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

Spongilla lacustris is a common freshwater sponge which becomes dark green at high illumination due to the presence of numerous symbiotic zoochlorellae. Oxygen metabolism of Spongilla from a shallow Danish stream was analyzed in relation to concentration of tissue chlorophyll, incident 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 Spongilla because of strong light attenuation in the sponge tissue, whereas 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 net production of Spongilla was low based on photosynthesis alone, and filtration of particles was probably needed to support the prolific growth observed in the stream. The estimated net gain from symbiotic algae could, however, substantially increase Spongilla growth in the light.