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UVB monochromatic action spectrum of the inhibition of photosynthetic oxygen production in the green alga Dunaliella salina Francesco Ghetti 1, Silvana Fuoco 1 and Giovanni Checcucci 1,2 1 C.N.R. Istituto di Biofisica, Pisa, Italy 2 Dipartimento di Etologia, Ecologia ed Evoluzione, Università di Pisa, Pisa, Italy Correspondence: Francesco Ghetti C.N.R. Istituto di Biofisica, Via San Lorenzo 26, 56127 Pisa, Italy e-mail: ghetti@ib.pi.cnr.it Abstract The effect of monochromatic UVB irradiation on the photosynthetic activity of the marine green alga Dunaliella salina was investigated by monitoring the rate of oxygen exchange. Samples were irradiated with narrow bands centred at different UV wavelengths (281, 290, 300, 310 and 322 nm). In a first set of experiments the samples were pre-irradiated keeping constant the irradiance and varying exposure time. By increasing UVB fluence, a wavelength-dependent decrease in the rate of oxygen production was observed. In a second set of experiments oxygen exchange was monitored simultaneously to UVB irradiation. At short wavelengths UVB irradiation caused a general decline of the rate of oxygen production and the decrement was faster with increasing UVB photon flux density. Action spectra derived from the two sets of experiments are in good agreement and indicate an increase of the UVB inhibiting effectiveness of about two orders of magnitude with decreasing wavelength from 320 to 280 nm. Keywords: action spectroscopy, Dunaliella salina; oxygen exchange; photosynthesis inhibition; UVB