7.7 Algal Toxins
In the aquatic ecosystem, pollution with microcystin, which are the toxins produced
by high fecundity, eutrophying blue-green algae, combined with anthropogenic
eutrophication, presents a major environmental problem. In eutrophic lakes during
the summer season, blue-green algae such as Microcyctis multiply irregularly to
large quantities, modifying the quality of the water and the biota of the aquatic
ecosystem every year. In particular, blue-green algae produce toxins such as
microcystin, and only very small amounts of these toxins that are present in lakes
are derived from nature. However, an impact assessment of the aquatic ecosystem,
especially the microbial ecosystem, with respect to contamination by microcystin,
has not been performed. Therefore, evaluation of the influence of the toxin
microcystin on microbial ecosystems, through examination of the biological interactions and material circulation among organisms and through the use of reduction
functions, is important.
7.7.1 Microcystin-LR
Based on the structural parameter (population density), there was no difference
observed between the populations of two microcystin-LR (where L stands for
leucine and R for arginine) systems (0.1 mg/L and 1 mg/L addition) and the control
system over time. The populations of Chlorella sp. and Scenedesmus quadricauda,
both chlorophytes, remained stable between 10
5 N/mL and 10
6 N/mL after the
addition of microcystin-LR. The population of Tolypothrix sp., a cyanophycean,
was also stably maintained between 10
4 N/mL and 10
6 N/mL. As for the
microanimals, the population of Cyclidium glaucoma decreased after the addition
of microcystin-LR, but because this behavior was also observed in the control
system, it was thought to simply reflect a natural change in the microcosm, and the
population was stable between 10 Nm/L and 10
2 N/mL. Additionally, the
populations of Philodina erythrophthalma and Aeolosoma hemprichi were stable
and ranged from 1 N/mL to 10 N/mL.
With the addition of 0.1 mg/L and 1 mg/L of microcystin, it became clear that
there was no remarkable change in the populations of this microcosm. Similarly, no
remarkable changes in the P/R ratio between the two addition systems (0.1 mg/L and
1 mg/L of microcystin-LR addition) and the control system, which involved the
addition of 480 mg/L of methanol, were recognized. However, there was a tendency
for the amount of respiration (R) to increase in comparison with the additive-free
control system, but it became clear that microcystin-LR did not have a remarkable
influence on the P/R ratio under the conditions of this experiment (Fig. 7.23).
From these results, the m-NOEC of microcystin-LR was estimated to be more than
1 mg/L. This value is larger than 1 ppb, which is the guideline for concentrations in tap
water established by the World Health Organization. It was made clear that microcystinLR does not have a significant influence on the microcosm.
7 Example Assessments of the Microcosm N-System
109
In the aquatic ecosystem, pollution with microcystin, which are the toxins produced
by high fecundity, eutrophying blue-green algae, combined with anthropogenic
eutrophication, presents a major environmental problem. In eutrophic lakes during
the summer season, blue-green algae such as Microcyctis multiply irregularly to
large quantities, modifying the quality of the water and the biota of the aquatic
ecosystem every year. In particular, blue-green algae produce toxins such as
microcystin, and only very small amounts of these toxins that are present in lakes
are derived from nature. However, an impact assessment of the aquatic ecosystem,
especially the microbial ecosystem, with respect to contamination by microcystin,
has not been performed. Therefore, evaluation of the influence of the toxin
microcystin on microbial ecosystems, through examination of the biological interactions and material circulation among organisms and through the use of reduction
functions, is important.
7.7.1 Microcystin-LR
Based on the structural parameter (population density), there was no difference
observed between the populations of two microcystin-LR (where L stands for
leucine and R for arginine) systems (0.1 mg/L and 1 mg/L addition) and the control
system over time. The populations of Chlorella sp. and Scenedesmus quadricauda,
both chlorophytes, remained stable between 10
5 N/mL and 10
6 N/mL after the
addition of microcystin-LR. The population of Tolypothrix sp., a cyanophycean,
was also stably maintained between 10
4 N/mL and 10
6 N/mL. As for the
microanimals, the population of Cyclidium glaucoma decreased after the addition
of microcystin-LR, but because this behavior was also observed in the control
system, it was thought to simply reflect a natural change in the microcosm, and the
population was stable between 10 Nm/L and 10
2 N/mL. Additionally, the
populations of Philodina erythrophthalma and Aeolosoma hemprichi were stable
and ranged from 1 N/mL to 10 N/mL.
With the addition of 0.1 mg/L and 1 mg/L of microcystin, it became clear that
there was no remarkable change in the populations of this microcosm. Similarly, no
remarkable changes in the P/R ratio between the two addition systems (0.1 mg/L and
1 mg/L of microcystin-LR addition) and the control system, which involved the
addition of 480 mg/L of methanol, were recognized. However, there was a tendency
for the amount of respiration (R) to increase in comparison with the additive-free
control system, but it became clear that microcystin-LR did not have a remarkable
influence on the P/R ratio under the conditions of this experiment (Fig. 7.23).
From these results, the m-NOEC of microcystin-LR was estimated to be more than
1 mg/L. This value is larger than 1 ppb, which is the guideline for concentrations in tap
water established by the World Health Organization. It was made clear that microcystinLR does not have a significant influence on the microcosm.
7 Example Assessments of the Microcosm N-System
109