Pj. Ralph et al., In situ underwater measurements of photosynthetic activity of coral zooxanthellae and other reef-dwelling dinoflagellate endosymbionts, MAR ECOL-PR, 180, 1999, pp. 139-147
Chlorophyll fluorescence was used to assess the iu situ photosynthesis of a
range of reef-dwelling endosymbionts. Such non-intrusive in situ measureme
nts became possible after the recent development of a submersible pulse mod
ulated fluorometer (DIVING-PAM, Walt, Germany). Several corals, a clam (Tri
dacna maxima) and an anemone (Heteractis rip.) all showed strong chlorophyl
l a fluorescence signals originating from the dinoflagellate endosymbionts.
Quenching analysis by the saturation pulse method revealed high quantum yi
elds and Light response curves characteristic of physiologically healthy su
n plants. Rapid light curves (RLC) were applied to assess the light saturat
ion behaviour of the different organisms in their rapidly changing natural
environment. The 3 corals (Acropora aspera, Goniastrea sp. and Porites sp),
the clam T. maxima, and the anemone Heteractis sp. all showed high photosy
nthetic activity. The corals had a maximum electron transport rate of 180 t
o 270 mu mol electrons m(-2) s(-1), the Heteractis sp, displayed a maximum
rate of approximately 120 mu mol electrons m(-2) s(-1), whilst the T. maxim
a showed no saturation up to 1900 mu mol quanta m(-2) s(-1), where a rate o
f 325 mu mol electrons m(-2) s(-1) was observed. Three species of corals sh
owed varying degrees of an apparent mid-day depression, occurring during th
e summer peak irradiance at low tide. Because quantum efficiency rapidly re
covered during the afternoon period, when the irradiance levels decreased a
gain, it is concluded from these preliminary investigations that the depres
sion resulted mainly from down-regulation of photosystem II. Depth did not
appear to influence the RLCs of A. aspera growing at 2, 5 and 10 m. Coral m
orphology influenced the apparent electron transport rate at different loca
tions within a single colony.