Photosynthetic rates of eight seagrass species from Zanzibar were limited by the inorganic carbon composition of natural seawater (2.1 m M , mostly in the form of HCO 3 − ), and they exhibited more than three time higher rates at inorganic carbon saturation (>6 m M ). The intertidal species that grew most shallowly, Halophila ovalis , Halodule wrightii and Cymodocea rotundata , showed the highest affinity for inorganic carbon ( K 1/2 = ca. 2.5 m M ), followed by the subtidal species ( K 1/2 > 5 m M ). Photosynthesis of H. wrightii , C. rotundata , Cymodocea serrulata and Enhalus acoroides was >50% inhibited by acetazolamide, a membrane-impermeable inhibitor of carbonic anhydrase, indicating that extracellular HCO 3 − dehydration is an important part of their inorganic carbon uptake. Photosynthetic rates of H. wrightii , Thalassia hemprichii , Thalassodendron ciliatum , C. serrulata and E. acoroides were strongly reduced by changing the seawater pH from 8.2 to 8.6 in a closed system. In H. ovalis , C. rotundata and Syringodiumisoetifolium , photosynthesis at pH 8.6 was maintained at a higher level than could be caused by the ca. 30% CO 2 concentration which remained in the closed experimental systems at that pH, pointing toward HCO 3 − uptake in those species. It is suggested that the ability of H. ovalis and C. rotundata to grow in the high, frequently air-exposed, intertidal zone may be related to a capability to take up HCO 3 − directly, since this is a more efficient way of HCO 3 − utilisation than extracellular HCO 3 − dehydration under such conditions. The inability of all species to attain maximal photosynthetic rates under natural conditions of inorganic carbon supports the notion that seagrasses may respond favourably to any future increases in marine CO 2 levels.
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