and Spitsyn, 1974a, 1974b; Giesy and Paine, 1977a). Cycling processes and biological uptake of Pu must be understood prior to environmental releases so that rational assessments of its hazards can be determined. Accumulation of plutonium by algae and bacteria is dependent upon the aqueous forms of plutonium (Andelman and Rozzell, 1970; Noshkin, 1972). While much is known about plutonium separations chemistry, little is’ known about the environmental chemistry and speciation of plutonium (Noshkin, 1972). Francis (1973) stated that chelation with naturally occurring organic soil components may be important in the movement of transuranics in natural food chains leading to man. Szalay (1964) suggested that humic acids be used in the disposal of radioactive wastes because these organic compounds immobilize radionuclides. Pillai and Mathew (1976) studied the effects of humics on Pu solubilization in sea water but emphasized that further information is needed on the chemical behavior and influence of humics on Pu behavior in water. Routson et al. (1976) reported that Put4 tends While Pu-humic interactions have been investigated Routson, etal., 1975; 1976), no studies have considered the effect ofhumics and fulvics on the availability of plutonium to aquatic biological systems. Recalcitrant, naturally occurring organic compounds, variously known as humics, fulvics, and tannics are of worldwide distribution in soil and aquatic systems. Because of their ubiquity, these large polyphenolic compounds are involved in the chemical processes of nearly all surface waters (Schnitzer and Khan, 1972). Organic ligans, which can form complexes, with metals are important in determining the form, movement, and availability of trace metals in natural waters (Rashid and Leonard, 1973; density of 4.5 x 10° cells + mi7l (K' = 3.6 day!) to make an density of 5 x 10° celis + mi~! in experimental flasks. Andelman, 1974; Pittwell, 1974; Jackson, 1975; Giesy, 1976). The distribution of organic carbon between various nominal diameter fractions varies spatially and temperally. Because of this variability in nominal diameter of organics an understanding of the effects of the various sized fractions must be obtained instead of total organic ligand. The surface waters of the Southeast are soft and contain high concentrations of refractory organic compounds. This coupled with the large number of present and projected nuclear power plants and nuclear fuel production and reprocessing facilities for the region makes crucial the understanding of Pu cycling and fluxing processes in aquatic ecosystems. The objective of this study was to determine the effects of various sized organics on Pu uptake by an algal and bacterial species. A. hydrophila were incubated with 237put4 for 6 hr on a rotary shaker at 200 rpm. Algal and bacterial cell densities were determined, using calibrated phytoplankton (0.1 ml) and Petroff-Hausser counting chambers respectively. Plutonium-237 was obtained from Oak Ridge National Laboratory and assayed by the Savannah River Laboratory. Plutonium was prepared as 23?pyt4 (Table 1). Plutonium stocks (1.1 x 1072 uci + ml)were stored in 0.5 N HNO3 in polyethylene. Water samples were collected from Skinface Pond, Aiken, Co., South Carolina. Particulates of nominal diameter > 0.15 um were removed and the remaining dissolved and colloidal constituents fractionated and concentrated by membrane ultrafiltration (Giesy and Briese, 1977; Giesy and Paine, 1977b). Uptake experiments were conducted in One milliliter of 237put4 100 ml medium in flasks. (bacterial The volume of NaOH required to adjust the experimental solutions containing organics and 237pyt4 spike had been previously determined. Present in experimental media at Organics were the concentration at which they were found in nature. The resulting experimental media contained 1.1 x 10-4 yi 237 py +4 at pH 4.5. Solutions were mixed and allowed to stand 5 min before algal or bacterial inoculations. Algae and bacteria were separated from experimental media by centrifugation into a phthalate separator (Giesy and Paine, 1977b). taining all components of Control cultures, con- the medium but no algal or bacterial cells, were handled in the same manner as experimental cultures to evaluate contamination in the separation procedure or Pu polymerization. Reagent blanks, controls and experimentals were analyzed for 237 pu using a GeLi detector an INOTEC 4000 channel pulse height analyzer. interfaced to Since all replicates could not be centrifuged concurrently, experimental units were blocked orthogonally over time. Significance of treatment effects were tested using standard analysis of variance Axenic cultures of Scenedesmus obliquus (Turp) Kutz (strain #1592) were obtained from the Indiana University Culture Collection. Aeromonas hydrophila (strain #7966) was obtained from the American Type Culture Collection. Stock, axemic 300 ml Erlenmeyer stock was added to AAP or AAP + 0.5% TBS uptake) and adjusted to pH 4.5 with 1.0 N NaOH using a microburette. The experimental design was a randomized block design. MATERIALS AND METHODS techniques and means separated with Student- Newman Keubs multiple range test. level. Significance was tested for at the a = 0.05 s. obliguus cultures were maintained in 200 ml Algal Assay Procedure (AAP) medium aerated with sterile air (Anon, 1971). Stock algal cultures were checked 532 initial cell S. obliquus and to form complexes with many organic Jigans such as those in soil. (Bondietti, et al., periodically for bacteria contamination by plating on peptose agar, incubating in tryptic soy broth (TSB) and microscopic examination. Stock and experimental cultures were incubated at 24 + 2°C under 4035 lux illumination from balanced spectrum "Growlux" fluorescent bulbs on a 16 hr light-8 hr dark regime. Algal inocula were taken from 10 day old S. obliquus cultures with cell densities of 4.2 x 10° cells + ml | and relative growthrate {K' 0.68 day “). Initial 8. obliquus cell densities were concentrated by centrifugation to between 1 and 2 x 10° cells « mi7! in experimental flasks. Stock A. hydrophila cultures were maintained in AAP-tryptic soy broth (TSB) medium (Difco). A. hydrophila inocula were drawn from 24 hr cultures with cell 533