Photosynthesis in a green alga (zoochlorella), Chlorella cf. vulgaris in the soft coral Sarcophyton sp.

An undescribed variety of the green alga Chlorella cf. vulgaris was found living in the octocoral (soft coral) Sarcophyton sp., which also has dinoflagellate zooxanthellae. The green alga is present in very small numbers and is capable of photoheterotrophy based on growth in the presence of an inhibitor of photosynthesis, DCMU, with glycerol provided as a carbon source. The presence of zoochlorellae (Chl a + b), in soft corals appears to have been unsuspected. The easily cultured green alga was unicellular, unflagellated and very small, c. 2 to 6 mu m in diameter and had a relatively high Chl b content (Chl b/a = 0.379 +/- 0.063). Rapid light curves using PAM fluorometry on the cultured cells showed that Yield vs. irradiance had a low maximum yield (Y-max, 0.388 +/- 0.0132; Irradiance 1/2 point, E-1/2 Ymax = 139 +/- 11.1 mu mol photon m(-2) s(-1)). The overall optimum irradiance (E-opt) for the alga was 314 +/- 18.8 mu mol photon m(-2) s(-1). The alga can photosynthesize at 50% or more of the optimum rate from 70 to 900 mu mol photon m(-2) s(-1) but would be severely inhibited under full sunlight. Most naturally occurring low irradiance habitats may have a low median irradiance but are exposed intermittently to very high irradiances and so a relatively high E-opt is advantageous in minimizing photochemical damage. Like the green algal symbiont Elliptochloris marina of sea anemones, Chlorella cf. vulgaris is not exclusively marine: it grows better in freshwater medium where it has different photosynthetic characteristics (higher Y-max, E-opt and ETRmax). Chlorella cf. vulgaris appears to be capable of photoheterotrophy and it is not clear if it is a symbiont, a commensal or a parasite.