Carbon acquisition strategies of the red alga Eucheuma denticulatum

Several species of Eucheuma (Rhodophyta) are commercially important and are cultivated in many places around the world for their content of carrageena n. In the present study a wild, native strain of E. denticulatum from Zanzi bar, Tanzania, was investigated for the presence of an external carbonic an hydrase (CA) and a potential membrane-bound HCO3- transport protein. The algae were brought to Sweden and cultivated under laboratory conditions . Photosynthetic activity was measured by observing changes in the pH of th e media. The presence of CA and a HCO3- transport protein was investigated using the inhibitors acetazolamide (AZ) and 4,4'-diisothiocyanatostilbene-2 ,2'-disulphonic acid (DIDS), respectively. The results indicate that Eucheu ma denticulatum has both an active external CA which was inhibited by the a ddition of AZ and a DIDS sensitive mechanism for anion exchange across the cell membrane. Both inhibitors could be washed away and the algae regained their full photosynthetic capacity. When AZ was washed away, the rate of pH increase was higher than in the control indicating the induction of carbon acquisition systems during the AZ treatment. The results also suggests tha t external CA dehydrates HCO3- to CO2 below a pH of 8.5-9.0. Above this pH, direct uptake of HCO3- is required. The presence of both of these mechanis ms for utilization of HCO3- are essential for the growth of Eucheuma which normally encounters low concentrations of CO2 in natural sea water. This is the first report on an active DIDS sensitive HCO3- transport mechanism in a red alga, which needs no induction.