Geomicrobiology is the scientific field at the intersection of geology and microbiology and is a major subfield of geobiology. It concerns the role of microbes on geological and geochemical processes and effects of minerals and metals to microbial growth, activity and survival. Such interactions occur in the geosphere (rocks, minerals, soils, and sediments), the atmosphere and the hydrosphere. Geomicrobiology studies microorganisms that are driving the Earth's biogeochemical cycles, mediating mineral precipitation and dissolution, and sorbing and concentrating metals. The applications include for example bioremediation, mining, climate change mitigation and public drinking water supplies.
Rocks and minerals Microbe-aquifer interactions Microorganisms are known to impact aquifers by modifying their rates of dissolution. In the karstic Edwards Aquifer, microbes colonizing the aquifer surfaces enhance the dissolution rates of the host rock.
In the oceanic crustal aquifer, the largest aquifer on Earth, microbial communities can impact ocean productivity, sea water chemistry as well as geochemical cycling throughout the geosphere. The mineral make-up of the rocks affects the composition and abundance of these subseafloor microbial communities present. Through bioremediation some microbes can aid in decontaminating freshwater resources in aquifers contaminated by waste products.
Microbially precipitated minerals
Some bacteria use metal ions as their energy source. They convert (or chemically reduce) the dissolved metal ions from one electrical state to another. This reduction releases energy for the bacteria's use, and, as a side product, serves to concentrate the metals into what ultimately become ore deposits. Biohydrometallurgy or in situ mining is where low-grade ores may be attacked by well-studied microbial processes under controlled conditions to extract metals. Certain iron, copper, uranium and even gold ores are thought to have formed as the result of microbe action.
Subsurface environments, like aquifers, are attractive locations when selecting repositories for nuclear waste, carbon dioxide (See carbon sequestration), or as artificial reservoirs for natural gas. Understanding microbial activity within the aquifer is important since it may interact with and effect the stability of the materials within the underground repository. Microbe-mineral interactions contribute to biofouling and microbially induced corrosion. Microbially induced corrosion of materials, such as carbon steel, have serious implications in the safe storage of radioactive waste within repositories and storage containers.
Environmental remediation Microbes are being studied and used to degrade organic and even nuclear waste pollution (see Deinococcus radiodurans) and assist in environmental cleanup. An application of geomicrobiology is bioleaching, the use of microbes to extract metals from mine waste.
Soil and sediment: microbial remediation
Microbial remediation is used in soils to remove contaminants and pollutants. Microbes play a key role in many biogeochemistry cycles and can effect a variety of soil properties, such as biotransformation of mineral and metal speciation, toxicity, mobility, mineral precipitation, and mineral dissolution. Microbes play a role in the immobilization and detoxification of a variety of elements, such as metals, radionuclides, sulfur and phosphorus, in the soil. Thirteen metals are considered priority pollutants (Sb, As, Be, Cd, Cr, Cu, Pb, Ni, Se, Ag, Tl, Zn, Hg). Soils and sediment act as sinks for metals which originate from both natural sources through rocks and minerals as well as anthropogenic sources through agriculture, industry, mining, waste disposal, among others.
Many heavy metals, such as chromium (Cr), at low concentrations are essential micronutrients in the soil, however they can be toxic at higher concentrations. Heavy metals are added into soils through many anthropogenic sources such industry and/or fertilizers. Heavy metal interaction with microbes can increase or decrease the toxicity. Levels of chromium toxicity, mobility and bioavailability depend on oxidation states of chromium. Two of the most common chromium species are Cr(III) and Cr(VI). Cr(VI) is highly mobile, bioavailable and more toxic to flora and fauna, while Cr(III) is less toxic, more immobile and readily precipitates in soils with pH > 6. Utilizing microbes to facilitate the transformation of Cr(VI) to Cr(III) is an environmentally friendly, low cost bioremediation technique to help mitigate toxicity in the environment.
Acid mine drainage
Another application of geomicrobiology is bioleaching, the use of microbes to extract metals from mine waste. For example, sulfate-reducing bacteria (SRB) produce H2S which precipitates metals as a metal sulfide. This process removed heavy metals from mine waste which is one of the major environmental issues associated with acid mine drainage (along with a low pH).
Bioremediation techniques are also used on contaminated surface water and ground water often associated with acid mine drainage. Studies have shown that the production of bicarbonate by microbes such as sulfate-reducing bacteria adds alkalinity to neutralize the acidity of the mine drainage waters. Hydrogen ions are consumed while bicarbonate is produced which leads to an increase in pH (decrease in acidity).
Microbial degradation of hydrocarbons
Microbes can affect the quality of oil and gas deposits through their metabolic processes. Microbes can influence the development of hydrocarbons by being present at the time of deposition of the source sediments or by dispersing through the rock column to colonize reservoir or source lithologies after the generation of hydrocarbons.
Metal resistance in bacteria
Bacteria have evolved a range of resistance mechanisms to cope with heavy metal toxicity in their environments. Metals like copper, zinc, and iron are essential at low concentrations, but excessive amounts can impair cellular processes or cause death. To survive, bacteria developed strategies to prevent metal uptake, detoxify metals inside the cell, or sequester them safely. These resistance mechanisms are crucial for environmental adaptation and bioremediation efforts.
Mechanisms of resistance
Efflux pumps Efflux pumps are one of the primary mechanisms that bacteria use to resist heavy metals. These protein complexes actively transport toxic metal ions out of the bacterial cell, often using energy from ATP hydrolysis or ion gradients. There are several families of efflux pumps involved in metal resistance, including P-type ATPases, Resistance-Nodulation-Division (RND) pumps, and Cation Diffusion Facilitators (CDFs). These pumps help to remove metal ions such as cadmium (Cd2+), zinc (Zn2+), and cobalt (Co2+) from the cell, preventing them from reaching toxic concentrations inside. The diversity of efflux pump systems allows bacteria to adapt to various metal pollutants in their environment and to survive in contaminated habitats like industrial waste sites or polluted waters. Some efflux systems are also capable of transporting antibiotics, adding an extra layer of resistance for bacteria in environments with both metal and antibiotic pressures.
Metal sequestration Once inside the bacterial cell, metal ions can disrupt critical cellular functions. To prevent this, bacteria often sequester metals by binding them to intracellular proteins, such as metallothioneins, which are small cysteine-rich proteins that have a high affinity for heavy metals. Similarly, phytochelatin-like molecules can also bind metal ions, effectively neutralizing them. This sequestration serves several purposes: it keeps metals away from reactive cellular components like proteins and DNA, reduces oxidative stress, and sometimes facilitates the transfer of metals to efflux pumps for removal. This strategy is particularly important in environments where the metals present are not easily removed through efflux alone, or when the metals must be temporarily stored until they can be safely eliminated from the cell. The sequestration of metals is not just a survival tactic; it can also enable bacteria to tolerate high concentrations of metals over extended periods.
Enzymatic detoxification Bacteria also possess enzymes that detoxify metals by chemically modifying them into less harmful forms. One example is mercuric reductase, an enzyme that reduces toxic mercury ions (Hg2+) to elemental mercury (Hg0), which is less toxic and can be volatilized out of the cell. Similarly, arsenate reductase converts arsenate () into arsenite (), a form that is less toxic and more easily expelled from the cell via efflux mechanisms. These enzymatic processes are vital for bacteria that inhabit environments contaminated with metals such as mercury, arsenic, or other heavy metals, where the ability to detoxify these substances is necessary for survival. Enzymatic detoxification also highlights the bacteria's ability to not only withstand metal toxicity but also adapt to the chemical diversity of the metal contaminants they encounter.
Mutations and gene transfer Bacteria can acquire metal resistance through both spontaneous mutations and horizontal gene transfer. Mutations in bacterial genes may lead to changes in the structure of metal-binding proteins, efflux pumps, or enzymes involved in detoxification, increasing the bacteria's ability to handle heavy metal stress. Horizontal gene transfer (HGT) plays a key role in the spread of metal resistance, as mobile genetic elements like plasmids, transposons, and integrons carry metal resistance genes (MRGs) between bacterial species. This allows bacteria to rapidly acquire and disseminate resistance traits across microbial communities. HGT also facilitates the co-transfer of other resistance genes, including those conferring resistance to antibiotics. This process often results in bacteria that can simultaneously resist both metals and antibiotics, complicating treatment efforts in clinical settings.
Environmental impact on resistance Environmental factors such as soil pH, temperature, nutrient availability, and agricultural practices can influence the spread and effectiveness of metal resistance in bacterial populations. Fertilizers and pesticides, for instance, can raise the concentration of heavy metals in the soil, creating selective pressure that promotes the survival of metal-resistant bacteria. In some cases, bacterial populations in these environments may also evolve resistance to antibiotics, a phenomenon known as co-selection. This process occurs because the same mechanisms that protect bacteria from heavy metals, like efflux pumps, may also expel antibiotics, leading to bacteria that are resistant to multiple threats simultaneously. This interplay between environmental contamination and bacterial resistance underscores the need for sustainable agricultural practices and effective waste management strategies to limit the spread of resistant bacteria.
Link between heavy metal and antibiotic resistance