Use of pulse amplitude modulated (PAM) ̄uorometry for in situ measurements of photosynthesis in two Red Sea faviid corals

Measurements of the photosynthetic activity of symbiotic zooxanthellae in corals under natural growth conditions has been limited until recently, and this is one of the ®rst reports on utilising a newly developed underwater pulse amplitude modulated (PAM) ̄uorometer (the Diving-PAM, Walz Gmbh, Germany) for such studies in situ. Photosynthetic responses to irradiance (photosynthetic photon ̄ux, PPF) of the two faviid corals Favia favus (ForskaÊ l) and Platygyra lamellina (Ehrenberg) were measured while snorkelling or SCUBA diving (in August 1997), and we report here the results in terms of e€ective quantum yields of photosystem II (Y ) and estimated rates of photosynthetic electron transport (ETR, calculated as Y ́ 0.5 ́ PPF ́ FA, where FA is the estimated fraction of light absorbed by the photosymbiont-containing tissue). Both species showed a reduction in Y with increasing actinic irradiances produced by the instrument above 500 lmol photons m s, and the corresponding ETR values yielded apparently typical photosynthesis versus irradiance (P-I ) curves, which saturated between 1500 and 2000 lmol photons m s. It was found that 30 s irradiation at each PPF level was sucient to give optimal ETR values and, therefore, each P-I curve could be obtained within a few minutes. In situ point measurements from various areas of colonies under ambient light showed average ETR values within the range expected from the P-I curves. In order to test the DivingPAM in an eco-physiologically relevant experiment, photosynthetic ETR versus PPF was measured for three sections of a large P. lamellina, each section of which received di€erent natural irradiance levels. The results clearly demonstrated adaptations to the ambient light ®eld in that vertical and downward-facing portions of the colony showed gradually lower maximal ETRs, steeper initial slopes of the P-I curves and, accordingly, lower light saturation points than upward-facing areas receiving higher light levels. Based on these trials, some evaluations are given as to the applicability of the Diving-PAM for photosynthetic measurements when monitoring similar corals.