communities, while Mn, Ni, and Cu pose considerable risks. The effects of chronic
irradiation were smaller than those of acute irradiation, and an acute-to-chronic ratio
was calculated to be 28 by dividing an acute dose by the chronic daily dose rate at
which the effect index was 10%. This ratio would be useful for community-level
extrapolation from acute to chronic radiation effects.
It is necessary to evaluate the combined effects of ionizing radiation and other
toxic agents on ecosystems because ecosystems are exposed to various factors. The
combined effects of γ-rays and acidification on an experimental model ecosystem
(i.e., the microcosm) that mimicked aquatic microbial communities were investigated. Microcosms, consisting of Euglena gracilis Z as a producer, Tetrahymena
thermophila B as a consumer, and Escherichia coli DH5α as a decomposer, were
loaded by the following treatments: (1) irradiation with 100 Gy
60 Co γ-rays;
(2) acidification of the culture medium to a pH ¼ 4.0, with a mixture of 0.1 N of
HNO 3 and 0.1 N of H 2 SO4 (1:1, v/v), which mimicked acid rain; and (3) irradiation
with 100 Gy γ-rays followed by the acidification of the culture medium (pH ¼ 4.0).
The γ-irradiation induced a temporary decrease in the cell density of Escherichia coli
but did not affect the cell densities of the other species. The concentrations of
chlorophyll a and ATP in the microcosm were not affected by γ-irradiation, and
chlorophyll a concentrations in Euglena gracilis cells were also not affected.
Acidification significantly decreased the cell density of Tetrahymena thermophila,
slightly decreased the cell density of Escherichia coli, and slightly increased the cell
density of Euglena gracilis. The concentrations of chlorophyll a and ATP in the
microcosm were increased by acidification, although chlorophyll a concentrations in
Euglena gracilis cells decreased. The combined exposure to γ-rays and acids
temporarily decreased the cell density of Escherichia coli, significantly decreased
the cell density of Tetrahymena thermophila, and slightly increased the cell density
of Euglena gracilis. The concentrations of chlorophyll a and ATP in the microcosm
were increased by this combined exposure, although chlorophyll a concentrations in
Euglena gracilis cells decreased. It was therefore concluded that the combined
exposure to γ-rays and acids had additive effects on cell densities, chlorophyll
a and ATP concentrations in the microcosm, and chlorophyll a concentrations in
Euglena gracilis cells (Fuma et al. 2010).
7.11.2 γ-Ray (
60
Co)
60 Co γ-rays were used to irradiate the microcosm at various stages of biological
succession and with various strengths, and the microcosm was transferred to a fresh
culture medium after a certain period of time. In the case of irradiation during the
initial stage or young stage, with various strengths, all microorganisms in the
microcosm became extinct at 3000 R and the system collapsed; it did not recover
by being transferred to a fresh culture medium. In contrast, the system did not
collapse with 1000,000 R irradiation during the stable stage. In the transferred,
fresh culture medium after 1 day since irradiation, there was an increased volume
7 Example Assessments of the Microcosm N-System
133
irradiation were smaller than those of acute irradiation, and an acute-to-chronic ratio
was calculated to be 28 by dividing an acute dose by the chronic daily dose rate at
which the effect index was 10%. This ratio would be useful for community-level
extrapolation from acute to chronic radiation effects.
It is necessary to evaluate the combined effects of ionizing radiation and other
toxic agents on ecosystems because ecosystems are exposed to various factors. The
combined effects of γ-rays and acidification on an experimental model ecosystem
(i.e., the microcosm) that mimicked aquatic microbial communities were investigated. Microcosms, consisting of Euglena gracilis Z as a producer, Tetrahymena
thermophila B as a consumer, and Escherichia coli DH5α as a decomposer, were
loaded by the following treatments: (1) irradiation with 100 Gy
60 Co γ-rays;
(2) acidification of the culture medium to a pH ¼ 4.0, with a mixture of 0.1 N of
HNO 3 and 0.1 N of H 2 SO4 (1:1, v/v), which mimicked acid rain; and (3) irradiation
with 100 Gy γ-rays followed by the acidification of the culture medium (pH ¼ 4.0).
The γ-irradiation induced a temporary decrease in the cell density of Escherichia coli
but did not affect the cell densities of the other species. The concentrations of
chlorophyll a and ATP in the microcosm were not affected by γ-irradiation, and
chlorophyll a concentrations in Euglena gracilis cells were also not affected.
Acidification significantly decreased the cell density of Tetrahymena thermophila,
slightly decreased the cell density of Escherichia coli, and slightly increased the cell
density of Euglena gracilis. The concentrations of chlorophyll a and ATP in the
microcosm were increased by acidification, although chlorophyll a concentrations in
Euglena gracilis cells decreased. The combined exposure to γ-rays and acids
temporarily decreased the cell density of Escherichia coli, significantly decreased
the cell density of Tetrahymena thermophila, and slightly increased the cell density
of Euglena gracilis. The concentrations of chlorophyll a and ATP in the microcosm
were increased by this combined exposure, although chlorophyll a concentrations in
Euglena gracilis cells decreased. It was therefore concluded that the combined
exposure to γ-rays and acids had additive effects on cell densities, chlorophyll
a and ATP concentrations in the microcosm, and chlorophyll a concentrations in
Euglena gracilis cells (Fuma et al. 2010).
7.11.2 γ-Ray (
60
Co)
60 Co γ-rays were used to irradiate the microcosm at various stages of biological
succession and with various strengths, and the microcosm was transferred to a fresh
culture medium after a certain period of time. In the case of irradiation during the
initial stage or young stage, with various strengths, all microorganisms in the
microcosm became extinct at 3000 R and the system collapsed; it did not recover
by being transferred to a fresh culture medium. In contrast, the system did not
collapse with 1000,000 R irradiation during the stable stage. In the transferred,
fresh culture medium after 1 day since irradiation, there was an increased volume
7 Example Assessments of the Microcosm N-System
133