PHOTOSYNTHETIC UTILIZATION OF INORGANIC CARBON BY SEAGRASSES FROM ZANZIBAR, EAST-AFRICA

Photosynthetic rates of eight seagrass species from Zanzibar were limi ted by the inorganic carbon composition of natural seawater (2.1 mM, m ostly in the form of HCO3-), and they exhibited more than three time h igher rates at inorganic carbon saturation (>6 mM). The intertidal spe cies that grew most shallowly, Halophila ovalis, Halodule wrightii and Cymodocea rotundata, showed the highest affinity for inorganic carbon (K-1/2 = ca. 2.5 mM), followed by the subtidal species (K-1/2 > 5 mM) . Photosynthesis of H. wrightii, C. rotundata, Cymodocea serrulata and Enhalus acoroides was > 50% inhibited by acetazolamide: a membrane-im permeable inhibitor of carbonic anhydrase, indicating that extracellul ar HCO3- dehydration is an important part of their inorganic carbon up take. Photosynthetic rates of H. wrightii, Thalassia hemprichii, Thala ssodendron ciliatum, C. serrulata and E. acoroides were strongly reduc ed by changing the seawater pH from 8.2 to 8.6 in a closed system. In H. ovalis, C. rotundata and Syringodium isoetifolium, photosynthesis a t pH 8.6 was maintained at a higher level than could be caused by the ca. 30% CO2 concentration which remained in the closed experimental sy stems at that pH, pointing toward HCO3- uptake in those species. It is suggested that the ability of H. ovalis and C. rotundata to grow in t he high, frequently air-exposed, intertidal zone may be related to a c apability to take up HCO3- directly, since this is a more efficient wa y of HCO3- utilisation than extracellular HCO3- dehydration under such conditions. The inability of all species to attain maximal photosynth etic rates under natural conditions of inorganic carbon supports the n otion that seagrasses may respond favourably to any future increases i n marine CO2 levels.