Environ. Sci. Technol. 2005, 39, 3056-3060
Modeling Cadmium Exchange by an Aquatic Moss (Fontinalis dalecarlica) L O U I S C R O I S E T I EÅ R E , † , ‡ L A N D I S H A R E , * , § A N D R EÄ T E S S I E R , § A N D SOPHIE DUCHESNE§ De´partement de Chimie-Biologie, Universite´ du Que´bec a` Trois-Rivie`res, C.P. 500, Trois-Rivie`res, Que´bec, G9A 5H7, Canada, and Institut National de la Recherche Scientifique - Eau, Terre et Environnement (INRS-ETE), Universite´ du Que´bec, 490 de la Couronne, Que´bec, Que´bec, G1K 9A9, Canada
Although aquatic mosses are widely used as metal biomonitors in rivers, there are few effective models to describe metal uptake and loss by these plants. To fill this gap, we exposed the aquatic moss Fontinalis dalecarlica for 28 d to three Cd concentrations (∼5-50 nM) in a flowthrough laboratory system. Cadmium accumulation by F. dalecarlica was rapid during the first few days of exposure and slowed thereafter but did not reach a steady state within our 1-month long experiment. This lack of a plateau in moss concentrations suggests that, for biomonitoring purposes, the duration of moss exposure should be considered either through a model of the type that we tested or by standardizing the exposure time of mosses transplanted in the field. During the subsequent 22-d elimination phase of our experiment, Cd concentrations in mosses did not return to their initial levels. This result suggests that a twocompartment model is likely to be more effective at describing Cd losses than would a one-compartment alternative. Indeed, predictions of a two-compartment model closely fitted our experimental data, which augurs well for the wider use of this model for other moss species and metals.
Introduction Mosses possess several qualities that have facilitated their use as metal biomonitors in running waters (1-4). First, they can accumulate metals to high concentrations, thereby facilitating the measurement of these contaminants (5, 6). Furthermore, they are tolerant to the metals that they accumulate and thus can survive at highly contaminated sites (4). Last, because mosses take up metals only from water, changes in their metal concentrations are expected to be easier to interpret than are those of organisms that take up metal from sediment or food (7). In Europe, where mosses are abundant and widespread in running waters, indigenous specimens are often collected in situ for monitoring metals (8). In contrast, their relative scarcity in Eastern North America has meant that these plants are more frequently transplanted from sites of abundance * Corresponding author phone: (418)654-2640; fax: (418)654-2600; e-mail:
[email protected]. † Universite ´ du Que´bec a` Trois-Rivie`res. ‡ Present address: INRS-ETE. § INRS-ETE. 3056
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to sites at which metal measurements are required (5, 9). To use mosses as biomonitors, environmental agencies need to know the length of time that transplanted mosses should be left in contaminated environments and how comparisons should be made among sites. An effective mathematical model describing metal exchange between mosses and water would be useful in this regard. In this study, we exposed Fontinalis dalecarlica to three concentrations of dissolved Cd that we maintained constant through the use of a flow-through system. This moss species is present in both North America (10) and Europe (11), and its congener Fontinalis antipyretica has been widely used in European rivers as a metal biomonitor (4, 12, 13). We used a two-compartment model with first-order kinetics to describe Cd accumulation and elimination in F. dalecarlica. Cadmium is a widespread byproduct of a range of mining and industrial activities (14) that is known for its toxicity in nature (15).
Methods Specimens of F. dalecarlica were collected in a stream located in the Mastigouche Reserve (46°50′41′′ N, 73°19′52′′ W) in the Province of Que´bec, Canada. This moss species is totally submerged and fixed to the riverbed by rhizoids. We collected the distal two-thirds of plants, rinsed them in river water, and then transported them to the laboratory in plastic bags filled with river water placed in coolers. In the laboratory, mosses were thoroughly cleaned with deionized water, and then leafy stems were isolated and placed in activated charcoal-dechlorinated tap water to acclimate to laboratory conditions for 2 weeks under conditions resembling those in their native stream, that is, at 14 °C, under an illumination of ∼35 W m-2, and a current speed of ∼0.2 m s-1. Water used for our experiment (Table 1) was tap water that had been dechlorinated by passing through a 30-L activated charcoal bed, heated to 14 °C, then fed by gravity through poly(vinyl chloride) piping to 35-L glass aquariums containing mosses. We maintained a flow rate of 0.6 L min-1 (measured twice daily), which corresponds to a water-renewal time of ∼35 min and a current speed of ∼0.2 m s-1 in the aquariums. In a previous study, we reported that variations in current velocity in the range of 0.01-7 m s-1 have no effect on Cd accumulation by mosses in our experimental system (16). We exposed F. dalecarlica (125 g fresh weight) to Cd at three nominal concentrations, that is, 4.5, 8.9, and 44.5 nM (0.5, 1.0, and 5.0 µg Cd L-1), under a 16:8 light:dark photoperiod at an illumination of ∼35 W m-2. These Cd concentrations are reported for Cd-contaminated sites in nature (17). Cadmium was added as CdCl2 (prepared daily) to the water intake of each aquarium using a peristaltic pump. Prior to the introduction of mosses, we flushed the experimental system for 2 d with the experimental Cd solutions to favor equilibration of surface sites with the metal. We exposed mosses in two aquariums for each Cd concentration tested and used one aquarium without Cd as a control. Mosses were kept in motion, and water mixing was assured, by bubbling each aquarium with filtered air. In the exposure phase of the experiment, we collected three moss samples (each ∼2 g fresh weight) from each aquarium at time 0 and at frequent intervals up to 28 d (12 sampling times). At 28 d, we transferred the remaining plants to Cd-free aquariums and continued sampling for a further 22 d to measure Cd efflux from mosses (nine sampling times). We sampled mosses more frequently at the beginning of both the exposure and the loss portions of the experiment. 10.1021/es049272i CCC: $30.25
2005 American Chemical Society Published on Web 03/17/2005
TABLE 1. Variables Related to the Quality of Water Used for Our Experiment variable (units)
mean ( SD (n)b
pH conductivity (µS cm-1) temperature (°C) Ca (µM) Mg (µM) Na (µM)a K (µM)a Cl (µM)a SO4 (µM)a
7.56 ( 0.09 (378) 59.1 ( 6.7 (378) 13.7 ( 0.5 (378) 148.2 ( 9.0 (175) 24.3 ( 0.8 (175) 151 12 110 127
a Values are from ref 16 for water from the same source. bCombined replicates from all times and treatment levels.
We collected water samples from the experimental system (using disposable plastic pipets) and placed them in acidcleaned high-density polyethylene containers; samples were taken at least once a day during the Cd-exposure phase of the experiment and at least once every 2 days during the Cd-efflux portion. These water samples were acidified with ACS-grade HNO3 to a pH < 2 prior to storage. Moss samples were dried at 65 °C, weighed, and digested for 16 h at 65 °C in closed high-density polyethylene test tubes containing 17% ACS-grade HNO3 (18). For measurement of elevated Cd concentrations, we used a flame atomicabsorption spectrophotometer (AAS, Perkin-Elmer model 5000 equipped with a deuterium background corrector), whereas for low Cd concentrations, we used a flameless AAS (Perkin-Elmer model 5000, graphite furnace equipped with a Zeeman-effect background corrector). Measurements of Cd in given moss samples by the two instruments differed by 0.05, Kruskal-Wallis).
FIGURE 1. Changes in mean ((SD) Cd concentrations in F. dalecarlica during the Cd-exposure and Cd-loss phases of our experiment for the three treatment levels, that is, [Cd2+] of 4.5 nM (A), 8.9 nM (B), and 44.5 nM (C). Cadmium concentrations in mosses are mean values of three measurements at each time in each of two replicate aquariums (n ) 6). Curves are model predictions generated using eq 6, for Cd-exposure, and eq 9, for Cd-loss; values of the mean rate constants used for modeling are given in Table 2. There was little variability in moss Cd concentrations ([Cd]moss) for a given sampling time and treatment level (Figure 1). Indeed, results for the duplicate aquariums at a given Cd exposure concentration were, in almost all cases, not significantly different for both the exposure and the loss portions of the experiment (ANCOVA, p > 0.05; square-root transformed data; time as covariable). The only case where a difference between replicate samples was observed was for the Cd-loss portion of the 44.5 nM Cd treatment-level, for which there was a slight (∼7%) but marginally significant difference (p ) 0.04, ANCOVA). For each aqueous Cd concentration, increases in [Cd]moss were initially rapid, then slowed after about a week, but continued to increase throughout the exposure period (Figure 1). Similar curves for [Cd]moss versus time were reported for the accumulation of Cd and other metals in various moss species by several researchers (20-23). In contrast, a linear uptake of Cd by F. dalecarlica over 28 d was reported by Gagnon et al. (24), which is likely explained by the large fluctuations in [Cd2+] in the semi-static experimental system used by these researchers. In the Cd efflux portion of our experiment, Cd loss was initially rapid, then slowed greatly thereafter; at the end of the efflux phase, [Cd]moss had not returned to the low levels measured prior to Cd exposure (Figure 1). To describe Cd uptake and loss by F. dalecarlica, we first tested the simplest alternative, that of a one-compartment model. However, because it did not satisfactorily explain our results for Cd-efflux (model curves not shown) and because VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Free Cd Ion Concentrations, [Cd2+], during the Cd-Exposure Phase of Our Experiment, Cd Uptake (ku) and Elimination (ku) Rate Constant for Each Moss Cd-Compartment, and Initial Cd Concentration ([Cd]in) in Each Moss Compartment at Beginning of Efflux Phasea estimated [Cd2+] ( SD nM (aquarium no.)
nominal [Cd2+] nM
