Rising pCO2 interacts with algal density to reversely alter physiological responses of Gracilaria lemaneiformis and Ulva conglobata

Abstract In nature growing of macroalgae increases their biomass densities, which alters the surrounding environments and in turn affects their photophysiologies. Photosynthetic performances were explored for two common and widely spread macroalgae species, a branched Gracilaria lemaneiformis (Rhodophyta) and a thin, sheet-like Ulva conglobata (Chlorophyta), responding to a matrix of biomass densities (1.0, 2.0 and 4.0 g L−1) and pCO2 (400 and 800 ppm). In all three biomass densities the photosynthetic capability of G. lemaneiformis was enhanced by elevated pCO2, but that of U. conglobata was reduced. The photosynthesis versus irradiance (P vs. E) curve-derived parameters [i.e., light-utilizing efficiency (α), saturating light irradiance (EK) and maximum photosynthetic rate (Pmax)] of G. lemaneiformis generally increased with increasing densities under both ambient and elevated pCO2, as did the dark respiration (Rd); but the α, EK, Pmax and Rd of U. conglobata decreased. The elevated pCO2 increased the α, EK and Pmax but decreased the Rd of G. lemaneiformis in all three densities, while the opposite effects occurred in the U. conglobata. Accordingly, the elevated pCO2 enhanced the integrated daily production of G. lemaneiformis by 62%, but reduced that of U. conglobata by 26%, indicating that rising pCO2 favors the photosynthesis of branched G. lemaneiformis but reduces that of sheet-like U. conglobata. We propose that the changes in algal density greatly mediate their physiological responses to increasing ambient CO2.