Photosynthetic utilisation of inorganic carbon by seagrasses from Zanzibar, East Africa

Photosynthetic rates of eight seagrass species from Zanzibar were limited by the inorganic carbon composition of natural seawater (2.1 mM, mostly in the form of HCO3-), and they exhibited more than three time higher rates at inorganic carbon saturation (>6 mM). 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 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-impermeable inhibitor of carbonic anhydrase, indicating that extracellular HCO3- 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 Syringodium isoetifolium, photosynthesis at 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 systems 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 the high, frequently air-exposed, intertidal zone may be related to a capability to take up HCO3- directly, since this is a more efficient way of HCO3- utilisation than extracellular HCO3- 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 CO2 levels.