DARK CARBON FIXATION STUDIES ON THE INTERTIDAL MACROALGA ASCOPHYLLUM NODOSUM (PHAEOPHYTA) 1

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Abstract
The characteristics of dark carbon fixation by Ascophyllum nodosum were investigated. In longitudinal profile the maximum rates of dark and light dependent fixation are found at the apex. The use of Michaelis‐Menten kinetics did not suitably describe the relationship between the uptake rate in the dark and the total inorganic carbon concentration. Dark fixation was saturated at a total inorganic carbon concentration [TIC] of 2.5 mM. The use of the Hill‐Whittingham equation to describe the uptake curve indicates that the process is diffusion limited. Comparisons of dark fixation at high (8.0) and low (5.2) pH suggest that bicarbonate ions are used as a source of inorganic carbon. The transfer of 14C, fixed in the dark, from the ethanol soluble to the insoluble fraction was relatively slow irrespective of the light treatment during the chase period. Ascophyllum nodosum displays a small diel fluctuation in the pH of aqueous extracts and titratable acidity similar to that displayed by CAM plants. The significance of dark fixation to the overall carbon budget is discussed.