Geomicrobiology

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 (AsO3−4) into arsenite (AsO−2), 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

The exposure of bacteria to heavy metals has been shown to promote resistance to both metals and antibiotics. Efflux pumps, which are key to metal resistance, can often expel not only metal ions but also antibiotics, creating a scenario where bacteria are resistant to a wide range of antibiotics in addition to heavy metals. This dual resistance is a growing concern for public health, as it complicates the treatment of infections in humans and animals. Furthermore, the use of antibiotics in agriculture and the environment can contribute to the selection of bacteria that are resistant to both environmental toxins and drugs, making it more difficult to control bacterial infections. The link between metal and antibiotic resistance highlights the importance of monitoring metal contamination in natural environments, as well as the development of strategies to mitigate the spread of multidrug-resistant bacteria.

Biofilm formation

Biofilms are slimy communities of bacteria that stick to surfaces. Under stressful conditions, like heavy metal exposure, bacteria often form biofilms to protect themselves. Instead of living freely (planktonic phase), bacteria in biofilms are sessile, anchored in place and surrounded by a protective matrix of exopolysaccharides (EPS).

Role of biofilms in metal resistance

Biofilms can act as protective barriers against heavy metals. By trapping metal ions in the EPS matrix, biofilms reduce the direct exposure of bacterial cells to toxic metals. This mechanism is especially important in metal-rich environments, where survival without biofilm formation would be difficult. In biomining operations, such as extracting copper from sulfide ores, biofilms formed by acidophilic bacteria help promote metal solubilization. Attachment to mineral surfaces and biofilm formation are critical early steps that influence the success of bioleaching. Similarly, in natural environments like soils and aquifers, bacteria form biofilms on mineral surfaces to stabilize themselves and improve pollutant degradation, aiding bioremediation efforts.

Mineral–bacteria interactions

Bacterial biofilms interact physically and chemically with minerals. The EPS matrix often incorporates positive ions like calcium (Ca²⁺) and magnesium (Mg²⁺), strengthening the biofilm structure. Mineral selectivity, or preference for certain minerals, can create hotspots for microbial life in places like the deep subsurface. These interactions not only stabilize biofilms but also affect contaminant fate and the efficiency of bioremediation strategies.

Early Earth history and astrobiology

A common field of study within geomicrobiology is origin of life on earth or other planets. Various rock-water interactions, such as serpentinization and water radiolysis, are possible sources of metabolic energy to support chemolithoautotrophic microbial communities on Early Earth and on other planetary bodies such as Mars, Europa and Enceladus.

Interactions between microbes and sediment record some of the earliest evidence of life on earth. Information on the life during Archean Earth is recorded in bacterial fossils and stromatolites preserved in precipitated lithologies such as chert or carbonates. Additional evidence of early life on land around 3.5 billion years ago can be found in the Dresser Formation of Australia in a hot spring facies, indicating that some of Earth's earliest life on land occurred in hot springs. Microbially induced sedimentary structures (MISS) are found throughout the geologic record up to 3.2 billion years old. They are formed by the interaction of microbial mats and physical sediment dynamics, and record paleoenvironmental data as well as providing evidence of early life. The paleoenvironments of early life on Earth also serve as models when searching for potential fossil life on Mars.

Extremophiles

Another area of investigation in geomicrobiology is the study of extremophile organisms, the microorganisms that thrive in environments normally considered hostile to life. Such environments may include extremely hot (hot springs or mid-ocean ridge black smoker) environments, extremely saline environments, or even space environments such as Martian soil or comets.

Observations and research in hyper-saline lagoon environments in Brazil and Australia as well as slightly saline, inland lake environments in northwestern China have shown that anaerobic sulfate-reducing bacteria may be directly involved in the formation of dolomite. This suggests the alteration and replacement of limestone sediments by dolomitization in ancient rocks was possibly aided by ancestors to these anaerobic bacteria.

In July 2019, a scientific study of Kidd Mine in Canada discovered sulfur-breathing organisms which live 2,400 metres (7,900 ft) below the surface, and which breathe sulfur in order to survive. These organisms are also remarkable due to eating rocks such as pyrite as their regular food source.