Toxic sulphide can accumulate in aquatic environments

when organic material is decomposed under oxygen-limiting conditions (Nedwell, 1982). (The term sulphide is intended to include undissociated H2S, the bisulphide anion HS− and the sulphide anion S2−.) In marine sediments, sulphide production is enhanced by bacterial sulphate reduction (Jorgensen and Fenchel, 1974; Nedwell, 1982). Species inhabiting intertidal flats or salt marshes are, therefore, frequently exposed to considerable concentrations of sulphide (Bagarinao, 1992; Grieshaber and Völkel, 1998). When the sulphide concentration in the sediment is high, sulphide can reach significant concentrations in the water column and may affect, for example, fishes living in shallow waters along the coast (Bagarinao, 1992). Tidal marsh inhabitants such as the California killifish (Fundulus parvipinnis) and the long-jawed mudsucker (Gillichthys mirabilis) exhibit very high sulphide tolerances, with 96 h LC50 values of 700 and 525 μmol l−1, respectively. In contrast, active swimmers and open coast inhabitants such as the speckled sanddab (Citharichthys stigmaeus) show little sulphide tolerance (Bagarinao and Vetter, 1989). Sulphide concentrations are generally lower in freshwater systems than in marine habitats. Nevertheless, sulphide can accumulate in the hypolimnion of eutrophic lakes and in fish farms (Reynolds and Haines, 1980; Bark and Goodfellow, 1985). Many freshwater fishes, including the rainbow trout (Oncorhynchus mykiss), are sensitive to sulphide, showing 96 h LC50 values in the low micromolar range (Smith et al., 1976). Some species, however, exhibit strikingly high sulphide tolerances. The common carp (Cyprinus carpio), for example, survives in highly polluted river water and is able to withstand sulphide concentrations of approximately 300 μmol l−1 for 1 month (Kumar and Mukherjee, 1988). Membranes are in general highly permeable to sulphide (Beermann, 1924) and it is, therefore, very likely that sulphide quickly penetrates the gill epithelium of fish. Sulphide entering the blood may interact with haemoglobin (Hb) by forming sulphaemoglobin (SHb), a green Hb derivative with covalently bound sulphur at the pyrrole of the porphyrin ring (Morell et al., 1967; National Research Council, 1979). Ferrous (FeII) SHb binds O2 with a much lower affinity than Hb: the P50 of SHb is approximately two orders of magnitudes higher than 1047 The Journal of Experimental Biology 203, 1047–1058 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 JEB2717