S. Beer et M. Ilan, IN-SITU MEASUREMENTS OF PHOTOSYNTHETIC IRRADIANCE RESPONSES OF 2 RED-SEA SPONGES GROWING UNDER DIM LIGHT CONDITIONS, Marine Biology, 131(4), 1998, pp. 613-617
Photosynthetic responses to irradiance by the photosymbionts of the tw
o Red Sea sponges Theonella swinhoei (Gray) and Cliona vastifica (Hanc
ock) growing under dim light conditions were measured in situ tin Sept
ember 1997) using a newly developed underwater pulse amplitude modulat
ed (PAM) fluorometer. Relative rates of photosynthetic electron transp
ort (ETR) were calculated as the effective quantum yield of photosyste
m II (Y) multiplied with the photosynthetic photon flux (PPF). Photosy
nthesis versus irradiance (P-I) curves; obtained within minutes, showe
d that individual specimens of both sponges, growing under very low li
ght conditions, feature lower light saturation points as well as lower
maximal ETRs than individuals growing under higher light. Evaluations
of such curves using low irradiances of the actinic light source (20
to 130 mu mol photons m(-2) s(-1)) showed a general decrease in Y,with
a shoulder from the lowest irradiance applied till 20 to 30 mu mol ph
otons m(-2) s(-1). Point measurements yielded ETRs close to what could
be estimated from the P-I curves. These point measurements also revea
led good correlations between the diurnally changing ambient irradianc
es (1 to 50 mu mol photons m(-2) s(-1)) and average ETR values for bot
h species. Further analysis showed that although Y values varied consi
derably between the different point measurements, they did not decreas
e significantly with light under these very low irradiances. Therefore
, PPF rather than Y seems to determine the in situ diel photosynthetic
performance at the low ambient irradiances experienced by these spong
es.