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.