Blue-diode Pulse Amplitude Fluorometry (PAM) was used to measure photosynthetic electron transport rate (ETR) of the soft corals Sarcophyton spp and Sinularia spp (Alcyoniidae, Cnidaria). Optimum irradiance (Eopt) of newly-collected material was ≈ 400 µmol quanta m-2 s-1 (≈ 18% sunlight, 400-700 nm) with substantial photoinhibition at higher irradiances. Maximum ETR (ETRmax) ≈ 100 µmol e- g-1 Chl a s-1. Open air aquarium-grown material had Eopt ≈300 µmol quanta m-2 s-1; ETRmax ≈ 50 µmol e- g-1 Chl a s-1. Based on light/dark O2 electrode methods in aquaria Gross Photosynthesis (Pg) and respiration was also measured. The PAM characteristics of the culturable zooxanthellae were similar but the Chl c2/a ratio was very different. An undescribed variety of the green alga Chlorella cf. vulgaris was also found living in the octocoral (soft coral) Sarcophyton sp. Its photosynthetic characteristics are comparable to the dinoflagellate zooxanthellae but 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. It is not clear if Chlorella cf. vulgaris is a symbiont, commensal or parasite. A similar green algal symbiont was not found in Sinularia.