The effects of light intensity and temperature on the calcification rate of Halimeda macroloba

As a phototrophic organism the calcareous green alga Halimeda macroloba is considered an important source of primary production. These algae live in a range of environments and help sequester CO2 through photosynthesis and calcification in coastal marine ecosystems. This study examined the calcification rate of H. macroloba under various light (50, 500, 900, 1200 mu mol photons m(-2) s(-1)) and temperature (25, 30, 35 degrees C) conditions. The rates of calcification, photosynthetic inorganic carbon (Ci) uptake, and relative electron transport rate (rETR) were measured using alkalinity titration methods and pulse-amplitude modulated (PAM) fluorometry in an experimental setup based on observation data; additionally, a future climate change scenario was simulated. The light intensity of 500 mu mol photons m(-2) s(-1) promoted high calcification and Ci uptake rates at all temperatures, with the highest rates at 25 degrees C. The very low light intensity of 50 mu mol photons m(-2) s(-1) was not enough to stimulate plant photosynthesis and calcification. The rates of both calcification and Ci uptake were significantly lower at all temperatures when plants were subjected to a high irradiance of 1200 mu mol photons m(-2) s(-1) than those in the other light conditions. Photosynthetic rETR seems to be dependent on light intensity, but might not reflect the high production of plants under intense light conditions. Finally, we discuss how Halimeda could contribute to CO2 sequestration in response to climate change.