8.7 Biogeochemical Cycles

Biogeochemical Cycles and Microbial Roles

Biogeochemical cycles describe how essential elements move through living organisms, the atmosphere, water, and soil. Microorganisms are the primary drivers of these cycles, carrying out chemical transformations that no other organisms can perform. Without microbial metabolism, elements like nitrogen and sulfur would accumulate in forms that plants and animals can't use, and ecosystems would grind to a halt.

The Carbon Cycle

Autotrophic microorganisms like cyanobacteria and algae fix atmospheric $CO_2$ into organic compounds through photosynthesis. This converts inorganic carbon into organic forms that other organisms can use as food and energy.

Heterotrophic microorganisms (bacteria and fungi) work in the opposite direction. They decompose organic matter and release $CO_2$ back into the atmosphere through cellular respiration, completing the cycle.

Methanogens are anaerobic archaea that produce methane ($CH_4$) during decomposition of organic matter in oxygen-free environments like wetlands and deep sediments. Methane is a potent greenhouse gas, so these organisms have a significant impact on climate.

The Nitrogen Cycle

Nitrogen makes up about 78% of the atmosphere, but most organisms can't use $N_2$ gas directly. Microbes solve this problem through a series of transformations:

1. Nitrogen fixation: Nitrogen-fixing bacteria (e.g., Rhizobium in root nodules of legumes, and free-living Azotobacter) convert $N_2$ into ammonia ($NH_3$), making nitrogen biologically available.
2. Nitrification: This is a two-step oxidation process. Nitrosomonas oxidizes ammonia to nitrite ($NO_2^-$), and then Nitrobacter oxidizes nitrite to nitrate ($NO_3^-$). Nitrate is the form most plants absorb.
3. Denitrification: Denitrifying bacteria like Pseudomonas reduce nitrate back to $N_2$ gas under anaerobic conditions, returning nitrogen to the atmosphere and completing the cycle.

The Sulfur Cycle

• Sulfur-reducing bacteria (e.g., Desulfovibrio) convert sulfate ($SO_4^{2-}$) to hydrogen sulfide ($H_2S$) in anaerobic environments like sediments and hot springs. This is a form of anaerobic respiration where sulfate serves as the terminal electron acceptor.
• Sulfur-oxidizing bacteria (e.g., Thiobacillus) reverse this process, oxidizing $H_2S$ back to sulfate under aerobic conditions. This makes sulfur available again for other organisms and completes the cycle.

The rotten-egg smell near marshes and hot springs? That's $H_2S$ produced by sulfur-reducing bacteria.

The Phosphorus Cycle

Unlike the other cycles, phosphorus has no significant gaseous phase. It cycles between rocks, soil, water, and organisms. Microorganisms contribute by solubilizing inorganic phosphorus (breaking it free from minerals) and mineralizing organic phosphorus (releasing it from decaying matter). Both processes make phosphorus available for plant uptake and influence its distribution in aquatic and terrestrial ecosystems.

Autotrophs vs. Heterotrophs in Cycling

These two metabolic groups depend on each other to keep biogeochemical cycles running.

Autotrophs produce organic compounds from inorganic sources. They use energy from light (phototrophs) or chemical reactions (chemolithotrophs) to fix carbon and other nutrients into the biosphere. They're the primary producers that build the base of food webs. Examples include cyanobacteria, algae, sulfur-oxidizing bacteria, and nitrifying bacteria.

Heterotrophs obtain energy and carbon by consuming organic compounds. They're responsible for decomposition and mineralization, breaking down dead matter and releasing inorganic nutrients back into the environment. Examples include most bacteria, fungi, and protozoa.

The relationship is cyclical: autotrophs fix inorganic elements into organic forms, heterotrophs break organic matter back down into inorganic nutrients, and those nutrients become available for autotrophs again.

Bioremediation with Microbes

Bioremediation is the use of microorganisms to degrade or detoxify environmental pollutants. It's often cheaper and less disruptive than physical or chemical cleanup methods.

There are two main approaches:

1. In situ bioremediation: Contaminated soil or groundwater is treated right where it is. This minimizes environmental disturbance and avoids the cost of excavation and transport. Nutrients or oxygen may be added to stimulate microbial activity at the site.
2. Ex situ bioremediation: Contaminated material is removed and treated in a controlled setting like a bioreactor. This allows more precise control over conditions (temperature, pH, oxygen levels) but costs more.

Contaminants that microbes can treat:

• Hydrocarbons from oil spills: Bacteria like Pseudomonas and Alcanivorax break down complex hydrocarbons into simpler, less harmful compounds. Alcanivorax populations naturally bloom in ocean waters after oil spills.
• Chlorinated compounds (e.g., PCBs): Dehalococcoides performs reductive dechlorination, stripping chlorine atoms from these toxic molecules and rendering them less harmful.
• Heavy metals (mercury, lead): Bacteria can transform metals into less toxic or insoluble forms through reduction, oxidation, or precipitation. This immobilizes the metals and reduces their bioavailability, though the metals themselves aren't destroyed.

Environmental Impacts of Microbial Processes

• Eutrophication: When excess nutrients (nitrogen and phosphorus from agricultural runoff or sewage) enter aquatic ecosystems, they fuel massive algal blooms. When those algae die, microbial decomposition consumes dissolved oxygen, creating hypoxic "dead zones" where fish and other organisms can't survive.
• Biomagnification: Certain pollutants increase in concentration at each level of the food chain. Mercury is a key example. Microbes in sediments convert inorganic mercury into methylmercury, a far more toxic and bioavailable form that accumulates in fish and eventually in humans.
• Redox reactions: Many biogeochemical transformations are oxidation-reduction reactions. Microbes use different electron donors and acceptors depending on what's available, and these redox reactions drive both energy flow and element cycling across ecosystems.
• Water quality: Microorganisms influence the hydrological cycle by cycling nutrients in aquatic systems, forming biofilms on surfaces, and affecting the breakdown of organic matter in water supplies.