Biochemical, histopathological, histochemical and bioenergetic parameters were
studied to determine their suitability as biological markers for hydrogen sulphide toxicity
detection using the river catfish Mystus nemurus (C&V) exposed to H2S in laboratory
experiments as well as those caught from the wild. In the laboratory, the toxic effects of
H2S to M nemurus juveniles were determined by using a flow-through bioassay
technique.
The 96-h LC₅₀ value of unionized H₂S was 3.20 µg/L, and 0.003 µg/L unionized
H₂S was recommended as the safety level for M nemurus juveniles under tropical
environmental condition. Sulphaemoglobin and thiosulphate concentrations significantly
increased (p<0.01) with increasing hydrogen sulphide (H₂S) concentrations and exposure
time. However, H₂S reduced the oxygen carrying capacity of haemoglobin by reducing
oxyhaemoglobin. Glutathione S-transferase (GST) specific activities significantly
increased (p<0.01) in fish exposed to H₂S higher than 30% of LC₅₀. Gill lesions such as epithelial separation, club-shaped lamellae and
interlamellar fusion were observed at different concentrations of H2S. The evidence
of neurotoxicity was elucidated by necrosis and damaged mitochondria in fish brain
tissue. Sulphur accumulation in gills progressively increased with the increase of H₂S
concentrations and exposure time.
The liver-somatic index (LSI) and growth rate significantly decreased (p<0.05)
with increased concentrations of H₂S and exposure time. Fulton's condition factor
failed to predict (p>0.1) stress effects in fish exposed less than six weeks to H₂S.
However, RNA-DNA ratios showed high correlations with H2S concentrations from
the second (r²= 0.83; p<0.01) to sixth week (r²=0.98;p<0.01) of exposure.
Thiosulphate and sulphaemoglobin showed positive correlations with H₂S
concentrations (r²= 0.79; p<0.01 and r²= 0.89; p<0.01 respectively). Sulphur
accumulation in gills was positively correlated with thiosulphate and sulphaemoglobin
concentrations in blood (r²= 0.74; p<0.01), indicating that these compounds resulted
from H2S exposure. In addition, H2S levels in water were directly correlated with
GST activities and sulphaemoglobin concentrations. However, H2S concentrations
showed an inverse relationship with oxyhaemoglobin concentrations.
The field study supported the laboratory findings for two indicators;
thiosulphate and sulphur accumulation, were potential biological markers for H₂S
toxicity. Other markers such as Fulton' s condition factor, liver-somatic index, growth
rate, RNA-DNA ratio, histopathology and histochemistry did not reflect specific toxic
effect, although they can be used to indicate the general health condition of fish
exposed to H₂S. Among all the indicators, thiosulphate was found to be the simplest
and fastest biological marker for detecting H₂S toxicity