Continuous Measurements of the Free Dissolved CO(2) Concentration during Photosynthesis of Marine Plants: Evidence for HCO(3) Use in Chondrus crispus.

An experimental system consisting of a gas exchange column linked to an assimilation chamber has been developed to record continuously the free dissolved CO(2) concentration in seawater containing marine plants. From experiments performed on the red macroalga Chondrus crispus (Rhodophyta, Gigartinales), this measurement is in agreement with the free CO(2) concentration calculated from the resistance to CO(2) exchanges in a biphasic system (gas and liquid) as earlier reported. The response time of this apparatus is short enough to detect, in conditions of constant pH, a photosynthesis-caused gradient between free CO(2) and HCO(3) (-) pools which half-equilibrates in 25 seconds. Abolished by carbonic anhydrase, the magnitude of this gradient increases with decreasing time of seawater transit from the chamber to the column apparatus. But its maximum magnitude (0.35 micromolar CO(2)) is negligible compared to the difference between air and free CO(2) (11.4 micromolar CO(2)). This illustrates the extent of the physical limiting-step occurring at the air-water interface when inorganic carbon consumption in seawater is balanced by dissolving gaseous CO(2). The direction of this small free CO(2)/HCO(3) (-) gradient indicates that HCO(3) (-) is consumed during photosynthesis.