Bioaccumulation of silver-110m, cobalt-60, cesium-137, and manganese-54 by the freshwater algae Scenedesmus obliquus and Cyclotella meneghiana and by suspended matter collected during a summer bloom event

Laboratory experiments were done to assess 110mAg, 60Co, 137Cs, and 54Mn uptake by two phytoplankton species, the chlorophyte Scenedesmus obliquus and the small diatom Cyclotella meneghiana. Mn and Co were characterized by similar uptake kinetic rates, 20–30 d21, whatever the algal species, whereas depuration rates were 3–60 d21. Silver uptake and depuration rates were very high (144–293 d21). However, Cs accumulation and depuration were very slow, with kinetic constants of 0.6–5 d21. Mn, Co, and Ag were more strongly accumulated by C. meneghiana than S. obliquus and vice versa for Cs. To evaluate the extrapolation of the kinetic rates fitted for S. obliquus and C. meneghiana to natural conditions, suspended solids were also collected during a bloom event and contaminated. Radionuclide exchange between three distinct compartments among the suspended solids was modeled: the kinetic rates fitted for S. obliquus and C. meneghiana were used to represent chlorophyte and bacillariophyte contamination, whereas kinetic rates describing a third compartment were estimated when possible. A third compartment was evidenced only for Mn and Co, whereas, for Ag, the chlorophyte and bacillariophyte compartments were sufficient to describe the particulate phase. For Cs, algae kinetic rates could not be used, so a single compartment was fitted. These experiments confirm the low affinity of Cs for phytoplankton and the high bioavailability of Ag. In the case of Co and Mn, several processes acting simultaneously govern the contamination of natural suspended solids. Waterways constitute an important group of natural resources, habitats, and living organisms that need protection from various stress factors, including radionuclide releases. Freshwaters can receive low-level radioactive liquid wastes discharged from nuclear facilities under normal operating conditions or may be accidentally contaminated such as occurred during the Chernobyl accident. Algae play a key role in the radioactive contamination of freshwater ecosystems, as a point of entry of pollutants within trophic nets and because they can accumulate radionuclides very quickly to a high level. Algal blooms in eutrophic rivers may lead to high biomasses whose impact on the contamination of higher trophic levels in freshwater bodies, as well as on the radionuclide fluxes from the water column to the sediment compartment by sedimentation or from the continent to the sea, must be considered. The behavior of manganese during phytoplankton blooms has been extensively studied using 54Mn to assess the importance of oxidation processes in its biogeochemical cycle (Sunda and Huntsman 1987; Kudo et al. 1992; Moffett 1994; Schoemann et al. 1998). The biological control of other radionuclides partitioning between liquid and solid phases has been very poorly studied. Moreover, most of the studies were undertaken in nutritive media whose chemical composition was completely different from that of natural waters and with green algae, which are not always representative of natural phytoplankton populations. Diatoms, for instance, may rep-