Inhibition of Energy Transfer Reactions in Cyanobacteria by Different Ultraviolet Radiation

Abstract

When Anacystis nidulans cells were irradiated with ultraviolet-C (UV-C, 254 nm), UV-B (285–325 nm) or UV-A (320–400 nm) radiation, the overall photosynthetic rate decreased progressively with time of treatment. Since a decrease in the rate of photosynthesis could be either due to a loss of excitation energy transfer from phycobilisomes (PBS) to chlorophyll and/or inhibition of various electron transfer reactions, both the fluorescence spectra and electron transfer reactions (rate of O2 evolution) were measured in UV irradiated cells. UV irradiation brings about drastic changes in the excitation and emission spectra of Anacystis cells. A comparative investigation on the nature of action of these different UV wavelengths indicates strong action by UV-C radiation on both PBS to Chl a energy transfer and Chl a-mediated electron transport reactions than that of UV-B and UV-A. UV-C radiation brings about drastic loss of phycobilins, as evidenced from absorption and fluorescence excitation spectra. In contrast to this, UV-B and UV-A treatment resulted in loss of energy transfer from allophycocyanin (APC) to Chl a. This was indicated by increased fluorescence emission from phycobilins with concomittant decrease in chlorophyll fluorescence.