S. Beer et al., USE OF PULSE AMPLITUDE-MODULATED (PAM) FLUOROMETRY FOR IN-SITU MEASUREMENTS OF PHOTOSYNTHESIS IN 2 RED-SEA FAVIID CORALS, Marine Biology, 131(4), 1998, pp. 607-612
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.