The mechanism of iodide accumulation by the brown sea weedLaminaria digitataThe uptake of131I

Abstract

Laminaria digitata frond rapidly absorbs radio-active iodine from aerated sea water. The absorption rate remains substantially constant throughout experiments lasting 2 or 3 h. After an hour the weed shows about thirty times as much activity as an equal volume of the surrounding sea water. The uptake rate is reduced if inactive iodide be added to the sea water to give a concentration of $10^{-4}$ $\text{M}$. The further observations support the view that iodide uptake proceeds by: (1) the oxidation of iodide to $\text{I}_{2}$, (2) the diffusion of $\text{I}_{2}$ (or its hydrolysis product HIO) into the tissues, and finally (3) the reduction of the penetrant $\text{I}_{2}$, or HIO within the tissues. Iodide added to aerated sea water is rapidly oxidized to $\text{I}_{2}$ in the presence of the weed and the agents and conditions mentioned in the succeeding paragraph as slowing or halting radio-iodine uptake have also been shown to prevent elementary iodine being formed. $^{131}\text{I}$ absorption is slowed or halted by thiocyanate, thiosulphate, metabisulphite, catechol, tyrosine and pyruvate in concentrations of $1-3\times 10^{-3}$ $\text{M}$. The full effect of these inhibitors persists only so long as they are present in the sea water about the weed. If an excess of inactive iodide be added to sea water, or if the weed be kept in the dark, then an atmosphere of $\text{N}_{2}$ interrupts the uptake of $^{131}\text{I}$; darkness alone, however, has no immediate influence on radio-iodine uptake. When elementary iodine rather than iodide be added to weed maintained in the dark under $\text{N}_{2}$ a rapid absorption of activity is initiated. After the weed has been placed in iodide-enriched sea water, analyses indicate that about three-quarters of the element lost from the water occurs in the tissues as iodide.