Based on $P_{1/2}$ values, relative affinities of alginate, polyguluronate, and polymannuronate for metal ions are, in order, as follows; 1) seaweed alginate: $Cu^{2+}$ > $Cd^{2+}$ > $Pb^{2+}$ > $Fe^{3+}$ >> $Zn^{2+}$ > $Sr^{2+}$ > $Ca^{2+}$ > $Co^{2+}$ >> $Cr^{6+}$ > $Mn^{2+}$ >> $Hg^{2+}$, $Mg^{2+}$, $Rb^+$, 2) polyguluronate: $Cd^{2+}$ > $Cu^{2+}$ > $Pb^{2+}$ > $Fe^{3+}$ >> $Ca^{2+}$ > $Sr^{2+}$, $Zn^{2+}$, $Co^{2+}$ >> $Mn^{2+}$ > $Cr^{6+}$ >> $Hg^{2+}$, $Mg^{2+}$, $Rb^+$, and 3) polymannuronate: $Cd^{2+}$, $Cu^{2+}$ > $Fe^{3+}$ > $Pb^{2+}$ > $Ca^{2+}$ > $Zn^{2+}$ > $Sr^{2+}$ > $Co^{2+}$ > $Cr^{6+}$ >> $Mn^{2+}$ >> $Hg^{2+}$, $Mg^{2+}$, $Rb^+$. Amounts of the metal ions, $Cd^{2+}$, $Cu^{2+}$, $Fe^{3+}$, $Pb^{2+}$, and $Zn^{2+}$, bound to 1 g of seaweed alginate, were measured as $363.5{\pm}45.0$, $226.3{\pm}9.2$, $1,299.4{\pm}$81.3, 500.7${\pm}$27.7, and 165.9${\pm}$11.4 mg, respectively. Amounts of the metal ions, $Cd^{2+}$, $Cu^{2+}$, $Fe^{3+}$, $Pb^{2+}$, and $Zn^{2+}$, bound to 1g of polyguluronate, were 354.5${\pm}$26.5, 177.6${\pm}$8.7, 1,288.6${\pm}$60.1, 424.0${\pm}$7.4, and 140.2${\pm}$28.5 mg, respectively, whereas those bound to 1 g of polymannuronate were 329.0${\pm}$10.3, 206.9${\pm}$1.9, 1,635.6${\pm}$11.1, 419.8${\pm}$12.6, and 251.0${\pm}$49.1 mg, respectively. Due to its higher solubility than alginate and higher affinity for metal ions than polyguluronate, polymannuronate can be used for bioremediation or biosorption of toxic and/or noble metal ions.