THE ACQUISITION OF INORGANIC CARBON BY MARINE MACROALGAE

Marine macroalgae inhabit three different environments: the eulittoral zone, rock pools, and the sublittoral zone. Many macroalgae exhibit C4 gas exchange characteristics, i.e., low CO2 compensation point, high pH compensation point, photosynthesis insensitive to changes in oxygen changes below 21 kPa, and a high affinity for inorganic carbon. It is concluded that in general eulittoral and rock-pool species are more C4-like than the subtidal species though there are interesting exceptions. Experimental evidence points to the following mechanisms being involved in inorganic carbon acquisition by macroalgae. The role of beta-carboxylation as the primary step in carbon fixation is only convincing in Udotea flabellum, while PGA is the first product of C-14 fixation in most species. Direct evidence of inorganic carbon accumulation is only available for Ulva fasciata, whereas Chondrus crispus does not have this ability. Studies show that carbonic anhydrase is prominent in the mechanism of carbon acquisition by Ascophyllum nodosum, and when inhibited the alga is dependent on CO2, whereas U. fasciata retains some ability to use bicarbonate ions. It is concluded that macroalgae display a range of inorganic carbon assimilation mechanisms that are active to varying degrees. The relationships between these mechanisms, the different macroalgal habitats, and carbonic anhydrase is discussed.