The overwhelming majority of biomass in oceanic food webs originates from phytoplankton. Photosynthetic primary producers and the consumers of the ecosystem they support can sink through the water column as detritus. In photosynthesis, sunlight is necessary for the synthesis of ATP and NADPH. Therefore, as we move further from sunlight, marine ecosystems should become less rich and dependent on the small incidences of energy provided by detritus. Azoic theory proposed by Edward Forbes in 1843 is now a superseded theory, but it argues that biodiversity and species richness decrease with depth. He estimated that marine life would not exist below depths of 550m. His findings supported his statement as detritus in nutrient deficient Aegean Sea was mostly all consumed by depths of 420m. One of many ways of debunking Azoic is the incidence of chemosynthesis. Which is necessary to the development of communities limited on an energy budget of marine snow and detritus. The role of chemosynthesis in creating life cannot be disputed as it precedes the existence of photoautotrophs. The origins of life on Earth are unknown but evidence suggests early life began at hydrothermal vents – fossilised microorganisms that are between 3.7 billion and 4.2 billion years old found in ferruginous sedimentary rocks, Nuvvuagittuq belt in Quebec, Canada.

What is chemosynthesis?

Chemosynthesis is a term coined by Wilhelm Pfeffer in 1897 to describe energy production by oxidation of inorganic substances.  Deep sea hydrothermal vent benthic communities were discovered in 1977. The vents occur where two tectonic plates are moving away from each other. First vents were discovered at a depth of 2,500m along the Galapagos Rift spreading centre using the Deep-Sea Research Vessel (DSRV). Seawater percolates into the lava of these vents, circulates within the earth’s crust, and escapes back onto the surface as up to 400°C superheated vent fluid. On its journey beneath the crust, the hot water absorbs minerals, which are expelled out of black and white smokers. Black smokers release sulphuric minerals such as hydrogen sulphide.

These minerals are nutrition for chemoautotrophs in the environment which are primitive bacteria and archaea. Their means of survival and supporting the ecosystem around them is to oxidise inorganic compounds. The black smokers expel extreme amounts of hydrogen sulphide (H2S). Thiomicrospira crunogenae oxidise sulphur by using hydrogen sulphide as an energy source. The reaction is similar to photosynthesis but note that solid sulphur is released as a product in the bacteria cytoplasm:

12H2S + 6CO2 → C6H12O6 + 6H2O + 12S

Symbiotic chemosynthesis

Large populations of Giant tubeworms (Riftia pachyptila) were found near black smoker vents on the Galapagos rift. Worms can grow over 2 metres long and early autopsies demonstrated that their gut contained sulphur crystals. Soon after discovery, graduate student Colleen Cavanaugh heard this information and theorised that sulphur oxidising bacteria was inside gut of the worm as solid sulphur is a product of a hydrogen sulphide chemosynthetic reaction. Research determined that these bacteria are a symbiont sealed inside a specialised organ called a trophosome. This symbiotic relationship the two-species share is a requirement of their extreme habitat. For biosynthesis to occur, there needs to be a balance in hydrogen sulphide availability and oxygen saturation. A black smoker is shooting out an abundance of sulphur which reacts with the oxygenated seawater to produce an anoxic environment. Giant tubeworms zoning will be located between a black smoker and where ambient seawater conditions begin. This zone creates optimal conditions and protection for both the host and symbiont. The circulatory system of the tubeworm has specifically adapted haemoglobin which can bind to both sulphur and oxygen at the same time. The Thiomicrospira crunogenae can use this to synthesise ATP and NADPH, then release energy and sulphur crystals out of the trophosome and into the gut.

Sulphur is released from basalt bedrock and absorbed by vent fluid. Sulphur reacts with the hydrogen in the vent fluid to create hydrogen sulphide which is released into the seawater. Giant tubeworms (Riftia pachyptila) zonation occurs due to an area having a balance of hydrogen sulphide and dissolved oxygen in the water.

Enceladus

Chemosynthesis is a remarkable means of harnessing energy in places where energy was thought to be scarce. It may be the largest indicator that life could exist on other terrestrial bodies in our solar system. Enceladus is the sixth largest moon of Saturn. A mean temperature of −198 °C suggests it is too cold and far from the sun for the sun to provide enough energy for the creation of life. It has liquid water in a subsurface Southern Ocean due to tectonic activity. The primary cause of this activity is tidal heating, a dissipation of energy generated from orbiting Saturn’s huge gravitational force. The energy produced by this process is estimated to be 15.8 gigawatts, almost 3 times the sum of all energy produced at yellow stone hot-springs.

One proposed theory of the structure of hydrothermal vents and a subsurface ocean near the south pole of Enceladus. The source of energy is tidal heating, which is generated by the force of Saturn’s gravitational pull.
Source: NASA – Jet propulsion laboratory, California institute of Technology.

Telemetry data from the NASA Cassini spacecraft in April 2017 detected significant values of molecular hydrogen being released from plumes indicated hydrothermal activity in the subsurface ocean. On Earth, molecular hydrogen (H2) could be fixed into chemical energy by methanotrophic archaea. Hydrocarbons such as methane (CH4) have been detected in trace amounts in the vapour expelled from geysers on Enceladus. As methane is a product of methanogenesis, this provides some evidence that Enceladus could support microbes which fix the hydrogen from the vents and dissolved carbon dioxide in the water. Heat, water, and suitable elements are the seeds of microbial life at hydrothermal vents. They are present on Enceladus and Europa. What remains undiscovered is the organism that can sow these seeds into life. Perhaps Cassini’s trail blazing research will lead to the discovery of a primitive chemotroph like Thiomicrospira crunogena that oxidises electron donors from hydrothermal vents beyond our planet.

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