Hemoglobins in Mollusc/Bacteria Symbioses

蛤蚌和贻贝生活在硫化氢和氧气不平衡共存的环境中，经常缺乏（或基本上缺乏）口腔和肠道，依靠改造后的鳃中共生细菌来获取碳营养。这种细菌通过氧化硫化氢和将二氧化碳固定成有机化合物来获取能量，这些有机化合物可以作为蛤蜊的食物。这些依赖共生体的动物的原型是由深海潜水器“阿尔文”号从太平洋海底的加拉帕戈斯群岛热液喷口收集的巨型白蛤。这个项目正在研究生活在红树林沼泽泥中的类似但可接近的物种。这些蛤鳃中的特殊细胞（细菌细胞）容纳了共生细菌，也含有高浓度的血红蛋白。目前正在确定血红蛋白是如何从海水中携带氧气和硫化氢，穿过细胞质到达共生细菌的。主要目的是重建血红蛋白介导的氧输送到细胞内共生体的分子机制。有人提出，蛤鳃中向细菌输送氧气的分子机制也适用于大豆和其他豆类的重要经济根模块，以及人类的心脏。在大豆根瘤中，豆红蛋白向细胞内细菌——根瘤菌——输送氧气，根瘤菌从大气中吸收氮并将其供应给植物。这些植物不需要氮肥。人体心脏中的肌红蛋白向细胞内线粒体输送氧气，使它们能够迅速制造ATP（肌肉收缩的燃料），以满足心脏的工作负荷。

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.