molecular weight fractions of naturally occurring colored organic acids are responsible for most of the chelating capacity in surface waters. and Ardakani (1972) Stevenson reported that metals bound to the larger fractions were less available to plants and less mobile in soil solution. larger diameter organics reduced otherwise sequestering Pu. Pu uptake, presumably In this study the by chelating an The smaller diameter organics greatly increased Pu uptake. This may be due to decreased Pu precipitation and adsorption to glass. Organic matter in soil solutions decreased Pu availability tc plants (Schulz et al., 1976). Pillai et al. (1976) found humics maintained more Pu in solution than sea water alone but precipitated removing Pu over a long period of time. Smaller diameter organics may also facilitate Pu uptake by crossing the cell membrane as a Pu-organic complex or by juxtapositioning Pu on the cell. Metal organic complexes may readily penetrate cell membranes, thus chelating agents may transport metals into cells in quantities greater than would normally occur Table 4. : cellular Effects of 4 organic fractions on A. hydrophila 237 Pu (Doyle etal., Low molecular weight humic acid fractions and fulvic acids may penetrate the cell membranes of phytoplankton concentration after 6 hr. 1974). weight humic acids (F I and F (Prakash et al., II) 1973) probably do not. but the higher molecular Prat et al. (1961) and Pr&t and Pospisil (1959) report that humic acids cannot penetrate plant cell 237pyt4 Removed From Medium Treatment Final cell _density (cell x 10/+ml~ Removed/Cell 1078 pei 237put4eceil”! % Total Removed 4.3 + 20" 14 + 6¢ 5.4 TSB + F I 5.1 + 2.24 742 B 3.2 TSB + F II 3.6 + 1,44 10 + 2 B 3.2 Tsp + Fort 3.1 + 1.2" 15 + 2 c 4.2 TSB + F IV 5.6 + 2.17 51 +9 26.0 TSB Means not significantly + 2 S.. *N = 4, a = 0.05, confidence intervals = by A, B or c. different from one anothér are denoted membranes. Humic substances that penetrate cell membranes are inhibitory to all cellular processes (Prdét, 1968). transport across the cytoplasmic membrane, while Saunders (1957) stated that humic acids may stimulate cell membranes of phytoplankton, thereby allowing an influx of bound metals. ACKNOWLEDGMENTS This research was supported by Contract E(38-1)-819 between the University of Georgia and the U. S. ERDA. R. M. Wallace, and E. L. Wilhite helped with Pu** preparation. R. Geiger and L. A. Briese aided in data collection and J. Cc. Corey helped with all phases of the study. 539 538 The important point is that all naturally occurring organics do net increase or decrease Pu uptake but some increase Pu uptake while others cause a decrease. Waris (1953) suggested that humic substances may directly affect the cytoplasmic membrane. Chaminade (1956) reported humic acids stimulate growth of violet epidermal cells by allowing mineral