Microscopic conquerors: The bacteria that cultured hidden kingdoms.
In the deep ocean, unknown to humankind until 40 years ago, are vast and bustling communities of marine life. Living below the depth penetration of the sun’s light, at depths where most food matter sinking from the surface decays beyond the quality to sustain life, are found large, diverse communities, such as those seen in the photograph below. Thus, organisms here are unable to sustain themselves on energy gleaned by plant or algal photosynthesis; unlike all known life before, these organisms are powered by the excess metabolic products of tiny living chemical factories; bacteria and archaea.
These single celled life forms produce the energy, which sustains these deep sea communities, they do this via the chemical process called chemosynthesis.
Chemosynthesis – Creating your own food.
Chemosynthesis are chemical processes seen in select species of archaea and bacteria. These complex chemical processes create biological energy from components regionally available in the water. All forms of chemosynthesis are based on using redox reactions to form carbohydrates from carbon dioxide and other dissolved compounds; which compounds depend on the species of bacteria.
In hydrothermal vent communities these redox reactions usually occur using sulphides, which are made abundant in the water by billowing jets of smoky, volcanically charged water, that erupt from the vents. There are 6 types of anaerobic chemosynthesis; with the most common being methanogenesis, where hydrogen (from water) and carbon dioxide undergo a series of reactions forming methane, water and sugars. There are 5 types of aerobic chemosynthesis. With the most common being the oxidation of sulphate and methane, these processes are not so dissimilar from the generation of carbohydrates facilitated by UV radiation, seen in plants, algae and some bacteria (elaborated in the image below).
Symbiosis – A mutual agreement.
As aerobic chemosynthesis requires carbon dioxide, the by-product of cellular respiration, aerobic chemosynthetic bacteria are prime candidates for living mutualistically with animals. This is when the bacteria are taken up into the cells, particularly the gills or other such areas of high surface area, and the excess by-products of chemosynthesis are used in cellular growth and metabolic function in the host organism. There are seven phyla known to host chemosynthetic bacteria, for example, molluscs, arthropods, sponges and worms.
There are three clades (genetic groups with a common ancestor) of mutualistic, chemosynthetic bacteria with the most common being Gammaproteobacteria, a sulphur-oxidising bacteria species of these Gammaproteobacteria occur both mutualistically and free living around hydrothermal vents.
To the keen reader, this relationship may be reminiscent of the well-known mutualism in which shallow water coral polyps have evolved to host zooxanthellae (a type of phytoplankton) in their cells to aid with meeting the energy requirements they have to grow and reproduce. However, unlike the corals many of these hydrothermal vent organisms rely solely on the energy generated by the bacteria. To such an extent where some species, such as the Riftia worms (see image above) lack a digestive track and anus, secreting any toxic waste products of metabolic function to the base of their tubes.
Some organisms maximise the potential of these microscopic assistants: The deep sea hydrothermal vent mussels of the genus Bathymodiolus found in the Mid-Atlantic Ridge and the Gulf of Mexico, have been found to host both the common sulphur-oxidising and methane-oxidising symbionts in their gill tissues. By utilising both these different types of symbiotic bacteria the mussels can overcome bacterial species specific limitations, which normally act as limiting factors on how much energy the bacteria can produce.
Could these bacteria have cultured more than just hydrothermal vent fauna?
Hydrothermal vents are strikingly similar to the current interpretations of early earth where magma erupted into the ocean. Prior to the shocking discovery of life surrounding deep sea hydrothermal vents, it was a common theory that life originated in the oceans. With time, this theory grew in to the proverbial primordial soup of pre-biotic life; organic polymers (such as long chain carbohydrates, polypeptides and nucleotides) which over time assembled into the archaea and bacteria (both being more primitive, simple cells) that may be recognised today, this concept is known as abiogenesis.
Not only are these habitats rich in rarer elements such as metals and sulphur, they also have high temperatures allowing metabolic and anabolic reactions to occur at an ideal rate for development without the aid of enzymes. This means the process of abiogenesis would have been happening at a faster rate at these sites than anywhere else, if they were indeed, happening in other habitats. The earliest known fossilised life forms that have ever been discovered were found in the igneous rock surrounding the World’s oldest known hydrothermal vent, these fossils were believed to be up to an astonishing 4.28 billion years old. The culmination of this evidence raised some fundamental questions about the origin of life on this planet, specifically, are these bacteria the descendants of the first biotic life on earth? This hypothesis has gained traction in modern science, with many accepting it as the most probable.
Why it matters.
Studying the evolution and genetic lineages of chemosynthetic bacteria and archaea may be the key to understanding the oldest life on earth and revealing more information on the development of life as we know it. Researchers are also looking into developing methods of culturing species of the non-symbiotic methanogenic bacteria that monopolise hydrothermal sites. These bacteria have the ability to anaerobically synthesise methane, which could be used for fuel, from carbon dioxide. This feat of bioengineering – if this process could be made wide-scale and streamlined enough – could provide a way of converting atmospheric carbon dioxide (at only the cost of collection) into valuable methane for fuel. Not only would this option be less ecologically damaging in terms of less drilling and fracking for natural gas supplies, but would only have a net greenhouse emission equal to the fuel used in production and transportation.
These deep-sea bacteria may not only be the source of life on earth but a potential control for climate change.