Journal of Environmental Quality 31:1570-1575 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Heavy Metals in the Environment
Relationships between Bacterial Tolerance Levels and Forms of Copper and Zinc in Soils
K. Saeki*,a,
T. Kunitob,
H. Oyaizuc and
S. Matsumotoc
a Biotron Institute, Kyushu Univ., Hakozaki, Fukuoka, 812-8581, Japan
b Center for Marine Environmental Studies (CMES), Ehime Univ., Bunkyo-cho 3, Matsuyama 790-8577, Japan
c Dep. of Applied Biological Chemistry, Graduate School of Agriculture and Life Sciences, The Univ. of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan
* Corresponding author (ksaeki{at}agr.kyushu-u.ac.jp)
Received for publication April 10, 2001.
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ABSTRACT
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The effects of various fractions of copper (Cu) and zinc (Zn) on soil bacteria were evaluated by the heavy metal tolerance level of the bacterial community (IC50) in soil samples collected near a mine. The IC50 values had no relationship with the total concentrations of Zn and Cu in the soils, but were weakly correlated with the 0.05 M CaCl2–extractable form of each metal in the soils (Cu: R2 = 0.670, p < 0.01; Zn: R2 = 0.453, p < 0.05). It was found that the IC50 correlated strongly with the total concentration of each metal in the extracts from water-saturated soil samples, described below as "soil solution" (Cu: R2 = 0.789, p < 0.01; Zn: R2 = 0.617, p < 0.01). The speciation of these metals in the soil solutions was estimated using an equilibrium thermodynamic computer model, SOILCHEM. Simulated free Cu ion ranged from 18 to 98% of total Cu, and organic complexes of Cu ranged from <1 to 56%. In all samples, Zn existing as the free ion was estimated to be more than 80% of total Zn in the soil solutions. The IC50 values were also correlated with the estimated free metal ion activities, but with slightly lower correlation coefficients than found for total concentration in the soil solutions (Cu: R2 = 0.735, p < 0.01; Zn: R2 = 0.610, p < 0.01). The results suggest that not only high metal ion activities, but also total dissolved metal concentrations in soil solutions may affect the bacterial community.
Abbreviations: CuS, total copper concentration in soil solution • IC50, heavy metal tolerance level of the bacterial community • pCu, copper ion activity • pZn, zinc ion activity • ZnS, total zinc concentration in soil solution
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INTRODUCTION
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MOST INVESTIGATIONS of the effects of metal pollution on soil microorganisms have focused on estimating the numbers, biomass, or activity of the microorganisms. It is well known that these variables can be influenced by different environmental factors, such as organic matter content and temperature, and not only by heavy metals. In addition, altered biological activity in polluted habitats can be caused by factors changed by the presence of metals (e.g., changes in pH or nutrient availability). Information obtained from any metal impact studies can therefore be difficult to evaluate because of the problems of separating the effect of metal toxicity from those of other environmental factors. Diaz-Ravina et al. (1994) evaluated the effects of heavy metals on the soil bacteria by the tolerance level of the community. The tolerance level (IC50) is the heavy metal concentration in growth media where a bacterial colony forming unit (CFU) decreases to 50% of that in unsupplemented media. A low IC50 value of a soil bacterial community means that metal-sensitive bacteria are dominant and that the community is affected at low metal levels. The method has an advantage in that it is uniformly adaptable to a variety of soil bacterial communities since the bacterial community extracted from the soil is examined under the same conditions. Thus, this value is rarely affected by the various soil properties other than the heavy metal effects (Baath, 1992; Baath et al., 1998). As a disadvantage, the IC50 measurement may be looking at a biased subpopulation of soil bacteria because many, probably most, species cannot be successfully cultured in the traditional growth medium.
It is important to evaluate which forms of heavy metals affect the indicator (IC50) in the soil bacterial community. In a previous study, we showed that IC50–Cu was positively correlated with the log concentration of soluble and exchangeable Cu (R = 0.757, p < 0.01) (Kunito et al., 1999). Furthermore, heavy metal ion activity in soil solution might have more toxic effects on soil bacterial communities than soluble and exchangeable forms in soil. Recently, heavy metal ion activities controlled by solubility–precipitation and sorption–desorption reactions in soil have been proposed to be one of the essential parameters for evaluating the toxicity of these metals to organisms (McBride, 1989; Sauve et al., 1998). Lighthart et al. (1983) reported that dissolved Cu was predicted to inhibit soil microbial respiration when the free ion activity reached a value of 0.01 to 0.1 mM in the soil solution. However, no effect of Cu ion activity in the soil solution was observed on the N and C mineralization rates in field-aged Cu-enriched soils (Minnich and McBride, 1986).
In the present study, soil samples with wide ranges of pH, organic carbon, and total heavy metal content were investigated to evaluate the influence of various Cu and Zn forms in the soils on IC50. The heavy metal forms included total soil concentrations, soluble and exchangeable forms, total concentration in soil solutions, and speciation in soil solutions.
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MATERIALS AND METHODS
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Soil Samples
We used eleven soil samples, collected at a depth of 0 to 10 cm near a copper mine in Akita Prefecture (Japan), contaminated by the heavy metals discharged from the mine.
Three soil samples of 500 g were randomly collected in each area (about 1 km2), then mixed to one sample. There was a distance of at least 50 m between sampling points. The soil samples SS-1, -2, -3, and -4 (Typic Hapludults), collected from a forest, were rich in organic matter. The other seven soils (Lithic Udorthents) were sampled from a place with little or no vegetation. Hence, these had little organic matter, and the pH values of SS-5, -6, and -7 were approximately 8 due to treating with lime to prevent Cu leaching from the soils.
The soils were passed through a 2-mm sieve and well dispersed. A portion of the soil samples was air-dried and analyzed for pH, total nitrogen, and organic carbon (Table 1). The amounts of soil organic C and total N were measured as described by Nelson and Sommers (1982). These soil analyses were done in triplicate.
Copper and Zinc Tolerance Levels of the Bacterial Community
The Cu and Zn tolerance levels of the bacterial community (Diaz-Ravina et al., 1994; Diaz-Ravina and Baath, 1996; Pennanen et al., 1996) were determined to evaluate the influence of the heavy metals on the bacteria in the soil. Tolerance levels were estimated by the plate count method following Diaz-Ravina et al. (1994), with the bacterial community extracted from the soil by homogenization. The bacterial numbers were estimated by a dilution plate-count technique with diluted TSB-agar media (0.2% typtic soy broth, 1% agar, 50 mg of cycloheximide L-1) supplemented with CuCl2 or ZnCl2 at four to six different concentrations. A control without any added metals was always prepared. The number of colony forming units in each metal-supplemented medium was expressed as a percentage of this control value. The heavy metal tolerance of each bacterial community was then estimated by calculating the logarithm of the added metal concentration, which resulted in 50% colony forming units (IC50) compared with the control value.
Copper and Zinc Extraction from the Soils
Total Cu and Zn concentrations in the soils were determined as described previously (Chino et al., 1992). The soils were heat-digested with mixed acid solutions (HNO3, HClO4, and H2SO4 = 10:4:1). Exchangeable forms of Zn were extracted from air-dried soil using a 0.05 M CaCl2 solution by shaking for 24 h. The extracts were then measured for Zn and Cu by atomic absorption spectroscopy (AAS) with D2–lamp background correction (Shimadzu [Kyoto, Japan] AA-680).
Copper and Zinc Analysis in Soil Solutions
Samples of soil solutions were obtained from a water saturation extract prepared by the modified method of Rhoades (1982). The water–soil mixture was centrifuged at 3500 x g; then the extract was filtered through a filter (0.45-µm pore size). Immediately after the filtration, the pH and electrical conductivity (EC) of the solutions were measured. Total Cu and Zn concentrations in the soil solutions (CuS and ZnS) were then measured by a graphite furnace AAS (Hitachi [Tokyo, Japan] Z-9000). Total concentrations for the other metals were determined by AAS or inductively coupled plasma atomic emission spectroscopy (ICP–AES), while the anions were measured by ion chromatography, and carbonate by a titration method (Hatano, 1986). Soluble organic carbon (SOC) was measured by wet combustion on an automatic C analyzer with an infrared detector (Shimadzu TOC-5000). Table 2 shows the data for the total soluble metal, ligand concentrations, pH, and electrical conductivity in the soil solutions.
Speciation of Cu and Zn in the soil solutions was simulated by SOILCHEM (Sposito and Coves, 1988) using the data in Table 2. SOILCHEM is a modified version of the multipurpose computer equilibrium chemical model GEOCHEM (Mattigod and Sposito, 1979). In the chemical speciation by SOILCHEM, a mixture model developed by Mattigod and Sposito (1979) was used for the simulation of complex formation between metals and organic matter in soil solutions. The inputs to the model also include the amount of organic C actually measured in the solutions. The metal ion activity was calculated with each ion activity coefficient from the data of the free ion concentrations estimated by SOILCHEM. The coefficients were given by the Davies equation:
 | [1] |
where fm is an activity coefficient of an ion M, Bdh is the Debye–Huckel limiting law parameter which was set at 0.512 as indicated by Sposito (1989), Zm is the valence of species M, and A is the ionic strength. The ionic strength was calculated by the equation of Marion and Babcock (1976):
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where EC is the electrical conductivity of the solution.
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RESULTS AND DISCUSSION
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Comparison of the Heavy Metal Tolerance Levels among the Soil Bacterial Communities
Higher IC50–Zn values were found in the soil samples SS-1, -8, and -9 than in SS-2, 3, -4, -6, and -7 (Table 1), indicating that the Zn tolerance levels of the soil bacterial communities were higher in SS-1, -8, and -9. Our previous study also reported that higher IC50–Cu values were found in SS-8 and -9 than in the others (Kunito et al., 1999). These results implied higher portions of both Cu- and Zn-tolerant bacteria in the bacterial communities of SS-8 and -9 soils than in the others. It suggested that the ratio of heavy metal–tolerant bacteria to total bacteria increased with heavy metal toxicity, accounting for survival and adaptation to the metals in the soils, resulting in the high IC50 value of the soil bacterial community. The high IC50 value means a strong effect of the metal on the soil bacterial community. The IC50–Cu value was much greater in SS-8 than SS-7, even though SS-7 had 40 times greater total concentration of Cu in the soil than SS-8.
Copper and Zinc Effects on Heavy Metal Tolerance of the Bacterial Communities
There was no significant relationship between log(total Cu) and IC50–Cu in the soils, nor between log(total Zn) and IC50–Zn (Fig. 1 and 2)
. Therefore, total heavy metal concentrations in soils cannot be used to evaluate the metal toxicities for soil bacteria.
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Fig. 1. Relationships between heavy metal tolerance level of the bacterial community (IC50–Cu) and Cu in various fractions in soils.
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Fig. 2. Relationships between heavy metal tolerance level of the bacterial community (IC50–Zn) and Zn in various fractions in soils.
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The Zn fractions extracted by 0.05 M CaCl2 (exchangeable Zn) ranged from 3.7 x 10-3 to 1.3% of total Zn in the soils. Figure 2 shows that IC50–Zn tended to increase with log(exchangeable Zn), although with a low correlation coefficient (R2 = 0.453). The relationship (R2 = 0.573) between the IC50–Cu and exchangeable Cu was also previously reported by Kunito et al. (1999), as shown in Fig. 1. These correlations imply that the exchangeable fractions of both metals included the portion of metals that affected microbial communities in the soils.
The highest concentrations of Cu and Zn in the soil solutions were observed in samples SS-8 and -9 (Table 2), whereas the highest total concentrations of Cu and Zn existed in SS-5, -6, and -7 (Table 1). In the present study, CuS and ZnS were not related to total Cu and total Zn (Fig. 3)
. That is, the soil solutions with high metal concentrations were not necessarily in the soils containing the largest amounts of total metals. The distribution of heavy metals, like Cu and Pb, in soil solutions was affected not only by total metal concentrations, but also by other factors such as pH and organic matter (e.g., Holm et al., 1995; Sauve et al., 1996, 1998). Figure 1 shows that the IC50–Cu of the soil bacterial communities significantly increased with the decrease in -log(CuS) in the soil solutions (R2 = 0.789, p < 0.01). Similarly, IC50–Zn was also significantly correlated to -log(ZnS) in the solutions (Fig. 2). The results implied that the soil bacterial community shifted to more tolerant flora due to larger amounts of Cu and Zn in the soil solutions. These relationships for both metals were statistically stronger than those between IC50 values and exchangeable metals, indicating that the exchangeable fractions included not only water-soluble metals but also additional metals that did not directly affect soil bacteria.
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Fig. 3. Relationship between the total metal concentrations in the soils and those in the soil solutions.
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It has been reported that heavy metal solubilities in soil were affected by dissolved organic matter (del Castilho et al., 1993; McBride et al., 1997) and by electrical conductivity in solutions (del Castilho et al., 1993). However, these factors did not correlate with CuS and ZnS in our study. There was a significant relationship between log(ZnS) and pH of the soil solutions (R2 = 0.446, p < 0.06). The effect of soil solution pH (Table 2) on the relationships between IC50 values and the heavy metal concentrations in the soil solutions was investigated by stepwise multiple regression analysis:
The regression analysis indicated that the hydrogen ions in soils have two opposing effects on the heavy metal toxicities on soil bacteria: increasing the solubility and competing with the metal ions. Recently, many researchers have found reduction of heavy metal toxicity or availability by competition with other cations, especially protons, for fish (Cusimano et al., 1986; Roy and Campbell, 1995), plants (Wright et al., 1987; Grauer and Horst, 1992), algae (Parent and Campbell, 1994), and insects (Hare and Tessier, 1996). In general, it has been expected that low soil pH elevates the solubilities of heavy metals by release from soil particles to the solutions, resulting in an enhancement of the toxicity of heavy metals on soil bacteria. However, the present results imply that there is a competition between protons and metal ions for bacterial uptake sites in soil environments.
Copper and Zinc Speciation in the Soil Solutions
The speciation study for Cu in the soil solutions, simulated by SOILCHEM, revealed that the ratio of free ion to total Cu varied with the soil samples from 18 to 98%, and Cu organic complexes from <1 to 56% (Table 3). McBride and Bouldin (1984), based on ion-selective electrode data, estimated that at least 99.5% of Cu in soil solution was in an organically complexed form. Fotovat and Naidu (1997) also reported that very small proportions of soil solution Cu occur as free cupric ions. The estimated fractions of organically complexed Cu in the soil solution Cu of the present study's soils, even of high pH soils (pH > 7), were relatively lower than those of the other studies described above. This difference may be caused by the smaller soluble organic carbon (SOC) content in the soil solutions. For example, the higher-pH soil samples SS-5, -6, and -7 had only small SOC (<2.0 mg L-1) in the soil solutions (Table 2), while all soil solutions of samples used by Fotovat and Naidu (1997) contained larger amount of SOC from 18 to 137 mg L-1. Therefore, considerable proportions of soil solution Cu consequently occurred as free cupric ion because there were low concentrations of organic ligands which can bind to Cu in the soil solutions. Zinc was present predominantly as the free ionic form (80%) in all solution samples (Table 3). Thus the effects of ZnS on soil microorganism were likely to be almost equal to that of Zn free ion in the soil solutions. Many researchers reported that most of the soil solution Zn existed as free ion (Holm et al., 1995; Dang et al., 1996; Fotovat and Naidu, 1997). The SO4 complex forms of both Zn and Cu are present at a relatively high level, compared with other inorganic complexes in the soil solutions. In addition, SOILCHEM calculated that in samples SS-5, -6, and -7, a portion of soil solution Cu was present as OH and CO3 complexes, and Zn as OH complexes.
The heavy metal ion activities (pCu and pZn) were almost the same as the logarithms of the free ion concentrations in the soil solutions (Table 3). A strong relationship was observed between pCu and pZn. As hydrogen ions enhance the metal solubilities, resulting in an increase in their activities, pCu and pZn can be expected to increase with decreasing soil pH. Significant correlations between the soil solution pH and free metal ion activities were found.
Effects of Metal Species and Free Metal Ion Activities on the Heavy Metal Tolerance Level of the Bacterial Community
In general, heavy metal toxicity and availability to organisms in aquatic and soil environments have been considered to depend on the free ion activity in solutions. Bacteria are affected by heavy metal free ion activity in sea water (Sunda and Gillespie, 1979), estuarine water (Jonas, 1989), and culture media (Shuttleworth and Unz, 1991). Heavy metal intake by plants from the soil is positively correlated with the free metal ion activity in media involving soil solutions, even when most Cd in soil solution exists as inorganic complexes (Bingham et al., 1983) and most Cu exists as organic complexes in soil solution (Sauve et al., 1996). In the present study, Fig. 1 shows that the IC50–Cu of the soil bacterial communities was significantly correlated with CuS (R2 = 0.789, p < 0.01) as well as pCu (R2 = 0.735, p < 0.01). Similarly, the IC50–Zn was significantly correlated both with the amount of free Zn ion in the solutions (pZn) and ZnS (Fig. 2). It appears that total dissolved and estimated free metals are essentially equally good predictors of bacterial metal resistance, and it would take a much larger data set to establish that the free metal estimate is actually a better predictor. Therefore, these results implied that the soil microbial communities may be affected not only by free metal ions but also by integrated metal species including free ions and other inorganic complexes in the soil solutions. Janssen et al. (1997a)( b) found that for earthworms, bioaccumulation factors of heavy metals could not be explained by ion activities. For plants, Cd uptake was not explained by Cd ion activity in soil solution, but by concentrations of Cd–Cl species (McLaughlin et al., 1997; Smolders et al., 1998).
The IC50–Cu values had no relationship with the amounts of the calculated Cu–organic complex forms in the soil solutions, implying that the Cu–organic complexes had no adverse effect on the soil bacterial community. The complexation of Cu and soil organic matter may alleviate the toxicity of Cu on the soil bacterial community. The IC50–Cu was also correlative with -log[CuCl+] (R2 = 0.368, p < 0.05), -log[CuNO+3] (R2 = 0.612, p < 0.01), and log[CuSO4] (R2 = 0.474, p < 0.05), as was IC50–Zn with -log[ZnCl+] (R2 = 0.632, p < 0.01), log[ZnNO+3] (R2 = 0.381, p < 0.05), and -log[ZnSO4] (R2 = 0.627, p < 0.01). These relationships can be attributed to the collinearities of the complexes with each metal free ion concentration or activity, for example, pCu was correlative with -log[CuCl+] (R2 = 0.922), log[CuNO+3] (R2 = 0.586), and -log[CuSO4] (R2 = 0.815), and pZn was correlative with -log[ZnCl+] (R2 = 0.977), -log[ZnNO+3] (R2 = 0.813), and -log[ZnSO4] (R2 = 0.874). Alternatively, there might be a possibility of effects of the inorganic metal complexes on the soil bacteria, but this cannot be established with correlation studies.
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CONCLUSION
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The sequence of effects of various fractions of Cu and Zn on the heavy metal tolerance level of the bacterial community (IC50), evaluated with correlation analysis, was: total concentrations in the soils <<< exchangeable fractions < calculated free metal ion activities = total concentrations in the soil solutions. The IC50 values were also weakly correlated with estimated Cl, NO3, and SO4 complexes with Cu and Zn in the soil solutions. The soil solutions in the present study had smaller amounts of soluble organic carbon than those of other reported studies, leading to small fractions of organic–Cu complexes in the solution Cu. In fact, the estimated organic–Cu complexes in the soil solutions were not correlated with the IC50–Cu of the bacterial community. These results may mean that the free Cu and Zn in the soil solutions have more importance than complexed forms in determining bacterial resistance. Regression analysis indicated that the hydrogen ions in soils have two opposing effects on the heavy metal toxicities on soil bacteria: increasing the solubility and competing with the metal ions.
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ACKNOWLEDGMENTS
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We are grateful to Dr. M. Abo, Prof. S. Yamazaki (The University of Tokyo), Dr. Y. Hosen (Japan International Research Center for Agricultural Sciences), and Dr. Shin-ichiro Mishima (National Institute of Agro-Environmental Sciences) for the use of the analytical equipment. T. Kunito was supported by the Japan Society for the Promotion of Science.
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ABSTRACT
INTRODUCTION
