The marine benthic green macroalga Caulerpa taxifolia is an invasive seaweed found in Europe, America, and Australia, and it forms into huge algal meadows on shallow seafloors with its stolon and rhizoid systems. It has bloomed along the coast of the South China Sea, causing serious environmental problems. However, its ecological impact has not been well studied. Therefore, this study investigated the changes in concentration of NH4-N and NO2-N in artificial seawater in which C. taxifolia was cultivated under laboratory conditions during the circadian rhythm. Results showed that concentrations of NH4-N and NO2-N decreased with the increase in culture time during the circadian rhythm. In 24 h, the NH4-N-removal efficiency increased during the light period, and the maximum reached 71.4%; that of NO2-N increased with time extension in the dark period, and the maximum reached 9.2%. The absorption of NH4-N and NO2-N by terminal stolon of C. taxifolia was different. NH4-N was absorbed more preferentially than NO2-N. However, there was no obvious correlation between NH4-N and NO2-N absorption. Therefore, the terminal stolon of C. taxifolia can be used to clean up inorganic nitrogen, and showed great application potential in the remediation of eutrophic waters as the algal-bacterial symbiotic system could facilitate NO2-N removal.

Codium fragile has attracted much attention due to its high economic and nutritional values. The light intensity and artificial aeration affect its growth and photosynthetic activity, which in turn affect its economic and nutritional values. The light intensities (30µmol·m− 2·s− 1, 60µmol·m− 2·s− 1, 90 µmol·m− 2·s− 1) and aeration regulation are investigated to the effect on the growth and photosynthetic physiology of C. fragile collected from the Bohai Sea, China. The results show that different light intensities have a highly significant effect on the maximal photochemical efficiency of PSII(Fv/Fm), photochemical quenching coefficient (qP), and a significant effect on the non-photochemical quenching coefficient (NPQ). They all decreased the least under 60µmol·m− 2·s− 1. The increase in the relative growth rate(RGR) of C. fragile during aeration was greater than that of the non-aeration group. At the same time, the Fv/Fm and qP decreased less than those of the non-aeration group. It shows that the aeration regulation had a highly significant effect on the wet weight, Fv/Fm and qP of C. fragile. Among the six groups, only aeration and light intensities of 60 and 90µmol·m− 2·s− 1 were suitable for the growth of C. fragile, because the Fv/Fm decreased less and the qP increased. The result shows that the interaction of the two environmental factors had a significant effect on the Fv/Fm and qP of C. fragile, while there was no significant effect on the wet weight and NPQ.