Photosynthesis in marine macroalgae: evidence for carbon limitation

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Abstract
During a shipboard expedition to Andros Island (Bahamas), photosynthetic measurements for the macroalgae Cladophoropsis membranacea (Chlorophyta), Dilophus guineensis, Turbinaria turbinata, and Lobophora variegata (Phaeophyta), and Laurencia papillosa (Rhodophyta), taken directly from their marine habitat, showed that only Cladophoropsis was saturated at seawater inorganic carbon levels (2.5 mM). The photosynthetic K0.5 values for inorganic carbon ranged from 1.1 to 3.2 mM. Decreasing the pH at 2.5 mM inorganic carbon, and thus enhancing the CO2 by 30-fold, only slightly increased photosynthesis, suggesting that bicarbonate was the major assimilated form of inorganic carbon. At 2.5 mM inorganic carbon, only Lobophora exhibited a Warburg effect on photosynthesis (49%), but at 0.5 mM, Turbinaria and Laurenica were also inhibited by O2. Ribulosebisphosphate carboxylase-oxygenase appeared to be the predominant carboxylation enzyme, but in Dilophus and Laurencia extracts, its activity was rivaled by phosphoenolpyruvate carboxylase and carboxykinase. Malate pools were detected in Turbinaria and Laurencia, and in the latter they were greater at night than during the day. However, this diel fluctuation was too small to implicate crassulacean acid metabolism. The data indicate that the bicarbonate concentration in seawater is insufficient to overcome O2 inhibition effects on photosynthesis, unless the macroalga has some form of CO2 concentrating system, based on bicarbonate uptake or C4 acid metabolism. In addition, the inorganic carbon in seawater may be a nutrient limiting the photosynthesis and productivity of certain macroalgae.