Abstract:To investigate the effects of increased atmospheric CO2 and N supply in seawater on the red alga Gracilaria lemaneiformis, the thalli were incubated in laboratory for 15 days under 4 different conditions of CoNo, C+No, CoN+ and C+N+ respectively, then the growth, nutrient uptake rates, contents of pigments and soluble proteins in alga were measured. The results show that either increased atmospheric CO2 or N supply can enhance the relative growth rate (RGR) of G. lemaneiformis, but they have no synergic effect. Elevated CO2 decreases, but N supply increases, the contents of pigments and soluble proteins. Increased atmospheric CO2 reinforces the activity of nitrate reductase (NRA) under the conditions of both enriched and unenriched N supply in G. lemaneiformis but stimulates the uptake of nutrient only in case of high N supply. In any case of CO2 concentration, enriched N supply improves the nutrient uptake rates of G. lemaneiformis. It is concluded that the growth ，NRA and nutrient uptake of the red alga G. lemaneiformis would be enhanced and the contents of pigments and souble proteins be changed by increased atmospheric CO2 or seawater eutrophication in future.