The Rate of Air-Sea CO₂ Exchange: Chemical Enhancement and Catalysis by Marine Microalgae

Ben J.H.Matthews

PhD Thesis, October 1999

Abstract

Carbon dioxide transfers across the air-sea interface faster than inert gases, due to its reaction with water within the surface microlayer. This reaction may also be catalysed by the enzyme carbonic anhydrase, produced in situ by marine microalgae to aid photosynthetic carbon uptake.

Over 600 CO₂ transfer velocity measurements were made in a laboratory tank with multiple headspaces, which was designed to investigate steady-state CO₂ fluxes between air and seawater, both with and without marine microalgae.

Uncatalysed chemical enhancement was greater at higher temperatures, lower pCO₂ and lower stirring speeds, as expected. Simultaneous influx and efflux measurements also demonstrated the influence of air pCO₂ on microlayer pH. However, measured enhancements were greater than predicted by a reaction-diffusion model, particularly at higher [OH-]. These discrepancies would be resolved if the reaction between CO₂ and OH- were faster than previously reported. Adding bovine carbonic anhydrase increased the transfer velocity as predicted using enzyme kinetic data. However this catalysis declined rapidly, particularly at higher temperatures.

During four blooms of Dunaliella and one of Emiliana Huxleyi in the tank, enhancement was significantly greater than in equivalent conditions without algae, indicating catalysis by carbonic anhydrase produced in situ. Catalysis increased as water pCO₂ fell, indicating greater physiological demand for enzyme as predicted. No catalysis effect was observed for Amphidinium and Skeletonema, and the results for Phaeodactylum and Phaeocystis were inconclusive. After these blooms, films of algae sometimes developed on the water surface. Simultaneous measurements in different headspaces (with and without films) indicated a direct CO₂ flux from the air into the film, unrelated to the ΔpCO₂ gradient. This unexpected phenomenon deserves further investigation.

Monthly maps showing the effect of enhancement on the net global atmosphere-ocean CO₂ flux were calculated for 132 scenarios, using satellite windspeed, temperature, and chlorophyll data and modelled pCO₂ data. Generally uncatalysed enhancement is greatest in warm equatorial waters and therefore decreases the net air-sea CO₂ flux. However catalysis by microalgae increases the net air-sea CO₂ flux because both the physiological demand for carbonic anhydrase and the chlorophyll distribution are biased towards low pCO₂ regions. Thus intercorrelation with ΔpCO₂ can increase the global impact of such kinetic processes.