USE OF PULSE AMPLITUDE-MODULATED (PAM) FLUOROMETRY FOR IN-SITU MEASUREMENTS OF PHOTOSYNTHESIS IN 2 RED-SEA FAVIID CORALS

Measurements of the photosynthetic activity of symbiotic zooxanthellae in corals under natural growth conditions has been limited until rece ntly, and this is one of the first reports on utilising a newly develo ped underwater pulse amplitude modulated (PAM) fluorometer (the Diving -PAM, Walt Gmbh, Germany) for such studies in situ. Photosynthetic res ponses to irradiance (photosynthetic photon flux, PPF) of the two favi id corals Favia favus (Forskal) and Platygyra lamellina (Ehrenberg) we re measured while snorkelling or SCUBA diving tin August 1997), and we report here the results in terms of effective quantum yields of photo system II (Y) and estimated rates of photosynthetic electron transport (ETR, calculated as Y x 0.5 x PPF x FA, where FA is the estimated fra ction of light absorbed by the photosymbiont-containing tissue). Both species showed a reduction in Y with increasing actinic irradiances pr oduced by the instrument above 500 mu mol photons m(-2) s(-1), and the corresponding ETR values yielded apparently typical photosynthesis ve rsus irradiance (P-I) curves, which saturated between 1500 and 2000 mu mol photons m(-2) s(-1). It was found that 30 s irradiation at each P PF level was sufficient to give optimal ETR values and, therefore, eac h P-I curve could be obtained within a few minutes. In situ point meas urements from various areas of colonies under ambient light showed ave rage ETR values within the range expected from the P-I curves. In orde r to test the Diving-PAM in an eco-physiologically relevant experiment , photosynthetic ETR versus PPF was measured for three sections of a l arge P. lamellina, each section of which received different natural ir radiance levels. The results clearly demonstrated adaptations to the a mbient light field in that vertical and downward-facing portions of th e colony showed gradually lower maximal ETRs, steeper initial slopes o f the P-I curves and, accordingly, lower light saturation points than upward-facing areas receiving higher light levels. Based on these tria ls, some evaluations are given as to the applicability of the Diving-P AM for photosynthetic measurements when monitoring similar corals.