Effects of UV radiation on photosynthesis and excretion of UV-absorbing compounds of Dasycladus vermicularis (Dasycladales, Chlorophyta) from southern Spain

Photosynthesis measured by chlorophyll fluorescence and oxygen exchange, pigment composition, and excretion of UV-absorbing compounds were tested in the green alga Dasycladus vermicularis (Scopoli) Krasser following exposure to a combination of ultraviolet radiation and photosynthetically active radiation (PAR) in the laboratory. A preliminary chromatographic analysis of a thallus extract resulted in three separated substances, one showing an absorption maximum at 332 nm and two at 348 nm (UV-A). In contrast, the compound or set of compounds excreted to the medium had a spectrophotometric absorption maximum at 387 nm, indicating degradation or chemical conversion of the in situ precursors. The total absorbances of these UV-absorbing compounds in seawater and in the thallus were higher when plants were exposed to PAR alone (P) or to PAR + ultraviolet A and B radiation (PAB). This suggests that accumulation and excretion of this substance respond either to UV-B radiation or to a low irradiance of PAR. Pulse amplitude modulated fluorescence (PAM) and oxygen evolution were also affected by exposure to PAB and P. After 72 h exposure to PAB and P, the optimal (Fv/Fm) quantum yield of fluorescence decreased between 15 and 30% compared to initial control, whereas lower photoinhibition of fluorescence occurred under PAR +UV-A conditions. Complete loss of O2 evolution was observed in thalli irradiated for 24 h with PAB, whereas no effects after UV-A exposure were observed. In general, following 24 h exposure to the different treatments, an increase in light requirements for compensation (Ic) and saturation (Ik) of photosynthesis was seen. Calculations of biologically effective UV doses using known action spectra revealed that fluorescence is impaired by 25% at UV-B doses above 900 kJ m−2