Response of the charophyte Nitellopsis obtusa to heavy metals at the cellular, cell membrane, and enzyme levels



The responses of the freshwater macroalga Nitellopsis obtusa to heavy metal (HM) salts of Hg, Cd, Co, Cu, Cr, and Ni were assessed at different levels: whole-cell mortality (96-h LC50), in vivo cell membrane (45-min depolarization of resting potential, EC50), and enzyme in plasma membrane preparations (K+, Mg2+-specific H+-ATPase inhibition, IC50). To measure ATPase activity, a novel procedure for isolation of plasma membrane–enriched vesicles from charophyte cells was developed. The short-term ATPase inhibition assay (IC50 from 6.0 × 10−7 to 4.6 × 10−4 M) was slightly more sensitive than the cell mortality test (LC50 from 1.1 × 10−6 to 2.6 × 10−3 M), and the electrophysiological test with the end point of 45-min depolarization of resting potential was characterized by less sensitivity for HMs (EC50 from 1.1 × 10−4 to 2.2 × 10−2 M). The variability of IC50 values assessed for HMs in the ATPase assays was close to that of LC50 values in the mortality tests (CVs from 33.5 to 83.5 and from 12.4% to 57.7%, respectively), whereas the EC50 values in the electrophysiological tests were characterized by CVs generally below 30%. All three end points identified two separate HM groups according to their toxicity to N. obtusa: Co, Ni, and Cr comprised a group of less toxic metals, whereas Hg, Cu, and Cd comprised a group of more toxic metals. However, the adverse effects within each group were discriminated differently. For example, the maximum difference between the highest and lowest LC50 for the group of less toxic metals in the long-term mortality test was approximately 60% of the response range, whereas the corresponding difference in IC50 values in the ATPase assay was 30%. In contrast, the LC50 values of the more toxic metals occupied only 10% of the response range, whereas the IC50 values were spread over 70%. Further investigation should be done of the underlying mechanism or mechanisms responsible for the observed differences in the dynamic range of a particular end point of the groups of toxicants of varying strength. © 2002 Wiley Periodicals, Inc. Environ Toxicol 17: 275–283, 2002; Published online in Wiley InterScience ( DOI 10.1002/tox.10058