Understanding Nature
About life enabling energy

Chemosymbiotic marine animals ... Animals, mostly bivalves, that digest sulfur and methane

Burgess COMMENTARY

Peter Burgess

Chemosymbiotic marine animals ... Animals, mostly bivalves, that digest sulfur and methane

The world first learned about chemosynthesis following the discovery of geothermal vents in deep ocean waters in 1977. Chemosynthesis is the process by which living creatures derive life-giving energy not by photosynthesis or the digestion of plant matter but by converting sulfur and methane compounds into nutrients. Light does not penetrate into the deep ocean, so photosynthesis is off the table. Deep ocean vents, though, are rich in heat, sulfur compounds, and methane compounds. Creatures such as tube worms have evolved to exist entirely on the sulfur and methane compounds emitted in great volume for the geothermal vents.

For some time, scientists assumed that only creatures living adjacent to geothermal vents utilized these sulfur and methane compounds. Recently, though, a variety of other marine animals have been found to utilize chemosymbiosis to provide at least a part of their energy needs. Mollusks are the most common of these hybrids. Numerous species of widespread lucinid clams contain unique hemoglobin which serves to transport hydrogen sulfide to autotropic bacteria that live independently in the clams’ tissues, particularly in the gut. As a result, the usual bacteria load in the gut is reduced and the gut itself is partially atrophied. These clams may be found in virtually all tropical and temperate marine waters, at depths varying from intertidal to deep.

Some mussels also exhibit chemosymbiosis. Biologists have discovered chemosymbiosis in a wide range of marine invertebrates, ranging from tiny worms to sponges to giant clams, living everywhere from mangrove swamps to the deep sea. There are estimated to be hundreds of chemosymbiotic bivalve species from at least six families. They have evolved symbiosis with various types of chemosynthetic bacteria in different manners, largely dependent on what chemical mixtures are available in each particular local environment. Unlike with the deep sea geothermal vents, the sulfur and methane for these chemosymbiotic marine animals is derived from such things as decaying animal and vegetable matter on the seafloor.