LONGITUDINAL PROFILES OF GROWTH, PHOTOSYNTHESIS AND LIGHT-INDEPENDENT CARBON FIXATION IN THE ANTARCTIC BROWN ALGA ASCOSEIRA-MIRABILIS

Thallus growth, photosynthetic oxygen evolution and rates of carbon fixation were determined along the lamina of the endemic Antarctic brown alga Ascoseira mirabilis (Ascoseirales), grown under simulated Antarctic conditions. The meristem is basally located and forms new blade tissue under spring-conditions. Light saturated net photosynthesis (P-max), measured as O-2 production, was higher in the intermediate region of the plant (9.8 mu mol O-2 g(-1) fw h(-1)). In general, photosynthetic parameters such as dark respiration, gross photosynthesis, photosynthetic efficiency (a) and photosynthetic light compensation (I-c) increased significantly towards the distal region. Carbon-fixation in A. mirabilis also showed thallus-dependent variation. Rates of light and light independent (dark) carbon fixation increased towards the distal regions ranging between 7.6-9.5 and 1.2-2.0 mu mol C g(-1) fw h(-1) respectively. The percentage of light independent carbon fixation (in relation to light C-14-fixation) also increased from the basal to the distal parts reaching 24% in the distal region of the thallus. The contents of Chl a and Chl c, were close to 0.37 and 0.14 mg g(-1) fw respectively and were notably uniform along the lamina. The results indicate that the formation of the blade by a basal meristem and the increase of light carbon fixation rates from base to the distal regions in A. mirabilis are similar compared with certain Laminariales, especially members of the genus Laminaria. However, light independent carbon fixation is highest in the meristems of Laminaria, opposite to the results obtained here for A. mirabilis.