Elevated CO2 exacerbates competition for growth and photosynthesis between Gracilaria lemaneiformis and Ulva lactuca

Abstract

Marine macroalgae with the same ecological niche generally have the same demands for various environmental factors, resulting in interspecific competitions among the algae. Because of the postulated global increase in CO 2 levels, co-existence of maricultured algae and native species seems like an inevitable challenge. In this study, comparative and interactive growth and photosynthetic responses of the maricultured macroalga Gracilaria lemaneiformis (Rhodophyta) and a common native epiphyte Ulva lactuca (Chlorophyta) to elevated CO 2 levels were investigated, in order to estimate the interspecific competition between these two algae. The results showed that U. lactuca exhibited a relatively higher growth rate and maximum net photosynthetic rate than G. lemaneiformis . Growth and photosynthesis of G. lemaneiformis were inhibited in the presence of U. lactuca . This inhibition of G. lemaneiformis was more severe when grown under elevated CO 2 levels. When U. lactuca was bicultured, elevated CO 2 levels caused a significant increase in light harvesting efficiency and maximum quantum yield. Moreover, U. lactuca showed relatively higher pigment contents and a higher correlation between chlorophyll a and maximum relative electron transport rates, indicating a relatively higher efficiency in light utilization. The interspecific competition between G. lemaneiformis and U. lactuca was most probably due to the rapid growth of U. lactuca . We believe that, on the basis of the predicted scenario of elevated CO 2 levels, the epiphyte U. lactuca would presumably have greater negative impacts on G. lemaneiformis maricultivation; therefore, more labor would be required to reduce the epiphytization of this species during G. lemaneiformis mariculture. • We investigated the comparative and interactive responses of G. lemaneiformis and U. lactuca to elevated CO 2 level. • We found U. lactuca exhibited a relatively higher growth rate and maximum net photosynthetic rate than G. lemaneiformis . • Growth and photosynthesis of G. lemaneiformis were inhibited in biculture, and this was more severe under elevated CO 2 level. • U. lactuca would presumably have greater negative impacts on G. lemaneiformis maricultivation in the scenario of CO 2 rising.