The ecophysiology of inorganic carbon assimilation by Durvillaea potatorum (Durvillaeales, Phaeophyta)

Abstract

Properties of inorganic C assimilation by the upper sublittoral brown macroalga Durvillaea polalorum (Labillardiere) Areschoug (Durvillaeales) were measured for comparison with the properties of C assimilation by eulittoral and sublittoral members of the Fucales and Laminariales. The dependence of O2 evolution on inorganic C concentration at pH 8.0 shows that seawater (2 mol m−3) inorganic C concentration is not quite saturating for light-saturated photosynthesis. The occurrence of HCO3−use was investigated by pH drift experiments, O2 evolution as a function of inorganic C concentration at an external pH of 9.2, and comparisons of the rate of photosynthesis at pH 8.0 and pH 9.2 with the rate of uncatalysed CO2 production from HCO3− at the inorganic C concentration in the O2 electrode chamber. All approaches suggest that the D. polalorum has a lower capacity to use external HCO3− than do many eulittoral brown macroalgae. The rate of dark inorganic 14C fixationis in the range commonly observed for brown macroalgae, i.e. higher than in red or green macroalgae either on an absolute basis or relative to the rates of photosynthesis or respiration. However, there is no significant diel variation in titratable acidity in D. polalorum such as occurs in many Fucaceae and is suggestive of a low-amplitude CAM-like metabolism. Overall, the characteristics of D. polalorum which were investigated more closely resemble those of the Laminariales and the Fucalean Halidrys siliquosa, which spend essentially all of their time submersed, than of the Fucaceae which spend some time emersed during most tidal cycles.