Photosynthetic utilisation of carbon and light by two tropical seagrass species as measured in situ

In situ measurements of seagrass photosynthesis in relation to inorganic ca rbon (Ci) availability, increased pH and an inhibitor of extracellular carb onic anhydrase were made using an underwater pulse amplitude modulated (PAM ) fluorometer. By combining the instrument with a specially designed Perspe x chamber, we were able to alter the water surrounding a leaf without remov ing it from the growing plant. Responses to Ci within the chamber showed th at subtidal plants of the seagrasses Cymodocea serrulata and Halophila oval is had photosynthetic rates that were limited by the ambient Ci concentrati on depending on the irradiance that was available during short-term photosy nthesis-irradiance trials. Relative electron transport rates (RETRs) at lig ht saturation (up to 500 mu mol photons m(-2) s(-1)) increased by 66-100% w hen the Ci concentration tion was increased from ca. 2.2 to 6.2 mM. On the other hand, intertidal plants of the same species exhibited a much lesser l imitation of photosynthesis by Ci at any irradiance (up to 1500 mu mol phot ons m(-2) s(-1)). Both species were able to use HCO3- efficiently, and ther e was stronger evidence for direct uptake of HCO, rather than extracellular dehydration of HCO3- to CO2 prior to Ci uptake. Subtidally, H. ovalis and C. serrulata grew to 10 and 12 m, respectively, where ambient irradiances w ere approximately 16 and 11% of those at the surface. Maximum RETRs (at lig ht saturation) were lower for these deep-growing plants than for the intert idally growing ones. For both species, the onset of light saturation of pho tosynthesis (E-k) occurred at approximately 100 mu mol photons m(-2) s(-1) for the deep water populations, which was four and two times lower than for the shallow populations of C. serrulata and H. ovalis, respectively. This, and the differences in maximal photosynthetic rates (RETRmax), reflects an acclimation of the deep-growing populations to the lower light environment . The results presented here show that photosynthesis, as measured in situ, was limited by the availability of Ci for the deeper growing plants in Zan zibar, while the intertidally growing plants photo synthesised at close to Ci saturation. The latter result is contrary to previous conclusions regard ing Ci limitations for these intertidal plants, and, in general, our findin gs highlight the need for performing similar experiments in situ rather tha n under laboratory conditions.