Short-Term Toxicity and Binding of Platinum to Freshwater Periphyton Communities S. Rauch,1,3 M. Paulsson,2 M. Wilewska,3 H. Blanck,2 G. M. Morrison3 1 2 3
Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Botanical Institute, Go¨teborg University, Box 461, 405 30 Go¨teborg, Sweden Water Environment Transport, Chalmers University of Technology, 412 96 Go¨teborg, Sweden
Received: 2 September 2003 /Accepted: 8 February 2004
Abstract. The release of platinum (Pt) from automobiles equipped with exhaust catalysts has resulted in increasing concentrations of this normally rare metal in the urban and roadside environment. Although concentrations are increasing, little is known about the environmental effects of Pt and its potential toxicity. This study was an investigation of Pt toxicity to naturally grown periphyton communities. Periphyton communities were exposed to Pt(II) and Pt(IV) in reference and stream waters. Uptake increased linearly with Pt concentration for both reference- and stream-water exposure. However, decreased photosynthetic activity was observed only for reference-water exposure. This difference was related to uptake by biotic components in reference water and binding to abiotic components in stream water.
Automobile catalysts were introduced in the mid-1970s in the United States to alleviate the emission of harmful pollutants from vehicle exhaust and to improve urban air quality. These devices employ platinum (Pt)-group elements to catalyze the removal of pollutants in the exhaust, but it has recently been found that a small amount of these metals is released during vehicle operation (Ko¨nig et al. 1992; Palacios et al. 2000; Moldovan et al. 2002), thus resulting in increased Pt concentrations in the urban and roadside environment (Go´mez et al. 2002; Rauch et al. 2001; Rauch and Hemond 2003; Rauch et al. 2004). Pt can be further transported to aquatic systems either through direct atmospheric deposition or through stormwater runoff (Laschka et al. 1996; Rauch et al. 2000a), and increased Pt concentrations have been reported in sediments (Rauch and Hemond 2003; Rauch et al. 2004), where Pt preferentially partitions, while water concentrations remain low (Rauch et al. 2000a). Although it was previously believed that the emitted Pt is relatively inert, it has now been demonstrated that Pt is subject to complex environmental transformations (Lustig et al. 1996),
and the occurrence of bioavailable species has been determined through the investigation of Pt uptake by freshwater macroinvertebrates and mussels (Moldovan et al. 2001; Zimmermann et al. 2002). A different uptake rate was found for Pt(II) and Pt(IV), and a charged species-related uptake mechanism has been suggested (Rauch and Morrison 1999). However, little is known about potential toxic effects of Pt on organisms. Exposure to soluble Pt under laboratory conditions has revealed the toxicity of Pt to aquatic life including the water flea, Daphnia magna (Lustig 1997; Biesinger and Christensen 1972), the freshwater isopod Asellus aquaticus (Rauch and Morrison 1999; Moldovan et al. 2001), the freshwater worm Variegatus lubriculus (Veltz et al. 1996), the green alga Scenedesmus subspicatus (Lustig 1997), and the marine bacterium Photobacterium phosphoreum (Microtox) (Rauch 2001). Laboratory tests have reported a toxicity concentration window of 14 to 520 g/L (0.7 to 2.7 mol/L). So far, the toxicity of Pt in the aquatic environment has been measured only with single-species tests under laboratory conditions, and there is no information about effects on natural communities under more realistic exposure conditions. This study investigated the uptake of soluble Pt (II) and (IV) by naturally grown stream periphyton communities (Paulsson et al. 2000; Blanck et al. 2003) as well its distributions in the community and effects on photosynthetic activity. The uptake was studied both in stream water and reference water, the latter being devoid of organic carbon and having low concentrations of other potential Pt ligands.
Materials and Methods Periphyton Community Periphyton samples were collected from La¨rjeån stream, a tributary to ¨ lv on the west coast of Sweden. It flows through an the large Go¨ta A agricultural area having relatively low exposure to Pt sources, i.e., traffic or hospitals. The stream is characterized by a near neutral pH (7.2 to 7.6); a good buffering capacity ([HCO3] ⫽ 0.8 mmol/L; hardness ⫽ 0.4 mmol/L); high turbidity, and nutrient ([N-NO3] ⫽ 0.02 mmol/L; [P-PO4] ⫽ 0.2 mol/L) and organic carbon concentrations
Short-Term Toxicity and Binding of Platinum
(DOC ⫽ 0.5 mmol/L) (Blanck et al. 2003; Haitzer et al. 2001; Akkanen et al. 2001). Periphyton communities in the stream, which have been the subject of previous investigations (Blanck et al. 2003; Guash et al. 1998), are composed of Achnanthes minutissima, Achnanthes sp. (highly dominant species), Cocconeis placentula, Cocconois sp, Gomphonema angustatum, and Gomphonema sp. (dominant species). Periphyton communities were colonized for 2 to 4 weeks during June and July 2001 on 1.5-cm2 glass disks in La¨ rjeån stream. The glass disks were mounted vertically on polyethylene holders that carried 10 disks each and were inserted into polyethylene racks. The racks floated parallel to the main current at a depth of approximately 0.5 m (Guasch and Tubbing 1996).
Exposure Atypically colonized periphyton glass discs were discarded, and the remaining discs were cleaned on all but the colonized side. Discs were then placed in scintillation vials with the periphyton-covered side facing upward. The samples were preincubated in 1 mL filtered (GF/F, Whatman and 0.2-m membrane filter; Schleicher and Schuell) stream or reference water and 1 mL Pt test solution in a geometric concentration series with exposure concentrations ranging from 10⫺9 to 10⫺6 mol/L. The chosen exposure concentrations ranged from 3 to 6 orders of magnitude higher than Pt measured in urban rivers (Laschka and Nachtwey 1993). It was necessary to have higher exposure concentrations to determine potential effects for short-term exposure. Control samples without added Pt were also included for both types of water. Five replicates were prepared for each concentration level. The samples were incubated for 23.5 hours in a specially constructed incubator in La¨ rjeån stream under ambient light and temperature conditions. The Pt(II) and Pt(IV) test solutions were prepared by dissolving sodium tetrachloroplatinate tetrahydrate and sodium hexachloroplatinate hexahydrate, respectively (Riedel-de-Hae¨ n, Germany), in 0.2 m filtered stream water or reference water. The reference water used in exposure experiments was prepared according to the International Organization for Standardization norm no. 6341 (1996) but was diluted 50 times to achieve low levels of potential ligands. Final concentrations were 0.04 mmol/L CaCl2,2H2O, 0.01 mmol/L MgSO4,7H2O, 0.016 mmol/L NaHCO3, and 0.0016 mmol/L KCl.
Periphyton Photosynthetic Activity Periphyton photosynthetic activity was measured as incorporation of radioactively labeled carbon added as 14C-bicarbonate. An H14CO⫺ 3 solution was prepared using a 200-fold dilution of an Amersham CFA stock solution (2 mCi/ml; Amersham Laboratories, UK) in ultraviolet light–treated deionized water adjusted to pH 9 with 0.1 mol/L NaOH. Fifty L H14CO3⫺ solution was added to each vial after 24 hours of preincubation with Pt. To avoid potential carbon deficiency during 14C incorporation, the toxicant solutions were changed after 23.5 hours. This procedure took approximately 0.5 hours, thus resulting in a total preincubation time of 24 hours. After 1 hour of incorporation with 14 C-carbon, 100 L 37% formaldehyde was added to terminate the incorporation process. To determine background activity, 2 samples were treated with 100 L 37% formaldehyde before the addition of 14 C. To avoid any 14CO2 release during transportation, 200 L 1 mol/L NaOH was added to all the samples. The solution was removed, and 1 mL concentrated acetic acid was added to drive off the remaining inorganic carbon, after which the samples were dried at 60°C under a stream of air. To enhance the release of incorporated 14C, the samples were extracted for at least 10 minutes in 1 mL dimethylsulfoxide (Filbin and Hough 1984).
The amount of 14C-carbon incorporated was measured in a scintillation spectrometer (Tri-Carb 2900 TR; Packard Instrument Company) after adding 9 mL scintillation solution (Ready Safe, Beckman). The activities, as disintegrations per minute, were calculated from counts per minute using external standard calibration and automatic quench compensation. The background activity was then subtracted from the dpm values.
Platinum Uptake and Analysis Pt uptake was measured parallel to the photosynthetic activity measurements. Three replicates were analyzed for each Pt concentration. After 25 hours of preincubation with Pt, the samples were immediately rinsed 3 times by quickly dipping the glass disk in filtered reference or stream water, after which the water was allowed to drain. The samples were placed in new scintillation vials and kept dark and cold at 4°C until analysis. Before analysis, samples were dried at 70°C until constant weight was achieved. Samples were prepared by closedvessel microwave digestion (CEM Mars5; Matthews, North Carolina). Disks were placed in digestion vessels (HP500) together with 8 mL Aqua regia and digested using a temperature and pressure ramp up to 210°C and 150 psi, respectively (Moldovan et al. 2001). After digestion, the solution was evaporated on a hot plate and redissolved in 1% HCl. Glass disks were reweighed after digestion to obtain the weight of the accumulated biomass. The disks were observed under a microscope to control the efficiency of the digestion procedure and either no algae or insignificant levels of algae could be found on the surface of colonized disks after digestion. Pt concentrations in digested samples were determined by quadrupole inductively coupled plasma-mass spectrometry (ICP-MS) with pneumatic nebulization (Elan 6000; Perkin Elmer Canada) using the instrumental parameters listed in Table 1. Platinum determination was subject to interference from HfO, which forms in the plasma and the interface; mathematic correction was applied according to Equation 1 (Parent et al. 1997; Rauch et al. 2001; Moldovan et al. 2001). Pt concentration on the disc was determined through external calibration using standards prepared from a 1000 g/L Pt ICP-MS standard (High Purity Standards, Charleston, South Carolina): IPt ⫽ IPt,s ⫺ (IHf.s ⫻ RHf,Pt),
where IPt equals true Pt intensity, IPt,s equals the apparent Pt intensity in the samples, IHf.s equals the apparent Hf intensity in the samples, and RHf,Pt equals the HfO/Hf ratio in an Hf solution (prepared from a 1000 g/L ICPMS standard; Sigma Chemicals, St. Louis, Missouri). In addition, laser ablation (LA)-ICPMS was used to investigate the spatial distribution of Pt on exposed discs. LA-ICP-MS uses a laser beam as a sampling probe for the direct analysis of solids by ICP-MS, thereby enabling the spatially resolved analysis of trace elements in solid samples (Durant 1999). LA was performed on a Cetac LSX-200 (Cetac, Omaha, Nebraska) coupled to the previously described ICPMS. The LSX-200 uses a quadrupled neodymium:yttrium aluminum garnet laser with an output of 266 nm and a beam diameter from 10 to 300 m. The glass disk was placed in a sealed sample cell and the laser focused onto the sample surface. The ablated material was carried in an argon stream into the plasma and analyzed. Instrumental parameters are listed in Table 2. The laser beam diameter used implied that several components of the periphyton communities were ablated simultaneously, e.g., algae and bacteria or algae and detritus. However, signal distribution and correlation between signals may be used to investigate detailed spatial distribution of elemental associations (Rauch et al. 2000b). The formation of oxide was lower when using laser ablation for sample introduction owing to dry plasma conditions, therefore interference from HfO did not affect the Pt signal significantly (Motelica-Heino et al. 2001).
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Table 1. ICP-MS operating conditions Parameter
Sample introduction Sample uptake Nebulizer Nebulizer gas ICP Rf power Plasma gas Auxiliary gas Cones Acquisition Analytes scanned Data acquisition Dwell time Sweeps per reading Readings per replicate Replicates
Fig. 1. (A) Pt(II) and (B) Pt(IV) uptake by periphyton communities exposed to Pt in reference water for 24 hours. Pt ⫽ platinum
Al ⫽ aluminum; Co ⫽ cobalt; Fe ⫽ iron; LA-ICP-MS ⫽ laser ablation-inductively coupled plasma-mass spectrometry; Mg ⫽ magnesium; Pt ⫽ platinum; Si ⫽ silicon.
LA-ICP-MS was performed on the samples exposed to 1000 nmol/L Pt in reference and stream water by scanning across the sample surface. Cobalt (Co), iron (Fe), aluminum (Al), and silicon were analyzed to investigate potential correlations with Pt.
Results and Discussion
Pt(IV) having a higher uptake rate than Pt(II) (Rauch and Morrison 1999). The uptake rate for exposure in stream water was lower than for exposure in reference water (Table 3). The main difference between the 2 waters for metal uptake was the occurrence of organic ligands in stream water. Although the experiments were performed with communities colonized at different times and may therefore have had a slightly different species composition as well as dry weight, the most likely explanation was that the relatively high dissolved organic matter and chloride concentrations of the stream water complexed the Pt, thus making it less available for uptake. A linear uptake was observed even for the lowest exposure concentration, which suggested that the main active ligands in water were humic acids, and it is possible that the periphyton matrix was a strong competitive ligand, for which Pt had a higher affinity, with competition, thus resulting in a lower uptake compared with exposure in reference water.
Platinum Uptake by Periphyton Communities The uptake of Pt by exposed periphyton communities is presented in Figures 1 and 2. Pt was found on all exposed disks, and uptake increased linearly with increasing concentration in the exposure range (1 to 1000 nmol/L). Uptake rates were determined by linear regression and are listed in Table 3. Uptake rates were similar for both Pt(II) and Pt(IV). This is in contrast to Pt uptake by the freshwater isopod Asellus aquaticus, which has been found to depend on Pt speciation with
Platinum Toxicity to Periphyton Communities There was no significant difference in photosynthetic activity between Pt-exposed periphyton communities and controls (Figures 3 and 4). However, there was a significant decrease in mean activity with increasing exposure concentration for exposure to both Pt(II) and Pt(IV) in reference water (Figure 3). A linear regression model was applied to the activity data demonstrating
Short-Term Toxicity and Binding of Platinum
Fig. 2. (A) Pt(II) and (B) Pt(IV) uptake by periphyton communities exposed to Pt in stream water for 24 hours. Pt ⫽ platinum Table 3. Uptake rates for Pt (II) and Pt (IV) exposure in reference and stream waters obtained by linear regressiona Uptake Rate (g/g DW)/(nmol/L) Pt Compound
R2 ⬎0.98, p ⬍0.001.
that photosynthetic activity decreases linearly with increasing exposure concentration (Table 4). For Pt(II) the photosynthetic activity starts to decrease from the lowest exposure concentration (Figure 3A), whereas for Pt(IV) the activity only decreases at higher exposure (Figure 3B), although similar Pt levels were measured on the disks. The steeper slope for Pt(IV) compared with Pt(II) (Table 4) suggested different toxicity mechanisms or affinity biotic ligands, with Pt(IV) being less toxic than Pt(II) at low exposure concentrations while having a similar availability and uptake rate. It is possible that the 2 forms of Pt have different targets in the community, e.g., Pt(IV) preferentially binds to the polysaccharide matrix, or that Pt(IV) exhibits fewer effects. It is known that Pt(II) has a higher affinity for amino acids and proteins, and therefore the reduction of Pt(IV) to Pt(II) may be a prerequisite for uptake and accumulation by living organisms (Rauch and Morrison 2000).
Fig. 3. Effect of 24-hour exposure to (A) Pt(II) and (B) Pt(IV) on photosynthesis in periphyton communities exposed in reference water. Dotted lines indicate 95% confidence limit for controls. Pt ⫽ platinum
Preliminary measurements of thymidine incorporation into bacteria were performed on periphyton communities exposed to Pt(II) and Pt(IV) under similar conditions. The measurements indicated an increased toxicity for both Pt(II) and Pt(IV) after exposure in reference and stream water. Decreases in both photosynthetic activity and thymidine incorporation suggested that both Pt(II) and Pt(IV) have effects on algae and bacteria in periphyton communities. The EC50 (concentration at which activity is 50% of control) was not reached in the exposure range and time used. It is difficult to compare these findings with previous studies using other organisms because different exposure times were used, and only single organisms (cells) were studied. The EC50 for the Microtox test, which uses the marine bacterium Photobacterium phosphoreum, is 1 mol/L for a 30-minute exposure (Rauch 2001). Periphyton communities were found to be more resistant than the bacterium used in the Microtox test, possibly because of the higher resistance of the periphyton matrix compared with a single bacterial species. Although it has been determined that Pt is taken up onto the colonized disks when exposed in stream water, no toxic effect could be observed. No trend could be determined for Pt(II), and an increasing trend was found for Pt(IV), although the linear regression was not as significant as for exposure in reference water. It can therefore be hypothesized that Pt is either in a nontoxic form or bound to surface biotic or abiotic components, i.e., silt or the polysaccharide matrix. If surface binding occurs, a slow migration inward to more sensitive targets in the periphyton
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Fig. 4. Effect of 24-hour exposure to (A) Pt(II) and (B) Pt(IV) on photosynthesis in periphyton communities exposed in stream water. Dotted lines indicate 95% confidence limit for controls. Pt ⫽ platinum
Table 4. Slope, correlation coefficient, and p value for linear regression applied to concentrationa and photosynthetic activityb
Fig. 5. Laser ablation of a colonized disk after 24-hour exposure to 1000 nmol/L Pt(II) in reference water. (A) Pt signal (as cps). (B) Signal ratios. The x-axis corresponds to real time during ablation, which is proportional to distance with a scanning speed of 5 m/s. Baseline signal before 100 s and after 600 s corresponds to preablation postablation times, respectively. The signal ratios are in the same order as the legend. Pt ⫽ platinum
Slope, Correlation Coefficient, and p Value Pt Compound Pt(II)
S ⫽ ⫺0.05 R ⫽ 0.79 p ⬍0.11 S ⫽ ⫺0.06 R ⫽ 0.93 p ⬍0.020
S ⫽ 0.01 R ⬍0.1 p ⬍0.89 S ⫽ 0.06 R ⫽ 0.66 p ⬍0.22
Independent variable. Percent of control, independent variable. S ⫽ slope; Pt ⫽ platinum; R ⫽ correlation coefficient. b
community is possible and would result in a delayed toxic effect. Strong affinity for the periphyton matrix may therefore lead to a higher potential toxicity because Pt was efficiently scavenged from the aqueous phase.
Platinum Distribution The distribution of platinum on colonized disks was investigated using LA-ICP-MS after 24-hour exposure to 1000 nmol/L Pt. Analysis of a colonized disk exposed to 1000 nmol/L Pt before
and after removing the periphyton layer with a stainless steel blade showed that Pt was predominantly present in periphyton colonies with ⬍5% of Pt adsorbed on the glass disk. Platinum could be found on the surface of colonized disks for both exposure in reference and stream waters (Figures 5 and 6), thus confirming uptake measurements (Figures 1 and 2). However, the overall signal for exposure in reference water was lower than for exposure in stream water. This can be related to the thicker colonization of stream water disks as was both observed visually and determined through dry weight measurement. It should be added that no correlation was observed between dry weight and toxicity. The use of relative signals (Figure 5B and 6B) compensated for this difference and for variations in thickness across the disks by using the ratio of 2 elements, which are affected in the same way by variations in sample thickness. Cobalt was used as a surrogate tracer for living organisms because of its incorporation as vitamin B12, a necessary constituent of all living organisms. The mean Pt-to-Co ratio was 3.69 and 2.38 for exposure in reference and stream water, respectively, indicating a higher amount of Pt in periphyton exposed in reference water as expected from uptake measurements. The Pt signal for reference-water exposure presents peaks (Figure 5A) that do not appear for stream-water exposure (Figure 6A). The peaks were either the result of heterogeneous sample distribution with thicker colonization for some areas of
Short-Term Toxicity and Binding of Platinum
There was a lower uptake rate for stream water, possibly because of the higher content of organic carbon and/or chloride. No significant toxicity was observed after exposure to Pt compared with unexposed communities. However, a significant linear decrease in photosynthetic activity was observed for exposure to Pt in reference water. Although the uptake mechanism did not depend on oxidation state, Pt(II) exhibited a higher toxicity compared with Pt(IV), perhaps because of the higher capacity of Pt(II) to bind to amino acids and proteins. Toxic effects could not be determined for exposure in stream water, despite uptake, indicating that Pt was adsorbed or bound to abiotic material and under natural conditions presents a lower toxicity to periphyton communities. Uptake by biotic components in reference water and binding to abiotic components in stream water was also supported by LA-ICP-MS analysis. This does not exclude that Pt may be toxic after longer exposure under natural conditions, during which Pt may be absorbed into cells. Further studies should therefore include exposure for longer periods. With Pt occurring mostly in a particulate form in the environment, exposure to Pt-containing particles should be another component of further studies.
Fig. 6. Laser ablation of a colonized disk after 24-hour exposure to 1000 nmol/L Pt(II) in stream water. (A) Pt signal (as cps). (B) Signal ratios. The x-axis corresponds to real time during ablation, which is proportional to distance with a scanning speed of 5 m/s. Baseline signal before 100 s and after 600 s corresponds to preablation and postablation times, respectively. The signal ratios are in the same order as the legend. Pt ⫽ platinum
the disk, or preferential binding at some sites on the disk, and may have been related to the different toxicities for reference and stream water exposure. Signal ratios (Figure 5B) showed that signals for different elements do not necessarily follow the same trend. There was, however, a good correlation between Pt and Co signals, whereas correlation with Al, Si, Mg, and Fe was lower. Although the plateau-shaped signal indicated that Pt was present across the disk, the occurrence of peaks and correlation with Co indicated that living organisms take up Pt when exposed in reference water. When the laser beam reached spots where biotic components were more concentrated, the ratio of Pt with abiotic components decreased, and the ratio of Pt to Co remained constant. The absence of signal peaks and the constant signal ratio for all elements, including Pt/Al and Pt/Fe, for the ablation of periphyton communities exposed in stream water (Figure 6) may have then been related to Pt binding to abiotic material, which could explain the absence of toxicity.
Conclusion Uptake of soluble Pt by periphyton was determined for exposure to both Pt(II) and Pt(IV) in reference and stream waters.
Acknowledgments. The authors acknowledge funding from the Swedish Environment Protection Agency and the Swedish Research Council for Environment, Agricultural Sciences and Planning. M. Wilewska was granted a scholarship from the Swedish Institute for the completion of this work.
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