H₂O₂-Induced Inhibition of Photosynthetic O₂ Evolution by Anabaena variabilis Cells

V. D. Samuilov^1*, K. N. Timofeev², S. V. Sinitsyn¹, and D. V. Bezryadnov¹

¹Department of Physiology of Microorganisms and ²Department of Biophysics, Faculty of Biology, Lomonosov Moscow State University, Moscow 119992, Russia; fax: (7-095) 939-3807; E-mail: vds@8.cellimm.bio.msu.ru

^* To whom correspondence should be addressed.

Received February 2, 2004; Revision received March 17, 2004

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Hydrogen peroxide inhibits photosynthetic O₂ evolution. It has been shown that H₂O₂ destroys the function of the oxygen-evolving complex (OEC) in some chloroplast and Photosystem (PS) II preparations causing release of manganese from the OEC. In other preparations, H₂O₂ did not cause or caused only insignificant release of manganese. In this work, we tested the effect of H₂O₂ on the photosynthetic electron transfer and the state of OEC manganese in a native system (intact cells of the cyanobacterium Anabaena variabilis). According to EPR spectroscopy data, H₂O₂ caused an increase in the level of photooxidation of P700, the reaction centers of PS I, and decreased the rate of their subsequent reduction in the dark by a factor larger than four. Combined effect of H₂O₂, CN^-, and EDTA caused more than eight- to ninefold suppression of the dark reduction of P700⁺. EPR spectroscopy revealed that the content of free (or loosely bound) Mn²⁺ in washed cyanobacterial cells was ~20% of the total manganese pool. This content remained unchanged upon the addition of CN^- and increased to 25-30% after addition of H₂O₂. The content of the total manganese decreased to 35% after the treatment of the cells with EDTA. The level of the H₂O₂-induced release of manganese increased after the treatment of the cells with EDTA. Incubation of cells with H₂O₂ for 2 h had no effect on the absorption spectra of the photosynthetic pigments. More prolonged incubation with H₂O₂ (20 h) brought about degradation of phycobilins and chlorophyll a and lysis of cells. Thus, H₂O₂ causes extraction of manganese from cyanobacterial cells, inhibits the OEC activity and photosynthetic electron transfer, and leads to the destruction of the photosynthetic apparatus. H₂O₂ is unable to serve as a physiological electron donor in photosynthesis.

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KEY WORDS: photosynthesis, photosynthetic oxygen, oxygen-evolving complex, Photosystem II, Photosystem I, hydrogen peroxide, manganese, cyanobacteria, Anabaena variabilis

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