IN-SITU MEASUREMENTS OF PHOTOSYNTHETIC IRRADIANCE RESPONSES OF 2 RED-SEA SPONGES GROWING UNDER DIM LIGHT CONDITIONS

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.