Sulphide metabolism in burrowing marine crustacea

Sulphide occurs widely in marine sediments and is highly toxic to most organisms. Its principal poisoning effect occurs at extremely low concentrations and is the result of inhibition of mitochondrial cytochrome c oxidase. Mud-shrimps, (Crustacea: Decapoda: Thalassinidea), construct burrows in sublittoral muddy sediments. The sediment in which they burrow may often be markedly reduced and conditions within the burrow are usually hypoxic and hypercapnic. Field measurements taken during this study also indicate significant exposure to sulphide in two species of mud-shrimp. Direct evidence has demonstrated that Callianassa subterranea may be exposed to potentially toxic levels of sulphide in the burrow water (37 +/- 44 muM, range 0 - 206 muM). Sulphide, thiosulphate and sulphite were also found in the haemolymph and tissues of freshly caught animals (Calocaris macandreae and Callianassa subterranea) indirectly indicating that they are exposed to ambient sulphide. The concentrations of sulphide found in the burrow water and thiol concentrations within the mud-shrimps also showed seasonal variations which may be correlated with the input of organic material. Mud-shrimps appear to be physiologically adapted to tolerate elevated levels of sulphide that they may encounter in their natural habitat. Laboratory experiments carried out on three species, Calocaris macandreae, Callianassa subterranea and Jaxea nocturna, have shown that they have a high tolerance of sulphide. High micromolar concentrations of sulphide (360 +/- 320 muM) were also found not to affect the irrigation behaviour of the mud-shrimp Calocaris macandreae suggesting tolerance of sulphide exposure. The mud-shrimp Calocaris macandreae can tolerate substantial exposure to sulphide before showing signs of anaerobic metabolism. An oxygen-dependent 'oxidation' mechanism appears to exist which defends mitochondrial cytochrome c oxidase from sulphide poisoning. Sulphide diffusing into the body tissues is oxidized primarily to thiosulphate, in the hepatopancreas and muscle tissues, and accumulates rapidly even during brief exposure to low micromolar concentrations of sulphide. Sulphite also appears as a secondary 'oxidation' product. No evidence of chemoautotrophic bacterial symbionts could be found and it is therefore proposed that the 'oxidation' of sulphide is facilitated by the animal's tissue. The 'oxidation' mechanism allows aerobic metabolism to be maintained despite the presence of sulphide although the mud-shrimp's sensitivity to sulphide is apparently greater at lower oxygen partial pressures. Sulphide also has a concentration-dependent capacity to either stimulate or inhibit aerobic metabolism in mud-shrimps. Aerobic metabolism appears to increase during exposure to sulphide, when oxygen is available, and can be maintained even under severe hypoxic and sulphidie conditions. The mud-shrimps resort to anaerobiosis (which may operate concurrently with aerobic metabolic pathways) when the 'oxidation' mechanism is saturated, when oxygen supply becomes limited and when intracellular sulphide concentrations become toxic. The possibility of metabolic depression to reduce energy expenditure may also occur in response to sulphide exposure and is discussed. Haemocyanin function was also found to be unaffected by sulphide and oxygen affinity increased in the presence of thiosulphate. This may be a physiological adaptation to increase the amount of oxygen available for sulphide 'oxidation' to produce the metabolites thiosulphate and sulphite during severely hypoxic and sulphidic conditions. Mudshrimps are therefore physiologically adapted to tolerate elevated levels of sulphide which they may encounter in their natural habitat. The 'oxidation' of sulphide in the mitochondria in a small number of other marine invertebrates has been established and it therefore seems likely that this may be the case in mud-shrimps. Indirect evidence suggests that the underlying biochemical mechanism of mitochondrial sulphide 'oxidation' is enzymatic ('sulphide oxidase'), although direct evidence of this, the isolation of such an enzyme and its characterization remain to be elucidated.