Summary
Highlights
The deep sea is a barren environment where biomass declines exponentially with depth. Organisms rely on 'marine snow' for food, as photosynthesis is impossible below 200 meters. However, specific regions on the deep seafloor use chemosynthesis, a process where life's energy comes from the Earth itself, not sunlight. These chemosynthetic oases include hydrothermal vents and cold seeps, which support unique ecosystems.
Chemosynthesis is similar to photosynthesis, but it uses chemical energy instead of light. This occurs at hydrothermal vents and cold seeps. Hydrothermal vents were discovered in 1977 near the Galapagos. Hundreds more have been found at depths of 2km+, along plate boundaries and seafloor spreading regions like the East Pacific Rise and Mid-Atlantic Ridge. They form when tectonic plate rifting allows magma to rise, heating seawater that percolates through fissures, picking up minerals, and jetting back out at high temperatures, forming 'black smokers' and 'white smokers' chimneys.
Hydrothermal vents exhibit a sharp temperature gradient. Microbes, including archaea and bacteria, are able to tolerate temperatures up to 122°C and perform chemosynthesis using hydrogen sulfide and methane to produce glucose. This process supports vast assemblages of animal life, creating an 'oasis of life'. Vent communities are densely populated, unlike the scattered life on the abyssal plain. Organisms like snails, shrimp, and crabs compete for resources, leading to resource partitioning and specialization. Examples include squat lobsters and limpets grazing microbial mats, muscles being suspension feeders, and Yeti crabs farming bacteria on their bodies. Giant tube worms form symbiotic relationships with microbes inside their tubes. Pompei worms tolerate extreme heat.
Life at vents shows zonation due to temperature gradients and microbial varieties. Primary consumers are abundant, along with higher trophic level organisms like octopuses and zarid fish. Deep-sea skates lay eggs near vents to use volcanic heat for accelerated development. The unique conditions of vents have led scientists to speculate that they could be where life on Earth originated, supported by the presence of primitive thermophilic microbes and chemical building blocks of life.
Cold seeps occur along continental margins and oceanic trenches where hydrocarbon-rich water escapes the seafloor. Unlike vents, they form at subduction zones where oceanic plates are subducted. Organic material buried under sediments degrades, producing methane. Tectonic compression forces this methane upwards. Anaerobic microbes oxidize methane using sulfate, producing hydrogen sulfide and bicarbonate, which fuel chemosynthetic microbes and form the base of the food web.
Cold seeps support diverse life, similar to hydrothermal vents, on what is otherwise a barren abyssal plain. Bathymodiolus muscles dominate, hosting endosymbiotic bacteria. Siboglinid tube worms burrow into soft sediments to access sulfides, forming long-lived 'bushes'. Yeti crabs farm bacteria on their bodies. These vibrant communities attract predators like octopuses and fish. Notably, cold seep life significantly reduces methane flux from the seafloor to the water column, acting as a 'benthic filter' that impacts global climate by consuming this potent greenhouse gas.
Variations in geological processes create different cold seep environments. Mud volcanoes form from methane gas and fluidized mud, offering challenging habitats. Methane hydrate beds, or 'methane ice', host specialists like ice worms that graze on chemosynthetic bacteria. Asphalt seeps, where petroleum deposits leak out and solidify, create unique underwater landscapes like 'tar lilies' that bacteria colonize. Brine pools, highly saline and anoxic lakes formed by salt diapirism, support chemosynthetic life only along their shores due to the toxic conditions.
Cold seeps are transitory. Methane oxidation produces bicarbonate ions, which react with calcium in seawater to form calcium carbonate. This creates carbonate reefs, chimneys, and spires that eventually block the seepage. This leads to ecological succession: bacterial mats and muscles appear first, followed by tube worms on forming carbonate outcrops. As seepage is blocked, muscles die off, but tube worms endure longer. Eventually, a lifeless rocky outcrop remains, which can then be colonized by stony corals, forming deep-sea coral gardens once supported by chemosynthetic activity.
Food falls, like whale carcasses and sunken wood, can create temporary sites of partial chemosynthesis. Whale falls undergo ecological succession: mobile scavengers strip flesh, followed by invertebrates. The 'sulfo-philic' stage begins when bone-eating 'osada' worms and anaerobic bacteria break down lipids in whale bones, producing sulfide. This sulfide supports bacterial mats, creating a localized chemosynthetic environment. These whale falls may serve as evolutionary 'stepping stones' for specialized vent and seep inhabitants due to their abundant and relatively short-lived nature.
Other food falls, such as smaller carcasses or those with unpalatable flesh (like sharks and rays due to high ammonia), do not support complex chemosynthetic communities. Wood falls, consisting of sunken trees or shipwrecks, also create fleeting oases. Specialized bivalves like Xylophaga bore into the wood and host endosymbiotic bacteria to digest cellulose and lignin. Squat lobsters also utilize bacteria to digest wood. These wood falls represent deep-sea animals adapted to terrestrial plants and create sulfidic hotspots that support chemosynthetic bacteria, analogous to terrestrial detrital communities.
Like whale falls, wood falls act as ecological 'stepping stones' for animals like Bathymodiolus muscles, which disperse larvae to new chemosynthetic habitats. Wood falls are generally short-lived, with wooden hulls decaying in under 100 years. However, in extreme environments like Antarctica, where wood-degrading organisms are absent due to the cold and lack of trees, wood can persist for centuries, as seen with the Endurance shipwreck. These metal shipwrecks also become oases for filter feeders and provide attachment points, allowing deep-sea dwellers to access nutrient-rich currents. Food falls, whether natural or man-made, highlight the interconnectedness of deep-sea ecosystems, serving as vital stepping stones and shelters, fostering biodiversity.