DIFFERENTIAL ABILITY OF MARINE AND FRESH-WATER MACROPHYTES TO UTILIZE HCO3- AND CO2

Six marine macroalgae and 2 angiosperms [Ceramium rubrum, Polysiphonia violaceae, Fucus vesiculosus, Chaetomorpha linum, Ulva lactuca, Enteromorpha sp., Ruppia maritima, Zostera marina] were examined for their ability to use HCO3- and CO2 for photosynthesis. All species used HCO3- despite wide taxonomic differences. They also used HCO3- with high affinity; natural HCO3- concentrations (2.2 mM) were close to saturation and the apparent half-saturation constants were low, i.e., K1/2 (HCO3-) = 0.54 to 0.80 mM HCO3-. Expressed as a ratio, the affinity for CO2 relative to HCO3- under rate-limiting concentrations was .apprx. 2 for the marine species compared to much higher values, 5.4-101, among freshwater species examined previously. This difference was due to a higher affinity for HCO3- among marine species; the affinity for CO2 was in the same range for marine and freshwater species, i.e., K1/2 (CO2) = 0.08-0.30 mM. The high affinity for HCO3- of marine species is consistent with the high and constant availability of HCO3- and the low availability of CO2 in seawater. In freshwaters, availability of HCO3- and CO2 varies considerably, depending on habitat. The difference in HCO3- affinity may be due to different electrogenic ion-pumps and thus mechanisms of HCO3- use, operating in freshwater and marine species. Photosynthetic rates in natural seawater were close to maximum at atmospheric equilibrium (pH 8.2-8.4) and declined at high pH approaching 0 between pH 9.5-10.5. This reduction may be due to the conversion of HCO3- to CO3-, followed by CaCO3 precipitation, and/or to a direct pH-effect. U. lactuca and Enteromorpha sp. retained photosynthetic activity at the highest pH tested (10.5), consistent with the high pH recorded in mats of these species.