PHOTOSYNTHESIS BY SYMBIOTIC ALGAE IN THE FRESH-WATER SPONGE, SPONGILLA-LACUSTRIS

Spongilla lacustris is a common freshwater sponge which becomes dark g reen at high illumination due to the presence of numerous symbiotic zo ochlorellae. Oxygen metabolism of Spongilla from a shallow Danish stre am was analyzed in relation to concentration of tissue chlorophyll, in cident light, and external CO2 concentration. Photosynthesis at light saturation increased linearly with chlorophyll content of the sponge. High light intensity was needed to saturate photosynthesis in green Sp ongilla because of strong light attenuation in the sponge tissue, wher eas isolated zoochlorellae in suspension saturated at low light. High CO2 concentration-substantially above air saturation-was also required to saturate photosynthesis of green Spongilla. We found that daily ne t production of Spongilla was low based on photosynthesis alone, and f iltration of particles was probably needed to support the prolific gro wth observed in the stream. The estimated net gain from symbiotic alga e could, however, substantially increase Spongilla growth in the light .