Copper‐induced release of complexing ligands similar to thiols by Emiliania huxleyi inseawater cultures

Abstract

Marine microalgae (Emiliania huxleyi) were grown in seawater enriched only with nitrogen and phosphorus, without control of free metal concentrations using synthetic chelators. Complexing ligands and thiol compounds were determined by cathodic stripping volt-ammetry. Copper was added to these cultures, and ligands were produced in response to the copper addition. Parallel measurements of thiols showed that glutathione and other unidentified thiols (electrochemically similar to thioacetamide) were produced by the algae at rates and concentrations similar to those of the complexing ligands. Smaller amounts of thiols were produced when ligands including thiols were added to the culture. The results indicate that thiols can account for a major part or most of the copper-complexing ligands produced by these algae. Furthermore, a feedback mechanism exists in which the production of thiol-type complexing ligands is controlled by the free copper concentration, production already being stimulated by an increase of [Cu 2ϩ ] from 0.4 to 1.5 pM. Incubations with added exudates, thiols, and salicylaldoxime (SA) showed much reduced copper toxicity even though copper uptake was increased by the exudates and the SA.Culture experiments have shown that marine and freshwater phytoplankton release complexing ligands into culture medium (e.g., van den Berg et al. 1979;Imber and Robinson 1983), which may be important because the availability of metals to algae is known to depend on their chemical composition (e.g., Brand et al. 1983). Therefore, the interaction of phytoplankton with trace metals is reciprocal: the biota affect the trace metal chemistry and the trace metals affect the biota.Using marine algal cultures where the metal chemistry was controlled by a synthetic chelator (usually ethylenediaminetetraacetic acid [EDTA]), the biological uptake of copper has been shown to be related to the cupric ion concentration, [Cu 2ϩ ] (e.g., Brand et al. 1986); toxicity occurs at pM levels of Cu 2ϩ . The concentration of Cu 2ϩ is typically lowered by by 2-3 orders of magnitude by organic complexation in seawater to a [Cu 2ϩ ] of 0.03-0.3 pM, which is thought to be sufficiently high to meet algal nutritional requirements but is below copper toxicity levels (Sunda 1995).

Distribution of the thiols glutathione and 3‐mercaptopropionic acid in Connecticut lakes

“…How GSH was released into surrounding environment from phytoplankton is not clear. The release of GSH may be associated with the normal metabolic processes of phytoplankton or as a response to enhanced Cu exposure (Leal et al 1999). It was suggested that GSH may be released from dead cells as they degrade (Leal et al 1999).…”

Toxicity and bioaccumulation of copper in three green microalgal species

“…3), indicating that C. lunula may have other mechanisms to detoxify Cu. It has been reported that production of glutathione and other thiols as the copper-complexing ligands were promoted by algae under the stress of accumulated Cu (Rijstenbil and Wijnholds, 1996;Leal et al, 1999). SOD, providing antioxidant protection in algae, increased during the first day of exposure to Cu (Okamoto and Colepicolo, 1998).…”

“…Binding to organic complexes, especially cell exudates, is another possible mechanism regulating the detoxification of metal ions (Xue et al, 1988;Xue and Sigg, 1990;Rijstenbil et al, 1994;Gerringa et al, 1995). It has been reported that microalgal production of glutathione, thiols or superoxide dismutase activity (SOD) can be stimulated by copper, which may also be a defense mechanism against copper toxicity (Rijstenbil et al, 1994(Rijstenbil et al, , 1998aRijstenbil and Wijnholds, 1996;Okamoto and Colepicolo, 1998;Leal et al, 1999).…”

Benthic fluxes of copper, complexing ligands and thiol compounds in shallow lagoon waters

“…Reactive thiol concentrations were determined by CSV as before (Leal et al, 1999). Voltammetric parameters were as for reactive copper with calibration against glutathione which was found to behave similarly to the unknown thiol species.…”

“…Metal complexing ligands (Croot et al, 2000;Moffett and Brand, 1996) and thiol compounds are also known to be released by marine phytoplankton (Dupont and Ahner, 2005;Leal et al, 1999) into surface waters. Specific thiols like glutathione have been found in oceanic (Dupont et al, 2006;Le Gall and van den Berg, 1998) and estuarine (Tang et al, 2004) waters.…”