Nutrient Cycles: Carbon, Nitrogen, and Microorganisms

The carbon cycle describes the movement of carbon atoms through the Earth's atmosphere, oceans, land, and living organisms. Carbon is a fundamental component of all organic molecules. Key processes include: 1. Photosynthesis: Plants and other producers absorb carbon dioxide (CO2) from the atmosphere or dissolved in water to produce organic compounds (e.g., glucose). 2. Respiration: All living organisms (plants, animals, microorganisms) release CO2 into the atmosphere or water as they break down organic compounds for energy. 3. Combustion: The burning of fossil fuels (coal, oil, natural gas) and biomass releases large amounts of CO2 into the atmosphere. 4. Decomposition: Decomposers (bacteria and fungi) break down dead organic matter, releasing carbon back into the soil and atmosphere (as CO2 through respiration). 5. Fossilisation: Over millions of years, under specific conditions, dead organic matter can be converted into fossil fuels, trapping carbon underground. Major carbon reservoirs include the atmosphere (as CO2), oceans (dissolved CO2, carbonates), land (in organic matter, soil, and fossil fuels), and living organisms.

The nitrogen cycle describes the movement of nitrogen through the atmosphere, soil, and living organisms. Nitrogen is an essential component of proteins, nucleic acids (DNA, RNA), and ATP. Atmospheric nitrogen (N2) is abundant but unusable by most organisms directly. Key processes, largely driven by microorganisms, include: 1. Nitrogen Fixation: Conversion of atmospheric nitrogen (N2) into ammonia (NH3) or ammonium (NH4+), a form usable by plants. This can occur through: * Atmospheric fixation: Lightning provides energy to convert N2 to nitrates. * Industrial fixation: The Haber-Bosch process produces ammonia for fertilisers. * Biological fixation: Carried out by nitrogen-fixing bacteria (e.g., *Rhizobium* in legume root nodules, *Azotobacter* free-living in soil). 2. Nitrification: A two-step process where ammonia/ammonium is converted to nitrites (NO2-) and then to nitrates (NO3-). * *Nitrosomonas* bacteria convert NH4+ to NO2-. * *Nitrobacter* bacteria convert NO2- to NO3-. Nitrates are the primary form of nitrogen absorbed by plants. 3. Assimilation: Plants absorb nitrates (NO3-) and ammonium (NH4+) from the soil and incorporate them into organic molecules (proteins, nucleic acids). Animals obtain nitrogen by consuming plants or other animals. 4. Ammonification (Decomposition): Decomposers (bacteria and fungi) break down dead organic matter and animal waste, releasing ammonia (NH3) or ammonium (NH4+) back into the soil. 5. Denitrification: Conversion of nitrates (NO3-) back into atmospheric nitrogen (N2) by denitrifying bacteria (e.g., *Pseudomonas*) under anaerobic conditions (low oxygen). This returns nitrogen to the atmosphere, completing the cycle.

Microorganisms are indispensable drivers of both the carbon and nitrogen cycles: In the Carbon Cycle: * Decomposers (bacteria and fungi): Break down dead organic matter, releasing carbon back into the soil and atmosphere (as CO2 through respiration). Without them, carbon would remain locked in dead biomass. In the Nitrogen Cycle: * Nitrogen-fixing bacteria (e.g., *Rhizobium*, *Azotobacter*): Convert atmospheric N2 into ammonia/ammonium, making it available to plants. This is the crucial first step for most biological nitrogen uptake. * Nitrifying bacteria (e.g., *Nitrosomonas*, *Nitrobacter*): Convert toxic ammonia/ammonium into nitrates, the form most readily absorbed by plants. * Decomposers (bacteria and fungi): Perform ammonification, breaking down organic nitrogen in dead organisms and waste into ammonia/ammonium, which can then re-enter the nitrification pathway. * Denitrifying bacteria (e.g., *Pseudomonas*): Convert nitrates back to atmospheric N2, completing the cycle and preventing excessive accumulation of nitrates in the soil.