Microbes and nutrient cycling

A nutrient cycle refers to the exchange of organic and inorganic matter in an ecosystem, resulting in the sequestration, elimination, recycling, and generation of particular substances and elements in the environment. Microbial life has long been known to play a vital role in consuming and regenerating resources in the environment on many levels and could be considered the primary tool through which nutrient cycling occurs.

Mushroom growing on the ground. Image Credit: Pratimaan/Shutterstock.com

How are nutrients recycled?

With respect to trophic levels, microorganisms are capable of being: primary producers, engaging in photosynthesis or other autotrophic processes; heterotrophic consumers who consume other microorganisms; or decomposers, breaking down dead plant and animal matter to recycle their components into nutrients. Therefore, the presence and character of microorganisms in a particular environment can directly influence the abundance of other life forms, both at micro and macro scales, by adjusting the availability and type of nutrients in the environment.

Microorganisms have populated the earth for more than 3 billion years, and thus predate the presence of animals and plants around which nutrient ecologies evolved. As discussed, microorganisms are able to obtain nutrients from the environment, using biochemical processes to generate energy. Over millions of years, these processes have shaped the Earth itself, causing and facilitating the transition of elements from forms in which they would otherwise remain forever locked into those accessible to other organisms and chemical processes, which concerns the field of geomicrobiology.

Microbial nitrogen cycle

Perhaps the best understood and most important microbial cycle network is that of nitrogen, an essential element of all living organisms and a key component of the most fundamental biomolecules such as nucleic acids.

The vast majority of nitrogen on Earth is in the form of atmospheric nitrogen, inaccessible to all but the very diverse nitrogen-fixing bacteria and archaea. These microorganisms generate ammonia from atmospheric nitrogen, which can then be used by other organisms and incorporated into their biomass. Ammonia can be oxidized to nitrate via nitrification, after which it is converted back to nitrogen gas by nitrification or anaerobic ammonium oxidation.

Microorganisms are involved in every step of nitrogen fixation, nitrification and denitrification, in some cases specializing in a particular aspect of one of these roles or performing them simultaneously. For example, some species of bacteria are able to fix nitrogen gas and denitrify at the same time, while others previously thought to exist only through nitrate oxidation have been shown to be able to survive in environments lacking nitrogen, switching to sulphide as the energy source. The source.

At least 14 novel redox reactions have been noted in the conversion of nitrogen compounds by microorganisms, spanning redox states from -3 to +5. Microorganisms use a wide variety of enzymes to carry out these reactions, and new interactions are continually being recognized.

Soil nutrients.

Soil nutrients. Image credit: Miha Creative/Shutterstock.com

Other geomicrobiological cycles

Likewise, microorganisms are involved in the biogeochemical cycle of carbon, oxygen, phosphorus, sulfur and other vital elements. While purely chemical and physical interactions between components of the atmosphere, hydrosphere and lithosphere also drive nutrient cycling on a global scale, microorganisms have played an indispensable role in the formation and maintaining the Earth as a place for life as we know it.

An example of how microorganisms have drastically shaped Earth’s environment is the Great Oxidation Event, where about 2.2 billion years ago, Earth’s atmosphere and oceans experienced a sudden and dramatic increase in oxygen concentration due to the increasing influence of photosynthetic cyanobacteria.

Previously, high concentrations of nitrogen, carbon dioxide, and carbon monoxide in the atmosphere would have generated a mildly reducing environment, then transitioning to a strongly oxidizing environment. Evidenced by the abundance of minerals containing reduced forms of metals dating from this period, and the sudden appearance of oxidized formations in the geological record.

Arsenic species in the reducing atmosphere before the event would have been largely incorporated into rock, but were strongly released into the newly oxidizing atmosphere and allowed to enter the oceans. An article by Chen et al. (2020) associates the evolution of arsenic detoxification biotools with this event, where strong selection pressure towards the development of arsenic-resistant genes would be favored. Microorganisms remain a key component of the arsenic cycle, modulating concentrations between the atmosphere, lithosphere and hydrosphere.

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