Photosynthesis and photosynthetic electron transport in the soft coral Sarcophyton spp

Abstract

Blue-diode Pulse Amplitude Fluorometry (PAM) and rapid light curves, fitted using the Waiting-in-Line equation, were used to measure photosynthetic electron transport rate (ETR) of the common soft coral Sarcophyton spp (Alcyoniidae, Cnidaria). Optimum irradiance (Eopt) of newly-collected material was 353 ± 28.9 µmol quanta m−2s−1 (≈18% sunlight, 400–700 nm), ½ optimum irradiance (E1/2−ETR) ≈82.0 µmol quanta m−2s−1 with substantial photoinhibition at higher irradiances. Maximum ETR (ETRmax) = 25.1 ± 1.28 µmol e– m−2s−1 (surface area basis) or 86.8 ± 4.44 µmol e– g−1 Chl a s−1 (Chl a basis). Due to strong midday inhibition total daily photosynthesis would be greater on cloudy days, during the overcast wet season and high tides. Open aquarium-grown material had Eopt ≈300 µmol quanta m−2s−1; ETRmax ≈21 µmol e– m−2s−1 or ≈50 µmol e– g−1 Chl a s−1. Chl a was ≈300 (field material) vs. ≈450 (aquaculture aquarium) mg m−2 but with no change in Chl c2/a ratio (0.521 ± 0.0181). Based on light/dark O2 electrode methods in aquaria Gross Photosynthesis (Pg) = 21.9 ± 6.85 µmol O2 g−1 Chl a s−1, Pnet = 15.1 ± 6.38 µmol O2 g−1 Chl a s−1; respiration −6.81 ± 2.23 µmol O2 g−1 Chl a s−1; P/R ratio = 3.72 ± 1.86 at an irradiance 192 ± 20.3 µmol quanta m−2s−1. The culturable dinoflagellate zooxanthellae material might not necessarily be representative of the resident zooxanthellae population. Their PAM characteristics were similar to the soft coral (Eopt ≈219 µmol quanta m−2s−1, ETRmax ≈138 µmol e– g−1 Chl a s−1), but the Chl c2/a ratio (≈0.16) was very different.