSII state. In contrast with diuron, it also demonstrated evidence of interaction with PSI. In the case of atrazine, 50% inhibition of the photocurrent is predicted for 29 ppb herbicide, giving a theoretical minimum detection below 1 ppb. The stronger binding affinity with diuron would lead to a much lower level of detection under the same conditions. The action of the different inhibition kinetics, light and dark affinities, and interactions at PSI and PSII allow distinction to be made between the different herbicides, which could be extracted from the photocurrent in suitable light/dark cycles.