Photosynthesis and photoinhibition in protoplasts of the marine brown alga Laminaria saccharina

The procedure for the isolation of protoplasts from adult sporophytes of Laminaria saccharina was reappraised, using the rate of oxygen evolution to assess protoplast viability. Modifications included the ionic composition of the osmoticum and culture of the protoplasts in darkness followed by increasing illuminations up to 25 micromoles m-2 s-1. The photosynthetic activity of freshly isolated protoplasts was identical to that of thallus pieces for at least 24 h. In association with the interconversion of violaxanthin and zeaxanthin (the xanthophyll cycle), the net photosynthesis (NP) and variable fluorescence (Fv) of the protoplasts drastically decreased upon illumination with oversaturating light. In contrast with thallus pieces, however, both NP and Fv were only partly restored in protoplasts under low light-conditions. In addition, culture under irradiances as low as 40 micromoles m-2 s-1 proved detrimental to protoplast survival. After 1 week in culture, photosynthetic activity of the living cells had decreased significantly, corresponding to a marked dedifferentiation of plastids. By the fourth week most of the surviving cells had divided, amounting to an overall plating efficiency of about 35%. These results suggest that freshly isolated protoplasts are physiologically representative of intact cells. Compared with thallus cells, however, protoplasts are more sensitive to light and progressively lose their tissue-specific functions.