High levels of natural radionuclides in a deep-sea infaunal xenophyophore

Abstract

While debate continues about the disposal of man-made radioactive wastes in the deep sea, one deep-sea rhizopod is storing naturally occurring radionuclides to high concentrations in its home centimetres below the floor of the Izu–Ogasawara Trench. The hadal infaunal xenophyophore Occultammina profunda fills its test, a network of sediment tubes, with spherical micro-packages of waste (stercomes) containing 450–520 d.p.m. per g dry wt of the natural radionuclide ²¹⁰Pb. The stercomes are stored in flimsy membrane tubes immediately next to a thread-like protoplasmic body (granellare), which has equally high ²¹⁰Pb levels. A model of stercome excretion suggests that O. profunda grows quickly and can produce transient subsurface peaks in the vertical distribution of ²¹⁰Pb in the sediment while causing little, if any, sediment mixing, a finding which could have profound consequences for the ²¹⁰Pb-modelling of deep-sea bioturbation (mixing of sediments by organisms). From the barium content of the granellare (21,000 parts per million, p.p.m.) and assuming that the ²²⁶Ra/Ba ratio in the surrounding sediment and sea water is maintained in the organism, it is inferred that the protoplasmic ²¹⁰Pb is supported by 320–350 d.p.m. per g dry wt of ²²⁶Ra concentrated in intracel-lular barite crystals. Consequently, O. profunda and xeno-phyophores in general are probably subject to unusually high levels of natural radiation (several Sv yr⁻¹), among the highest reported for any living organism.