Nutrient Cycles

Carbon Cycle

The carbon cycle describes the movement of carbon through ecosystems, involving both biological and abiotic processes. Microorganisms play a central role in carbon cycling, facilitating the transformation of carbon between its various forms.

• Photosynthesis: Autotrophic microorganisms (e.g., cyanobacteria, algae) convert atmospheric CO2 into organic matter using sunlight.

• Decomposition: Heterotrophic microbes break down organic matter, releasing CO2 back into the atmosphere.

• Methanogenesis: Certain archaea produce methane (CH4) from CO2 and H2 under anaerobic conditions.

• Syntrophy: Cooperative interactions between different microbial species enable the breakdown of complex organic compounds, often leading to methanogenesis.

Example: In wetlands, methanogenic archaea and syntrophic bacteria collaborate to convert organic matter into methane.

Key Equations

• (Methanogenesis)

• (Photosynthesis)

Nitrogen Cycle

The nitrogen cycle involves the transformation of nitrogen between its various chemical forms, many of which are mediated by microorganisms. This cycle is essential for the synthesis of amino acids, nucleic acids, and other biomolecules.

• Nitrogen Fixation: Conversion of atmospheric N2 to ammonia (NH3) by diazotrophic bacteria (e.g., Rhizobium).

• Nitrification: Oxidation of ammonia to nitrite (NO2-) and then to nitrate (NO3-) by nitrifying bacteria.

• Denitrification: Reduction of nitrate to N2 gas by denitrifying bacteria, returning nitrogen to the atmosphere.

• Ammonification: Decomposition of organic nitrogen to ammonia by heterotrophic microbes.

• Anammox: Anaerobic oxidation of ammonia to N2 gas by specialized bacteria.

Example: Azotobacter species fix atmospheric nitrogen in soil, supporting plant growth.

Key Equations

• (Nitrogen Fixation)

• (Nitrification)

• (Denitrification)

• (Anammox)

Sulfur Cycle

The sulfur cycle tracks the movement of sulfur through ecosystems, with microorganisms facilitating both oxidation and reduction reactions. Sulfur is vital for proteins and other cellular components.

• Sulfur Oxidation: Chemolithotrophic bacteria oxidize reduced sulfur compounds (e.g., H2S) to sulfate (SO42-).

• Sulfur Reduction: Sulfate-reducing bacteria convert sulfate to hydrogen sulfide (H2S) under anaerobic conditions.

• Organic Sulfur Compounds: Microbes degrade organic sulfur compounds (e.g., cysteine, methionine) to release inorganic sulfur.

Example: Desulfovibrio species reduce sulfate to H2S in sediments.

Key Equations

• (Sulfur Oxidation)

• (Sulfur Reduction)

Humans and Nutrient Cycles

Mercury in the Environment

Mercury is a toxic heavy metal found in various environmental compartments. Human exposure occurs through industrial emissions, contaminated water, and food (especially fish).

• Toxic Forms: Methylmercury (CH3Hg+) is the most toxic form, bioaccumulating in organisms.

• Bacterial Resistance: Some bacteria possess genes (e.g., mer operon) enabling them to detoxify mercury by reducing Hg2+ to volatile Hg0.

Example: Pseudomonas species can resist mercury toxicity by enzymatic reduction.

Human Impacts on Carbon and Nitrogen Cycles

Human activities significantly alter nutrient cycles, affecting ecosystem health and climate.

• Carbon Cycle: Fossil fuel combustion increases atmospheric CO2, contributing to global warming.

• Nitrogen Cycle: Excessive use of fertilizers leads to nitrate pollution and eutrophication in aquatic systems.

Example: Agricultural runoff causes algal blooms due to elevated nitrogen levels.

Summary Table: Microbial Roles in Nutrient Cycles

Additional info: The above notes expand on brief learning objectives by providing definitions, examples, and key equations for each nutrient cycle, as well as a summary table for quick comparison.