Enzyme activities suggest that the NAD-ME C4 type CCM exist in Ulva sp.

Abstract

Ulva sp. is a large intertidal species of green algae that has been a causative species in green tide in the Qingdao sea area of China since 2008. To explore the physiological basis of outbreaks, we investigated the photosynthetic carbon sequestration mechanism of Ulva sp. Growth and breeding capacity, chlorophyll fluorescence parameters, chlorophyll content, and enzyme activities of carbonic anhydrase and other important enzymes in the C4 pathway were measured when different inorganic carbon sources were provided (0, 400, 1500, 20,000 μL·L−1 of CO2 or HCO3−). When CO2 was absent (0 μL·L−1 CO2), chlorophyll fluorescence parameters [Fv/Fm, Y (II), Y(I)] and chlorophyll content decreased significantly, but NPQ parameters and MDH and PEPC enzyme activities increased significantly and PEPCK enzyme activity was not affected significantly. When HCO3− was added, chlorophyll content and NAD-ME enzyme activity decreased significantly. These results indicated that lack of CO2 could stimulate photoprotection of Ulva sp. containing decreasing the synthesis of chlorophyll and reducing the PSII reaction center of the open state. Ulva sp. may not have the PEPCK C4 type CCM and might use the NAD-ME C4 type CCM to tolerate the stress caused by the lack of CO2.