Hemoglobins in Mollusc/Bacteria Symbioses

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Clams and mussels living in environments where hydrogen sulfide and oxygen coexist in disequilibrium frequently lack (or essentially lack) both mouth and guts and rely for their carbon nutrition on symbiotic bacteria housed in the modified gill. The bacteria derive energy by oxidizing hydrogen sulfide and fixing carbon dioxide into organic compounds which serve as food for the clam. The prototype of these symbiont-dependent animals is the giant white clam collected by the deep submersible Alvin, from the Galapagos hydrothermal vents on the floor of the Pacific Ocean. Similar but accessible species living in the mud of mangrove swamps are being studied in this project. Specialized cells (bacteriocytes) in the gills of these clams house the symbiotic bacteria and also contain hemoglobin at great concentration. It is being determined how the hemoglobin carries both oxygen and hydrogen sulfide from the sea water across the cytoplasm of the cell to the symbiotic bacteria. The primary aim is to reconstruct the molecular mechanism of hemoglobin-mediated oxygen delivery to the intracellular symbiont. It is proposed that the same molecular mechanism which delivers oxygen to bacteria in the clam gill serves also in the economically important root modules of soybeans and other legumes, and in the human heart. In the soybean root nodule, leghemoglobin delivers oxygen to intracelluar bacteria, Rhizobium, which fix nitrogen from the atmosphere and supply it the plant. These plants do not require nitrogen fertilizer. Myglobin in the human heart delivers oxygen to intracellular mitochondria, enabling them to make ATP, the fuel of muscle contraction, rapidly enough to meet the work load of the heart.