15 PER CENT CALCIUM CARBONATE 100 QO e |Of—E bere cn ~ | OO — a oO Kk aq a 10.0 i NOlLVWHiN3ZINOD -— 100 WNidved 4O 10.0 1.0 = SAILVTSEY > 3 Lor oO . }— 2 a <a [ | YSAIVMVSAS OL = |} PETT | {| j oe JU itt 0.1 0.0! o ° S MTTEEMaMa, ide a ee ee el CALCIUM O CONCENTRATION (gm/gm) Fic. 10.—The variation in the [Ra]/[Ca] ratio with the degree of calcification of the algae ilarly, the concentration factors for thorium (Figure 11) are always greater than 1.0 and are higher than those found for radium. In contrast, the concentration factors for uranium relative to calejum from seawater are most frequently less than 1.0 (Figure 12), but do inerease slowly to a maximum of 10.0 when the algae contain less than 10% calcium carbonate. From these data it may be concluded that there are two mechanisms involved in the concentration of ra- dium, thorium, and uranium by these algae. These are (1) ion exchange or coprecipitation of the ion with the calcium carbonate matrix, and (2) some form of complex formation with either the protein nitrogen or some other componentof the organic fraction. The concentration of radium and thorium appears to be related more to the latter than the former since in general the [M]/[Ca] ratio increases with decreasing concentration of calcium carbonate in the organism. In the case of radium, the variation in the [Ra]/[Ca] ratio is very obviously dependent on phyta (Figure 10). We have already considered in detail the be- havior of the highly calcified members of the Chloro- phyceae and concluded that there the concentration of radium is due to mechanism (1). Similarly, it has been shown that members of the Rhodophyceae con- centrate radium proportionally to their calcium con- tent (Figure 7), but their [Ra]/[Ca] ratios are significantly higher than those, for example, of Halimeda opuntia, which suggests that some of the radium is being complexed by the organic fraction of the orga- nism. The concentration of radium by the Phaeophyceae has been shown to be proportional to the concentration of protein nitrogen and is, therefore, largely fixed by organic complexation. The behavior of radium is consistent with its chem- ical properties in seawater. Under normal conditions In seawater radium is almost certainly present as its free ion because, as its concentration is so low Oe