Toxicity of parathion-methyl to a cyanobacterium
201
HPC. As extreme values computed by the probit
Acknowledgements
method such as LC05 and LC95 are not statistically
dependable, they were not computed here.
We are grateful to Dr M D P Rao, Principal,
Khallikote College, Berhampur for facilities and part
of the work was done with the help of a major
research project on pesticides from Department of
Environment, Government of India, New Delhi to
RNP. This piece of work is a part of PhD thesis of S
Panigrahi (Berhampur University, 2001).
At sub-lethal concentrations in agar media both
formation and germination of akinetes of the
cyanobacterium were suppressed. Both these
processes individually require suitable combinations
of growth or climatic parameters (Nichols &Adams,
1982; Herdman, 1988). The presence of a toxic
chemical should alter the physiological balance
entailing failures of both processes. Earlier studies
with the fungicides, 2-PAM (Pyridinium-
2-aldoxime, methyl iodide) and MEMC
(methoxyethyl mercuric chloride), suppression of
akinete formation and germination were recorded
with the same cyanobacterium (Panigrahi et al.,
1998). This could create a disturbing condition in
life cycles of similar N2-fixing cyanobacteria, when
a residual insecticide such as MEMC is used.
The reducing activity of heterocysts (which lack
PS II activity and do not liberate O ) in the present
study was less readily affected by2the insecticide,
probably due to the delay in the penetration of the
insecticide and TTC into heterocysts, which have
thick walls. A high concentration of the insecticide
(five times more than the LC100 levels obtained)
caused an impairment of the heterocyst
dehydrogenase activity. The process of N -fixation
was found to be highly sensitive initially, as2the levels
below the HPC impaired the process, resulting in a
significant reduction in the total N-content of
cultures. But at longer incubation times,
detoxification probably caused a progressive
increase in the total N-content.
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