Effects of Dissolved Inorganic Carbon and Oxygen Content on the Photosynthetic Characteristics of two Common Tropical Seagrasses Halophila ovalis (R. Br.) Hook. and Thalassia hemprichii (Ehrenb.)

Abstract

We investigated the effects of dissolved inorganic carbon (DIC) and O2 concentrations on photosynthetic characteristics of two common tropical seagrasses, Halophila ovalis and Thalassia hemprichii. In the first series of experiments, leaf segments were incubated in 2.2 mM DIC or DIC-free media. Relative electron transport rates (rETR), non-photochemical quenching (NPQ), and maximum quantum yield of photosystem II (Fv/Fm) were measured under saturating irradiance at air-equilibrated O2 (~300 µM), 3 % of air-equilibrated O2 (~10 µM), and finally at restored air-equilibrated O2 (~300 µM). DIC limitation reduced rETR and increased NPQ, while low O2 reduced rETR and Fv/Fm, indicating that both carbon assimilation and O2 can act as electron sinks. In the second series of experiments, leaf segments were exposed to irradiance of either 400, 1200, 2000 or 2800 μmol photons·m-2·s-1 for 40 min in four different conditions: 1) 2.2 mM DIC, air-equilibrated O2; 2) DIC-free, air-equilibrated O2; 3) 2.2 mM DIC, 3 % of air-equilibrated O2; and 4) DIC-free, 3 % of air-equilibrated O2.  Photoinhibition due to high light exposure was detected; however, no significant effect of DIC or O2 concentration was observed. This result indicates that alternative electron flow was not a crucial component of photoprotective mechanisms at high irradiance.