Lysosomes and sulfide-oxidizing bodies in the bacteriocytes of Lucina pectinata, a cytochemical and microanalysis approach

Abstract

Lucina pectinata is a large tropical clam living deeply burrowed in the black, reducing mud of mangrove swamps. It is known to possess hemoglobin in the cytoplasmic areas of its bacteriocytes, which harbor sulfide-oxidizing bacteria. The bacteriocytes also possess lysosome-like microbodies containing either membrane whorls or electron-dense granules in which free heme compounds have been identified. The cytochemical detection of acid phosphatase and arylsulphatase through EDX (energy-dispersive X-ray) microanalysis strongly suggests that the bacteriocytes of L. pectinata contain, in fact, two different types of microbodies. Some of these (devoid of dense granules) possess a variable amount of lysosomal enzymes and occasionally a limited quantity of iron, which may result from a recycling process of hemoglobin. Their main function seems to be the digestion of a limited proportion of symbiotic bacteria. They represent genuine secondary lysosomes with a functionally acidic pH. The second type of microbodies is characterized by dense granules containing sulfur and iron hemes but no lysosomal enzymes. Their sulfide-oxidizing activity was substantiated by benzyl viologen assay, with Na₂S as a substrate. These microbodies appear to be similar to the sulfide-oxidizing bodies (SOBs) described in the bacteriocytes of other bivalve species with symbiotic thioautotrophic bacteria; however, their sulfide-oxidizing activity appears to be non-enzymatic. They are discrete organelles, characterized by a functionally basic pH and pseudoperoxidasic activity, and have been termed SOBs. Therefore, the bacteriocytes of L. pectinata possess at the same time functional lysosomes and functional SOBs.