The different responses of growth and photosynthesis to NH4+ enrichments between Gracilariopsis lemaneiformis and its epiphytic alga Ulva lactuca grown at elevated atmospheric CO2.

We investigated how elevated CO 2 affects the responses of Gracilariopsis lemaneiformis and Ulva lactuca to NH 4 ⁺ enrichments. All algae were incubated under four nutritional conditions (zero addition, 100, 500, and 2500 μM NH 4 ⁺), and two CO 2 levels (390 ppm and 1000 ppm). The growth, photosynthesis, and soluble protein contents of both species increased under the eutrophication condition (100 μM NH 4 ⁺). However, the growth and carotenoid contents of the two species declined when NH 4 ⁺ concentration increased. Under the super eutrophication condition (2500 μM NH 4 ⁺), all indexes measured in G. lemaneiformis were suppressed, while the growth and photosynthesis in U. lactuca changed indistinctively, both compared with the control. Moreover, under the super eutrophication condition, elevated CO 2 reduced the suppression in the growth of G. lemaneiformis , but decreased the growth of U. lactuca. Nonetheless, G. lemaneiformis displayed much lower growth rates than U. lactuca under the super eutrophication and elevated CO 2 condition. • Responses of two algal species to NH 4 ⁺ and CO 2 enrichments were investigated. • 100 μmol L⁻¹ NH 4 ⁺ enhanced growth and photosynthesis in both algal species. • 2500 μmol L⁻¹NH 4 ⁺ was toxic to Gracilariopsis lemaneiformis but not Ulva lactuca. • Elevated CO 2 reduced the toxicity of 2500 μmol L⁻¹ NH 4 ⁺ to G. lemaneiformis. [ABSTRACT FROM AUTHOR]