PHOTOSYNTHESIS AND PHOTOINHIBITION IN PROTOPLASTS OF THE MARINE BROWNALGA LAMINARIA-SACCHARINA

The procedure for the isolation of protoplasts from adult sporophytes of Laminaria saccharina was reappraised, using the rate of oxygen evol ution to assess protoplast viability. Modifications included the ionic composition of the osmoticum and culture of the protoplasts in darkne ss followed by increasing illuminations up to 25 mu mol m(-2) s(-1). T he photosynthetic activity of freshly isolated protoplasts was identic al to that of thallus pieces for at least 24 h. In association with th e interconversion of violaxanthin and zeaxanthin (the xanthophyll cycl e), the net photosynthesis (NP) and variable fluorescence (Fv) of the protoplasts drastically decreased upon illumination with oversaturatin g light. In contrast with thallus pieces, however, both NP and Fv were only partly restored in protoplasts under low light-conditions. In ad dition, culture under irradiances as low as 40 mu mol( m(-2) s(-1) pro ved detrimental to protoplast survival. After 1 week in culture, photo synthetic activity of the living cells had decreased significantly, co rresponding to a marked dedifferentiation of plastids. By the fourth w eek most of the surviving cells had divided, amounting to an overall p lating efficiency of about 35%. These results suggest that freshly iso lated protoplasts are physiologically representative of intact cells. Compared with thallus cells, however, protoplasts are more sensitive t o light and progressively lose their tissue-specific functions.