Geomicrobiology is the study of the role of microbes in the geological and geochemical processes that shaped the earth and continue to function today. Microbes play a vital role in the recycling, generation, sequestration and elimination of a wide variety of substances and chemicals in the environment Going through biogeochemical cycles that cover the atmosphere, hydrosphere and deep lithosphere.
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When did geomicrobiology become a field of study?
The term geomicrobiology was coined in the 1950s to describe the study of these processes, although scientists had been studying soil and aquatic microbiology since the 19th century. Christian Ehrenberg identified in 1836 that Gallionella ferruginea has been associated with the presence of iron deposits from peatlands, although at the time he misidentified this bacterium as a protozoan.
Later in 1887 Sergei Winogradsky noted that Beggiatoa bacteria could oxidize hydrogen sulfide gas to solid elemental sulfur, then in 1888 this Leptothrix ochracea bacteria has been implicated in releasing iron carbonate from rock to generate iron oxide.
The role of bacteria in precipitation of calcium and manganese in the oceans, methane formation, weathering of rocks, and the nitrogen cycle became clearer over the following decades, often preceded by the recognition of l metabolic activity of microorganisms in the laboratory and the subsequent realization of the extent that these processes played in the formation of the earth and its ecosystems.
Even in the last 20e century, the discoveries concerning the role of microorganisms in the formation of the earth were recognized. For example, it was long believed that microorganisms would be unable to survive outside a relatively narrow pH and temperature range, but the discovery of extremophilic bacteria that thrive in strongly acidic or alkaline environments around sites of mine drainage as well as identifying those who live around hot thermal vents and in frozen conditions at the poles have changed that view.
Reports that the influence of microbial life extended miles below the earth’s surface and the seabed in the 1990s and 2000s further fueled the realization that many aspects of geology previously considered entirely to be the result of physical and chemical forces were in fact due to the actions of microorganisms over millions of years.
The field of geomicrobiology had historically established that subterranean anaerobic life is organized into zones on a “thermodynamic scale”, where highly energetic iron reducers exclude sulfate reducers, which similarly exclude methane-producing microorganisms. . However, the environmental pH can alter the metabolic pathway which is the most thermodynamically favorable.
For example, under acidic conditions, iron reducers hold an additional energy advantage that is not present under alkaline conditions. In this way, certain types of microorganisms can become dominant in a particular geographic region, although long-term bioreactor experiments reveal that the three types of energy producers work in symbiosis to balance the availability of resources, rather than to compete directly.
The upper oceanic crust is continually created at mid-ocean ridges, where high-temperature basalt-seawater reactions generate energy to sustain life, with heat-generating hydrothermal currents that recycle and collect sediment. Anaerobic and aerobic microorganisms thrive in these environments and contribute to the recycling of hydrogen, carbon, and sulfur, and are believed to constitute the majority of microbial life in the oceans.
As the basalt rock is pushed back from the ridge, it cools and experiences cracking and oxidation over a period of approximately 10 million years, at which time the intensity of the basalt-seawater reactions and therefore the circulation of fluids and sediments decreases. Over 90% of the oceanic lithosphere is over 10 million years old and deficient in organic sediments and abundant in dissolved oxygen. It was therefore believed that relatively few microorganisms would live under these conditions.
In an article by Suzuki et al. (2020) Microbial cells are identified in samples of basaltic lava from the subsoil dating back more than 10 million years, living on abundant marine sediments rich in iron. Dissolved molecular oxygen has been found to penetrate the basalt rock, supporting aerobic microbes throughout the overlying sediment. Most of the microorganisms identified in these ancient basalts engage in heterotrophy and methanotrophy, feeding on dissolved organic matter brought in by seawater flows or generated by weathering of rocks.
The authors suggest that this realization could have implications for the possibility of life on Mars, whose basaltic crust formed around 4 billion years ago and was once covered with oceans like on Earth, providing a model around which astrobiologists could investigate the past existence of life on other planets.