Sulphur-oxidising bacteria and haemoglobin in gills of the bivalve mollusc Myrtea spinifera

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Abstract
The gills of M. spinifera carry large numbers of Gram-negative bacteria within the epithelial cells of the subfilamentar region and contain elemental S. Hb which has a high affinity for O2 is present in the gill tissue. Homogenates of gill tissue fix CO2 via ribulosebisphosphate carboxylase and contain 3 enzymes concerned in the oxidation of S, adenylylsulfate reductase, sulfate adenylyltransferase and sulfate adenylyltransferase (ADP). Fixation of CO2 by whole gills is enhanced by free S-. Whole gills accumulate labeled S when supplied with Na235S, and a major proportion of the labeled S is metabolized to non-volatile compounds or is bound to the tissues. The bacteria are evidently chemoautotrophs and presumably obtain energy for fixation of CO2 by oxidation of reduced S compounds. The hemoglobin is not apparently involved in transport of S- but may control O2 tension close to the bacteria. The sediment in which the bivalves live is low in S- compared with some coastal muds and the hydrothermal vents. The interstitial water contains less than 1 .mu.M of dissolved S- and the sediment releases less than 700 .mu.moles dm-3 of dissolved S- when treated with dilute acid. Habitats with similar low concentrations of dissolved S- are widespread in nature. An ability to exploit such low levels of reduced S must be mutually advantageous to host and bacteria, though as yet there is no direct evidence for transfer of organic matter from bacteria to host. To obtain sufficient energy for growth and maintenance of the association, the bacteria must have access to the sediment-bound S-.