Frond development and CO2-fixation in Laminaria hyperborea.

Discs punched out of different zones of the found of Laminaria hyperborea (Gunn.) Fossl. were exposed to H(14)CO3 (-) in the light and in the dark for various lengths of time. Photosynthetic rates in young and old parts were in the same range, 12-39 μmol CO2 dm(-2) h(-1) at 4°C, whereas dark fixation was remarkably higher in the growing zone than in the old frond: 13-28% of the corresponding light fixation in the young phylloid and only 2-6% in the old one. In all parts of the frond the reductive pentose phosphate cycle is the main system of carboxylation, and is accompanied by a notable primary synthesis of amino acids. In the growing zone heavy and primary labelling of malate and aspartate parallel to 3-PGA is due to the undiminshed activity of the dark fixation system (PEP-carboxykinase) in the light. Mannitol synthesis seems to be enhanced with increasing age of the phylloid, but rapid and primary (14)C-incorporation into amino acids was found in all parts of the frond.The data presented here suggest a possible significance of the high dark fixation rates for the growth of the sublittoral Laminaria hyperborea. Translocation of mobilized storage material from the old phylloid might supply the young tissue with reduced material during the growth season when the light intensity is low in the habitat of this alga.