mol of total gadolinium atoms per a liter (mol/ L). The same volume of distilled water was added to each system for controls. There were three replicates for each treatment.
57. An Ecological Evaluation of Gadolinium Toxicity Using an Aquatic Microcosm
Shoichi Fuma, Hiroshi Takeda, Kiriko Miyamoto, Ket Yanagusawa, Yoshikazu Inoue, Nobuyoshi Ishii, Kazunori Sugai, Chitose Ishii and Zen'ichiro Kawabata* (*Kyoto University)
Keywords: aquatic microcosm, ecological assessment, Escherichia coli, Euglena gracilis, gadolinium, model ecosystem, Tetrahymena thermophila
Gadolinium (Gd) is one of the rare earth metals. It has been used as an admixture to superconductors, magnets, fluorescent materials, electronic materials, glasses and so on. It has also been recently used as a paramagnetic contrast-enhancing agent in nuclear magnetic resonance imaging (MRI). It is possible that natural ecosystems will be damaged by the increasing industrial and medical uses of gadolinium. However, there are few trials for ecological evalua tion of gadolinium toxicity, especially at the community level. The authors investigated gadolinium toxicity to microbial communities of an aquatic microcosm.
The microcosm used in this study was developed by Kawabata et a1.(V. Protozool. Res., 5, 23-26, 1995) It consists of flagellate algae Euglena gracilis Z as a producer, eiliate protozoa Tetrahymena thermophila B as a consumer and bacteria Escherichia coli DH5 a as a decomposer. The details of procedures of the microcosm's construction, its incubation and measurement of the population densities of each microorganism are in the literature (for example, Fuma et al., Int. J. Radiat. Biol., 74, 145-150, 1998)
The population change of each organism in the mi crocosm reaches a steady state 50 days after inoculation as a result of interactions between the species. All species can co-exist in the microcosm for as long as one year. In the case that each organism is cut tured alone in the same medium and conditmons as the microcosm, T. thermophila dies out without reaching a steady state. T. thermophila cannot extst without E. coli, because T. thermophila grazes E. coli as its staple food. The microcosm is maintamned with energy of protease peptone in the early stage of culture. After exhaustion of proteose peptone, ut ts maintained with energy which Eu. gracilis fixes by photosynthesis. Each species is supported with metabolites or the breakdown products of the other two species. The microcosm is therefore considered to simulate a basic process in aquatic microbial cornmunities. It also makes the microcosm available as an ecotoxicological test tool because there is good repeatability in these population changes in the mi crocosm and pure-culture systems, respectively.
In this study, the microcosm system, and pureculture system of Eu. gracilis and E. coli were ex posed to gadolinium on the 56th, 58th and 59th day after the beginning of the culture, respectively. As for T. thermophila pure-culture system, T thermophila was inoculated to the microcosm medium to which gadolinium had been added. Gadolinium was added to each system in the form of GdCl3 solution at nominal concentrations of 50 100 300 and 1000 mol of total gadolinium atoms per a liter (mol/ L). The same volume of distilled water was added to each system for controls. There were three replicates for each treatment.
Fig. 22 shows the changes in the population densi ties of the three species in the microcosm after exposure to gadolinium. In controls, the populations of each species remained almost constant for the dura tion of the experiment. At 50 mol/ L gadolinium, the population of no species in the microcosm were affected significantly. However, the population of E. coli showed a tendency to be larger than the controls, though this increase was not statistically significant. At 100 mol/ L, the population of E. coli temporar ily decreased compared with controls, though this decline was not statistically significant. However the population of E. coli recovered to the control lev els soon, and after the 32nd day of exposure they were maintained at larger levels than controls. The populations of the other two species in the microcosm were not affected. At 300 mol/L, E. coli almost died out just after exposure. The population of T. thermophila decreased, and became smaller than controls on the TLh day. This decline was maintained until the 50th day. However, the populations of T thermophila recovered to control levels on the 87th day. The populations of Eu. gracilis at these three grades of concentrations were not affected signifi cantly. At 1000 mol/L, all species died out just after exposure.
Each species in the microcosm did not respond to gadolinium in the same manner as its constituents in pure-culture systems. For example, at 50 and 100 mol/ L gadolinium, T. thermophila was not af fected in the microcosm, while it died out earlier than controls in the pure-culture systems. At 300 mol/ L, T. thermophila did not die out in the microcosm, though it temporarily decreased compared with controls. On the other hand, at the same ton centration of gadolinium, pure-cultured T. thermophila did not grow at all, and died out much earlier than controls. At 300 mol/ L, Eu. gracilis was not af fected in the microcosm, while it died out in the pure-culture systems. This mitigation of gadolinium toxicity to T. thermophila or Eu. gracilis in the microcosm might arise from co-existence of other species. That is, there is the following possibility: (1) Coexisting species decreased gadolinium concentrations in the medium by absorption or adsorption of gadolinium. (2) Co-existing species transformed a chemical form of added gadolinium to a less toxic one. For example, Gd3+ might be chelated with metabolites or breakdown products of co-existing speaies. This is supported by the fact that adding organic ligands which can form a gadolinium organic species complex led to a great reduction of the gadolinium bioconcentration in algae (Sun et al. 1997), which is expected to reduce gadolinium toxic ity to the algae. For another example, at 100 mol /L, the populations of E. coli in the microcosm tem porarily decreased compared with controls, while E. coli cultured alone was not affected. This enhancement of effects on E. coli in the microcosm might arise from transformation of Gd3+ to more toxic chemical forms by co-existing Eu. gracilis or T. thermophila.
These results indicate that the microcosm responded to gadolinium at the communuty level despite its simplicity as shown to 
-rays and acidification. This suggests that the evaluation of gadolinium ecotoxicity by this microcosm test us more realistic than single-species tests, which are generally used for ecotoxicity evaluatton.
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| Fig.22. Effects of gadolinium on the populations in the microcosm. Solid lines represent results of the microcosm exposed to gadolinium. Broken lines represent results of controls. Values are the mean of three replicates. Asterisks indicate statistically significant differences from controls (p<0.05, Student's t-test). |
Publications:
1)Fuma, S., Takeda, H., Miyamoto, K., Yanagisawa, K., Inoue, Y., Sato, N., Hirano, M., Kawabata, Z. Int. J. Radiat. Biol., 74, 145-150, 1998.
2)Fuma, S., Miyamoto, K., Takeda, II., Yanagisawa, K., Inoue, Y., Sato, N., Hirano, M., Kawabata, Z.:In: Inaba, J., Nakamura, Y. (eds). Comparative Evaluation of Environmental Toxicants-Health ef fects of Environmental Toxicants Derived from Advanced Technologies. Kodansha Scientific Ltd.,Tokyo, pp 131-144, 1998.