In the deep sea, biomass declines exponentially, leading to adaptations in deep-sea organisms. They rely on marine snow, a downward-drifting supply of organic material, as their primary food source. Photosynthesis becomes impossible below 200 meters, and darkness prevails at around 1,000 meters. However, there are chemosynthetic oases on the deep sea floor where primary production occurs through a different process. These oases rely on chemosynthesis, deriving energy from the Earth itself rather than sunlight. This series of films explores the formation, ecology, threats, and the importance of stewardship for these unique environments.
Into the Abyss: Chemosynthetic Oases
Chemosynthesis is a process similar to photosynthesis, but it derives energy from chemical sources instead of sunlight. In the deep sea, primary production fueled by chemical energy occurs in specific environments like hydrothermal vents and cold seeps. Hydrothermal vents were discovered in 1977 and are found along oceanic spreading ridges and plate boundaries. These vents occur due to fissures in the Earth's crust, allowing hot magma to rise, heat seawater, and release mineral-rich fluids back into the ocean. These fluids precipitate minerals, forming chimney structures on the seabed. Different types of vents exist, emitting hot plumes with varying mineral compositions. Despite the extreme temperatures near the vents, prokaryotic microbes can tolerate and carry out chemosynthesis. They use hydrogen sulfide and methane to create glucose from water and dissolved carbon dioxide. The chemosynthetic primary productivity at these sites supports a diverse community of specialized organisms, forming an oasis of life in the deep sea.
Hydrothermal vents in the deep sea host unique and abundant communities of organisms that rely on chemosynthetic microbes as a source of food. These vents are characterized by intense interspecific competition among species sharing the same resource. However, instead of leading to extinction, this competition promotes specialization and resource partitioning, allowing different species to coexist. Various organisms have developed different adaptations to acquire the resources available at the vents. For example, squat lobsters and limpets graze on microbial mats, suspension feeders like deep-sea mussels feed on free-living microbes, and yeti crabs farm bacteria on their bodies. Giant tube worms form an endosymbiotic relationship with microbes, absorbing chemicals from the vent fluids. Pompeii worms also farm bacteria but can tolerate higher temperatures. The presence of a temperature gradient and varying microbe abundances lead to zonation and geographic isolation of species within the same vent system. Predators such as octopuses and zoarcid fish prey on the primary consumers at the vents. The zonation of life at vents also affects non-vent deep-sea fauna, with higher abundances near the vents due to exported organic matter. Over 590 animal species have been identified at hydrothermal vents, with a majority being unique to this environment. The unique conditions of hydrothermal vents, rich in energy and nutrients, have led scientists to speculate whether they could be the birthplace of life on Earth. Evidence suggests that some primitive organisms and chemical building blocks of life are found at the vents. In conclusion, hydrothermal vents support specialized ecosystems in the deep ocean, providing insights into the necessary conditions for life to thrive.
Cold seeps are regions on the sea floor where cool, hydrocarbon-rich water escapes, providing conditions for chemosynthetic assemblages to form. Unlike hydrothermal vents, cold seeps form at the continental margin through geological processes involving organic material degradation and the release of methane. Chemosynthetic microbes utilize methane and hydrogen sulfide, resulting in primary productivity and the establishment of diverse ecosystems. Cold seeps are characterized by high biodiversity compared to the low productivity of the surrounding abyssal plain. Bathymodiolus mussels dominate these communities by forming endosymbiotic associations with chemosynthetic microbes, and they support other species such as grazers and scavengers. Tube worms, similar to those found at hydrothermal vents, also inhabit cold seeps but have adaptations to soft sediments and can burrow into the sediments to obtain sulfides. Yeti crabs at cold seeps exhibit trophic adaptations similar to those at hydrothermal vents, farming bacteria on their bodies. Predators like octopuses, fish, and larger crabs complete the food chain at cold seeps. Importantly, the abundant life at cold seeps reduces the release of methane into the water column, serving as the only biological methane sink in the ocean and contributing to the regulation of global climate.
Cold seep environments are diverse and exhibit variations in geological processes, leading to distinct habitats and unique communities. Different types of cold seeps include mud volcanoes, gas hydrate beds, asphalt seeps, and brine pools. Mud volcanoes are formed when methane gas and fluidized mud erupt from the sea floor, creating cone-shaped structures. Despite the challenges posed by the flowing mud, bacterial mats and tube-worm meadows can be found along the borders of mud volcanoes, thriving on methane. Gas hydrate beds consist of frozen methane combined with water and form mounds in high-pressure, low-temperature conditions. Ice worms inhabit these hydrate mounds, burrowing into the hydrate and grazing on chemosynthetic bacteria. Asphault seeps occur when petroleum deposits leak from deep beneath the sea floor, leaving behind solidified hydrocarbons resembling cooled lava fields. Organisms, poorly understood, have been found living on and around the asphault, creating a unique cold-seep habitat. Brine pools form when salt flats are pushed to the surface, creating dense, anoxic, hyper-saline water with dissolved methane. Organisms are limited to the shores of brine pools due to the toxic nature of the brine. Cold seeps are considered islands of abundance, isolated patches along continental margins, and are transient, with seepage eventually ending as carbonate reefs and structures block the flow. This self-destructive process leads to ecological succession, with bacteria, mussels, tube worms, and stony corals colonizing the seep site at different stages. Cold seep environments highlight the adaptability of life in challenging conditions and the crucial role they play in regulating climate through methane consumption.
Food-fall events on the sea floor can create temporary sites of partial chemosynthesis, such as sunken whale carcasses and sunken wood. Whale carcasses go through stages of ecological succession, with mobile scavengers stripping the flesh, followed by invertebrates colonizing the bones and utilizing the organic material. In the sulfophilic stage, chemosynthetic life is supported as bone-eating worms burrow into the skeleton, allowing anaerobic bacteria to exploit the lipids and produce sulfide. This sulfide supports chemosynthetic microbial mats and a community of animals that survive on the energy released by the microbes. These temporary chemosynthetic environments may represent evolutionary stepping-stones for specialized vent and seep inhabitants. With many whale carcasses settling on the sea floor each year, whale-falls are relatively common and may bridge the gap between non-chemosynthetic scavengers and specialized vent and seep animals. Other types of food-falls, such as smaller carcasses, are not as conducive to complex communities. Whales are considered keystone species, providing important ecosystem services even in death.
Wood-falls in the deep ocean create unique ecosystems that support diverse biological communities. When trees sink to the ocean floor, they provide a concentrated source of nutrients and a solid surface for sessile animals to anchor to. Wood-fall specialists, such as the bivalves of the genus Xylophaga and the Giant Shipworm, have evolved traits to consume and digest the wood with the help of endosymbiotic bacteria. These wood-fall ecosystems resemble detrital communities on land, with crustaceans playing a dominant role in both. The microbial life associated with wood-falls performs anaerobic digestion, producing sulfide as a byproduct and supporting chemosynthetic bacteria. Bathymodiolus mussels and other animals rely on these chemosynthetic microbes for nutrition, and their presence at wood-falls indicates the importance of these isolated food sources for deep-sea biodiversity. Wood-falls may serve as ecological stepping-stones for dispersal between chemosynthetic habitats, similar to whale-falls. However, wood-falls are often short-lived, and the utilization of wood by deep-sea organisms accelerates its decay. In rare cases, such as the wreck of Ernest Shackleton's ship Endurance, wood can endure for a long time, providing a solid surface for attachment and support for filter-feeding organisms. Metal shipwrecks also host diverse communities, albeit without chemosynthetic activity. These wrecks serve as attachment points and provide access to nutrient-rich currents for deep-sea organisms. Overall, food-falls demonstrate the adaptability of deep-sea life and the interconnectedness of different ecosystems in the ocean.