Response of chlorophyll d-containing cyanobacterium Acaryochloris marina to UV and visible irradiations

Abstract

We have previously investigated the response mechanisms of photosystem II complexes from spinach to strong UV and visible irradiations (Wei et al J Photochem Photobiol B 104:118-125, 2011). In this work, we extend our study to the effects of strong light on the unusual cyanobacterium Acaryochloris marina, which is able to use chlorophyll d (Chl d) to harvest solar energy at a longer wavelength (740nm). We found that ultraviolet (UV) or high level of visible and near-far red light is harmful to A. marina. Treatment with strong white light (1,200μmolquantam(-2)s(-1)) caused a parallel decrease in PSII oxygen evolution of intact cells and in extracted pigments Chl d, zeaxanthin, and α-carotene analyzed by high-performance liquid chromatography, with severe loss after 6h. When cells were irradiated with 700nm of light (100μmolquantam(-2)s(-1)) there was also bleaching of Chl d and loss of photosynthetic activity. Interestingly, UVB radiation (138μmolquantam(-2)s(-1)) caused a loss of photosynthetic activity without reduction in Chl d. Excess absorption of light by Chl d (visible or 700nm) causes a reduction in photosynthesis and loss of pigments in light harvesting and photoprotection, likely by photoinhibition and inactivation of photosystem II, while inhibition of photosynthesis by UVB radiation may occur by release of Mn ion(s) in Mn4CaO5 center in photosystem II.