Effects of solar radiation on the Patagonian macroalga Enteromorpha linza (L.) J. Agardh Chlorophyceae

The photosynthetic performance of Enteromorpha linza (L.) J. Agardh - Chlor ophyceae was determined with a portable PAM instrument in situ and under se minatural radiation conditions in Patagonia, Argentina. Solar radiation was measured in parallel with a three-channel radiometer, ELDONET (Real Time C omputer, Mohrendorf, Germany), in three wavelength ranges, UV-B (280-315 nm ), UV-A (315-400 nm), and PAR (400-700 nm). The effective photosynthetic qu antum yield decreased after 15-min exposure to solar radiation when the tha lli were kept in a fixed position but recovered in the subsequent shade con ditions within several hours. A 30-min exposure of free floating thalli, ho wever, caused less photoinhibition. The photosynthetic quantum yield of E. linza was also followed over whole days under clear sky, partly cloudy and rainy conditions in a large reservoir of water (free floating thalli) and i n situ (thalli,crowing in rock pools). Most of the observed effect was due to visible radiation; however, the UV wavelength range, and especially UV-B , caused a significant reduction of the photosynthetic quantum yield. Fluen ce rate response curves indicated that the species is a typical shade plant which showed non-photochemical quenching at intermediate and higher irradi ances. This is a surprising result since these al-ae are found in the upper eulittoral where they are exposed to high irradiances. Obviously they util ize light only during periods of low irradiances (morning, evening, high ti de) while they shut down the electron transport chain during intensive expo sure. Fast induction and relaxation kinetics have been measured in these al gae for the first time and indicated a rapid adaptation of the photosynthet ic capacity to the changing light conditions as well as a fast decrease of PS II fluorescence upon exposure to solar radiation. There was a strong ble aching of chlorophyll due to exposure to solar radiation but less drastic b leaching of carotenoids. (C) 2001 Elsevier Science B.V. All rights reserved .