Accumulation and depuration kinetics of ¹³⁷Cs in freshwater algae and macrophyte

Laboratory uptake and depuration kinetics of ¹³⁷Cs were determined in freshwater green filamentous algae Spirogyra sp. and macrophyte Hydrilla verticillata by using the radiotracer technique. This study provides the first comprehensive comparative analysis of two distinct groups of freshwater plant, free floating algae and a submerged vascular plant, that accumulate radioactive ¹³⁷Cs over time. The results showed that both plants take up ¹³⁷Cs from water in direct proportion to its dissolved concentration during the experiment. At equilibrium, the concentration factors were found to be 4.80 ± 0.03 (mL g^-1) and 3.15 ± 0.06 (mL g^-1) while uptake rate constant k_u was 0.58 ± 0.09 (d^-1) and 0.21 ± 0.04 (d^-1) for Spirogyra sp. and H. verticillata, respectively. Depuration kinetic parameters indicated that most of the radiocesium was rapidly lost from plants during the first days of depuration, and only 20-40% of ¹³⁷Cs was retained in both plants at the end of the experiment. The depuration rate constants (k_e) and the biological half-lives (T_b1/2) of ¹³⁷Cs were observed to be 0.16 ± 0.01 (d^-1) and 0.09 ± 0.08 (d^-1) and 4.29 ± 0.01(d^-1) and 7.70 ± 0.04 (d^-1) for Spirogyra sp. and H. verticillata, respectively. Differences in ¹³⁷Cs concentration between both species growing under similar environmental conditions suggest that the uptake rate of ¹³⁷Cs is mainly influenced by plant morphology. The results presented in this study suggest that both Spirogyra sp. and H. verticillata could serve as suitable bioindicators for monitoring ¹³⁷Cs contamination in freshwater systems.