ARTICLE pubs.acs.org/est
Release of Reduced Inorganic Selenium Species into Waters by the Green Fresh Water Algae Chlorella vulgaris Denina Bobbie Dawn Simmons†,§ and Dirk Wallschl€ager*,‡ †
Environmental and Life Sciences Graduate Program, ‡Environmental & Resource Sciences Program and Department of Chemistry, Trent University, 1600 West Bank Drive, Peterborough, ON, Canada K9J 7B8 ABSTRACT: The common green fresh water algae Chlorella vulgaris was exposed to starting concentrations of 10 μg/L selenium in the form of selenate, selenite, or selenocyanate (SeCN-) for nine days in 10% Bold’s basal medium. Uptake of selenate was more pronounced than that of selenite, and there was very little uptake of selenocyanate. Upon uptake of selenate, significant quantities of selenite and selenocyanate were produced by the algae and released back into the growth medium; no selenocyanate was released after selenite uptake. Release of the reduced metabolites after selenate exposure appeared to coincide with increasing esterase activity in solution, indicating that cell death (lysis) was the primary emission pathway. This is the first observation of biotic formation of selenocyanate and its release into waters from a nonindustrial source. The potential environmental implications of this laboratory observation are discussed with respect to the fate of selenium in impacted aquatic systems, the ecotoxicology of selenium bioaccumulation, and the interpretation of environmental selenium speciation data generated, using methods incapable of positively identifying reduced inorganic selenium species, such as selenocyanate.
’ INTRODUCTION Selenium (Se) is of high current environmental interest in North America because it is released accidentally from several large-scale industrial production processes, including mining, agricultural irrigation, petroleum refining, and coal combustion, into aquatic ecosystems. In some cases, Se may then bioaccumulate in the aquatic food chain to the point where its concentrations in the eggs and/or ovaries of higher aquatic organisms reaches levels that may cause reproductive effects.1 Algae play a crucial role in this bioaccumulation process because they show very high Se bioaccumulation from the surrounding water and thereby determine the amount of Se available to higher organisms, which tend to take up Se predominantly or exclusively from their diet. Previous studies suggest that different algal species have vastly different uptake capacities for Se,2 so the nature and composition of the phytoplankton community in a Se-impacted aquatic system is one of the key factors regulating to what extent anthropogenically emitted Se bioaccumulates. Another key factor influencing Se bioavailability to algae is its chemical speciation. Ambient waters and industrial discharges are known to contain selenite and selenate in varying proportions,3 and different algal species show preferential uptake for either one or the other of these inorganic Se oxyanions.4,5 The overall hydrochemistry of the water body can also influence the availability of Se species to algae, e.g., in the case of high sulfate concentrations suppressing the uptake of selenate,2 its chemical analog. Certain industrial process waters, e.g., those from petroleum refineries and gold and silver mines, contain a third inorganic Se species, selenocyanate (SeCN-),6 but to date, there is no analytical evidence of its occurrence in impacted ecosystems nor data regarding its relative bioavailability to algae. Finally, there are reports of significant fractions of operationally defined “organic” Se in ambient waters,7 but to date, the volatile methylated Se species dimethylselenide (DMSe) and dimethyldiselenide (DMDSe), r 2011 American Chemical Society
which can be produced and released by algae,8 are the only positively identified organic Se species in ambient waters, and they occur at extremely low concentrations.9 However, because organic Se species, such as selenomethionine (SeMet), have much higher bioavailability to algae than inorganic Se species,10 it is important to investigate if and which organic Se species occur in Se-impacted waters. The interaction between different algal and Se species has been studied intensively, but because of the low concentrations of both Se and algae in most ambient waters, these studies have often been conducted in the laboratory. One key problem associated with many of those studies was that unrealistically high Se exposure concentrations were used (often in the mg/L range) (e.g., ref 5) because unimpacted waters typically contain
