CO2 Storage and CO2 Concentrating in Brown Seaweeds II. Function in Gas Phase

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CO2 concentrating occur in a great number of vascular plants, both limnetic and terrestric, as well as in several both limnetic and marine microalgae. Using the terminology of Raven et al. (1), biochemical CO2 concentrating predominates in vascular plants, while biophysical CO2 concentrating is the only kind hitherto found in microalgae. Very few direct measurements of CO2 concentrating have been carried out on marine macroalgae (2). HCO3 − is frequently utilized by several littoral marine macroalgae (3,4), and biophysical CO2 concentrating is probably widespread in green macroalgae (5,6). Although evidence for HCO3 − uptake has been presented for Rhodymenia palmata (7), the mechanism for HCO3 − utilization in red macroalgae is very little investigated. In general, HCO3 − utilization, on a surface basis, appears to be much less efficient in littoral red macroalgae than in littoral brown and green macroalgae (8). Also from the results of pH drift experiments (3) different mechanisms for HCO3 − utilization between littoral red, green and brown macroalgae has been suggested. These data as well as recent investigations on Chondrus crispus (9), may be interpreted as if red algae rely on carbonic anhydrase (CA) for their HCO3 − utilization rather than on any CO2 concentrating mechanism, at least their CO2 concentrating is different from that of green and brown macroalgae. A biochemical CO2 concentrating mechanism similar to that of the C4 metabolism of certain terrestrial plants, has repeatedly been suggested for littoral algae (10,11), but has not been possible to verify in pulse chase experiments (12).