The effects of full-spectrum solar radiation and of solar radiation deprived of total u.v. radiation (λ<395 nm) on the endemic Mediterranean red alga Rissoella verruculosa (Bertoloni) J. Agardh were studied in situ in early summer, from sunrise to sunset. Photosynthetic performance, pigment content and the activities of enzymes related to nutrient uptake, were monitored under both radiation conditions throughout a daylight period.

The doses of solar radiation, measured on the day during which the experiments were carried out (24 June 1997), were 9228·25, 1109·70 and 13·03 kJ m−2 for PAR (λ=400–700 nm), u.v.-A (λ=315–400 nm) and u.v.-B (λ=280–315 nm), respectively. Under these conditions, a clear daily variation in photosynthetic performance was found. However, no significant differences were detected between the two radiation conditions. At noon, strong sunlight impaired O2 evolution by 75%, but complete recovery occurred during the afternoon, reaching similar values to those measured in the early morning. By contrast, photoinhibition of optimal quantum yield (Fv/Fm), and the relative electron transport rate was followed by only a slight recovery during the afternoon. The rate of photosynthesis in air (simulating emersion conditions), estimated by CO2 exchange, showed a negative balance at noon, which was accentuated in plants exposed to the full-solar spectrum.

Significant changes in the activities of nitrate reductase and carbonic anhydrase were found throughout the day under both radiation conditions. In thalli receiving solar radiation deprived of u.v., total CA activity decreased throughout the day. However, in thalli exposed to full-spectrum solar radiation, the activity of the enzyme tended to increase in the afternoon, correlating with an increase in NR activity.