Mercury uptake and removal by Euglena gracilis
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O R I G I N A L PA P E R
Silvia Devars · César Avilés · Carlos Cervantes · Rafael Moreno-Sánchez
Mercury uptake and removal by Euglena gracilis
Received: 22 November 1999 / Revised: 23 June 2000 / Accepted: 23 June 2000 / Published online: 2 August 2000 © Springer-Verlag 2000
Abstract The uptake and removal of mercury (added as HgCl2) from the culture medium by Euglena gracilis was studied. In cultures initiated in the light, cells accumulated a small fraction of the added heavy metal (5–13%). Mercury was both biologically and nonbiologically volatilized, and cell growth was partially inhibited; under these conditions the glutathione content was 3.2 nmol/106 cells. In contrast, in cultures initiated in the dark, mercury uptake by cells was two to three times higher, biological volatilization remained unchanged and nonbiological volatilization and growth were negligible; the glutathione content diminished to 1.4 nmol/106 cells. Biological mercury volatilization depended on cell density and metal concentration, but was light-independent. Thus, volatilization of mercury by Euglena appeared not to be an effective mechanism of resistance, whereas a high intracellular level of glutathione and a low mercury uptake seemed necessary for successful tolerance. Key words Euglena gracilis · Mercury volatilization · Glutathione
Introduction Mercury concentrations in human body tissues vary from picogram to microgram quantities per gram (Tsalev 1985). Mercury is also accumulated by some aquatic plants (Shirastava and Rao 1997) and fungi (Gadd 1993), but rarely by algae (De Filippis and Pallaghy 1994; Wilde and Beneman 1993), although there is a report indicating a significant accumulation of Hg2+ by Chlorella (Wilkinson et al.
S. Devars (✉) · C. Avilés · R. Moreno-Sánchez Departamento de Bioquímica, Instituto Nacional de Cardiología, Juan Badiano 1, Col Sección XVI, México, D.F. 14080, México e-mail: [email protected], Tel.: +52-5-5732911 ext. 1298, Fax: +52-5-5730926 C. Cervantes Instituto de Investigaciones Químico-Biológicas, Universidad Michoacana, Morelia, Michoacán, México
1989). Accumulation of mercury can be expected in cells exposed to appropriate doses of HgCl2, since lipid bilayer membranes are highly permeable to HgCl2 (Gutnecht 1981; Karniski 1992). Accumulation of heavy metals in algae, as in higher plants, induces phytochelatin synthesis (Gekeler et al. 1988). Phytochelatins are low molecular mass peptides, derived from polymerization of glutathione, that bind heavy metals with high affinity, protecting cells from their toxic effects. However, mercury is a poor phytochelatin inducer in both higher plants (Grill et al. 1987; Zenk 1996) and algae (Howe and Merchant 1992). Cell wall (Cappolino et al. 1997) and glutathione binding of mercury (Blaska and Shaikh 1991; Lorscheider et al. 1995) might be alternative protective mechanisms. In bacteria, resistance to mercury is related to enzymatic reduction of Hg2+ to volatile Hg0 (Silver 1996), a reaction that operates within minutes; in contrast, formation of Hg0 in
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