Humans aren’t the only users of fossil fuels. In many parts of the ocean, natural gas (methane) is constantly bubbling out of the sediment. These areas are known as cold seeps and are often a marker of productive fossil fuel reservoirs in the crust underneath. The name cold seep is somewhat of a misnomer (they are often slightly warmer than surrounding waters), but they are indeed much cooler than the more famous hydrothermal vents, which form due to geothermal activity. They are often found in shallower waters than hydrothermal vents, which generally occur in the deepest regions of the ocean where the earth’s crust is rifting. As with hydrothermal vents, cold seeps provide a unique opportunity for ecosystems to arise which are based on chemical energy, rather than solar-powered photosynthesis like the rest of the biosphere. Unusual life forms harness the chemical energy of the methane and sulfide emitted at these seeps, and many can also be found at the more famous hydrothermal vents.
Bathymodiolus mussels, vesicomyid clams, and other bivalves thrive at these seep environments, and have evolved to partner with bacteria in their gills and stomachs which can directly consume and produce energy from the reaction of methane and sulfate continuously being sourced from the seep underneath, creating bicarbonate and sulfide as products. This form of metabolism, chemosynthesis, is distinct from the familiar photosynthesis/respiration that most life-forms at the surface use to create energy. The partnership between a multicellular animal and chemosynthetic bacteria is called chemosymbiosis. These reactions are of course being utilized by all sorts of microbes not partnering with bivalves, but the bacteria that take on metazoan hosts have the advantage of a stable environment and a constant flow of fresh seep gas brought in by the bivalves’ gills. The bivalves feed on the products of the microbe’s hard work. As they are some of the only inhabitants that can tolerate the oxygen-free, toxic environment of seeps, they form dense thick shell beds wherever a seep appears.
Other chemosymbiotic groups use the hydrogen sulfide byproducts from seeps, which they oxidize into elemental sulfur. The most prominent of these are the vestimentiferan tubeworms, which lack mouth or anus and are totally dependent on the activity of the bacteria that they house in a modified digestive tract. Some of these worms have been found in seeps in extreme environments, such as the truly cold cold seeps of the Arctic. They are less spectacular in appearance than their bright red counterparts from hydrothermal vents, but are believed to live for an extremely long time, depending on the consistency of the seep that they inhabit.
At the extreme pressures and low temperatures of the deep ocean, methane can freeze in combination with water molecules, forming structures called clathrates. Much of the deep ocean floor is dusted with deposits of clathrates. Microbes which feed on clathrates are believed to be a food source for grazing polychaete “ice worms.” These unusual organisms can survive up to 96 hours without a whiff of oxygen, an unbelievable feat for a moving, multicellular organism.
Cold seep environments are perhaps merely one specific manifestation of a vast, poorly understood collection of biota which do not depend on the sun for their energy. We do not yet understand many of the deep ecosystems which may be present within the earth’s crust, in the deep ocean and trapped under polar ice. NASA studies cold seep and hydrothermal environments as the best analog for the conditions life could experience in the frozen oceans of Europa and Enceladus. It is eerie to think that such alien ecosystems exist merely a few kilometers off of our familiar shores, and were using fossil fuel energy far before humans figured out how to combust it.