Two modes of bicarbonate utilization in the marine green macroalga Ulva lactuca

Abstract

ABSTRACTThe green marine macroalga Ulva lactuca L. was found to be able to utilize HCO3− from sea water in two ways. When grown in flowing natural sea water at 16°C under constant dim irradiance, photosynthesis at pH8.4 was suppressed by acetazolamide but unaffected by 4,4′‐diisothiocyanostilbene‐2,2′‐disulphonate. These responses indicate that photosynthetic HCO3− utilization was via extracellular carbonic anhydrase (CA) ‐mediated dehydration followed by CO2 uptake. The algae were therefore described as being in a ‘CA state’. If treated for more than 10 h in a sea water flow‐through system at pH9.8, these thalli became insensitive to acetazolamide but sensitive to 4,4′‐diisothiocyanostilbene‐2,2′‐disulphonate. This suggests the involvement of an anion exchanger (AE) in the direct uptake of HCO3−, and these plants were accordingly described as being in an ‘AE state’. Such thalli showed an approximately 10‐fold higher apparent affinity for HCO3− (at pH9.4) than those in the ‘CA state’, while thalli of both states showed a very high apparent affinity for CO2. These results suggest that the two modes of HCO3− utilization constitute two ways in which inorganic carbon may enter the Ulva lactuca cells, with the direct entry of HCO3−, characterizing the ‘AE state’, being inducible and possibly functioning as a complementary uptake system at high external pH values (e.g. under conditions conducive to high photosynthetic rates). Both mechanisms of entry appear to be connected to concentrating CO2 inside the cell, probably via a separate mechanism operating intracellularly.