In this study,the ability of macroalgae Gracilaria sp.of removing eutrophication factors and toxic heavy metals Al,Cr,and Zn in a closed cultivation system is reported.The results show that the concentration of the three heavy metals decreased significantly during the experimental period in an algal biomass dependent manner.The biofiltration capacity of the alga for Al,Cr,and Zn is 10.1%-72.6%,52.5%-83.4% and 36.5%-91.7%,respectively.Using more materials resulted in stronger heavy metal removal.Additionally,the concentration of chl-a,TN,TP and DIN of water samples from aquariums involving large,medium,and small algal biomass cultivation increased first and then decreased during the experiment.COD value of all three groups decreased with time and displayed algal biomass dependency:more algae resulting in a greater COD value than those of less biomass.Furthermore,changes in COD reflect an obvious organic particles deprivation process of algae.This is the first report on heavy metal removal effect by Gracilaria species.The results suggest that macroalgae can be used as a biofilter for the treatment of nutrient-enriched or heavy-metal polluted water,to which an appropriate time range should be carefully determined. In this study, the ability of macroalgae Gracilaria sp. Of removing eutrophication factors and toxic heavy metals Al, Cr, and Zn in a closed cultivation system is reported. Results show that the concentration of the three heavy metals decreased significantly during the experimental period in an algal biomass dependent manner. The biofiltration capacity of the alga for Al, Cr, and Zn is 10.1% -72.6%, 52.5% -83.4% and 36.5% -91.7%, respectively. Using more materials resulted in strong heavy metal removal . Additionally, the concentration of chl-a, TN, TP and DIN of water samples from aquariums involving large, medium, and small algal biomass cultivation increased first and then decreased during the experiment. COD value of all three groups decreased with time and displayed algal biomass dependency: more algae resulting in a greater COD value than those of less biomass. More advanced, changes in COD reflect an obvious organic particles deprivation process of algae. This is the first report on heavy metal remov al effect by Gracilaria species. The results suggest that macroalgae can be used as a biofilter for the treatment of nutrient-enriched or heavy-metal polluted water, to which an appropriate time range should be determined determined.