Bryozoans increase available CO2 for photosynthesis in Gelidium sesquipedale (Rhodophyceae)

A possible benefit of the presence of the epiphytic bryozoan Electra pilosa (L.) for the red macroalga Gelidium sesquipedale (Clem.) Thuret et Bornet is described. Absorption spectra and photosynthetic parameters of O-2 evolu tion vs. irradiance curves were determined for both epiphytized and nonepip hytized thalli. The absorptance of G. sesquipedale thalli for PAR was not m odified by the presence of the epiphyte. Gross photosynthetic rates at satu rating light were approximately doubled in epiphytized thalli. Photosynthes is by G. sesquipedale was enhanced when CO2 concentration was increased in the medium by a decrease in PH. On the other hand, an increase in pH from 8 .1 to 8.7 produced a significant reduction of the O-2 evolution rates indic ating that G. sesquipedale has a very low capacity to use HCO3-. The decrea se ill photosynthesis at high pH was higher in nonepiphytized thalli than i n epiphytized ones, suggesting that the amount of available CO2 is higher i n the presence off. pilosa. This positive effect was attributed to the CO2 released by respiration of the epiphyte.