Anaerobic respiration with special reference to dissimilatory nitrate reduction

Anaerobic respiration with special reference to dissimilatory nitrate reduction

Microorganisms usually use one of three sources of energy - Phototrophs capture radiation energy from the sun. Chemoorganotrophs oxidize organic molecules to liberate energy, while chemolithotrophs employ inorganic nutrients as energy sources.  Microorganisms also vary in the electron acceptors used by chemotrophs.  Three major kinds of acceptors are employed and as per the different modes are Fermentation, aerobic respiration and anaerobic respiration.

In fermentation, the energy substrate is oxidized and degraded without the participation of an exogenous or externally derived electron acceptor. Usually, the catabolic pathway produces an intermediate such as pyruvate that acts as the electron acceptor. Fermentation normally occurs under anaerobic conditions, but also occurs sometimes when oxygen is present.

When the energy-yielding metabolism make use of exogenous or externally derived electron acceptors, the metabolic process is called respiration and is divided into two - aerobic respiration and anaerobic respiration.  In aerobic respiration, the final electron acceptor is oxygen and in anerobic respiration, the final electron acceptor is a different exogenous acceptor, mostly an inorganic molecule such as (NO₃^_, SO₄^2_, CO₂, Fe₃^_, SeO₄^2_, etc), or organic acceptors such as fumarate.

The amount of energy is different for fermentation and respiration.  Since the electron acceptor in fermentation is at the same oxidation state as the original nutrient and there is no overall net oxidation of the nutrient, only a limited amount of energy is made available. The acceptor in respiratory processes has reduction potential much more positive than the substrate and thus considerably more energy will be released during respiration.

Fermentation - an energy-yielding process in which organic molecules serve as both electron donors and acceptors.

Respiration - an energy-yielding process in which the acceptor is an inorganic molecule, either oxygen (aerobic respiration) or another inorganic acceptor (anaerobic respiration).

Anaerobic respiration is a fascinating and essential process in various ecosystems, particularly where oxygen is scarce or absent. It allows organisms to generate energy (ATP) by breaking down organic molecules and instead of using oxygen, it utilizes other inorganic molecules such as nitrate, sulfate, and CO2, metals, etc or sometimes organic compounds as the final electron acceptor. 

Key Characteristics of anaerobic respiration:

• Absence of Oxygen: The defining feature is the lack of oxygen as the final electron acceptor.
• Electron Transport Chain: Unlike fermentation (which also occurs without oxygen but doesn't use an electron transport chain), anaerobic respiration utilizes an electron transport chain to generate a proton motive force, which drives ATP synthesis via chemiosmosis.
• Lower ATP Yield: Compared to aerobic respiration, anaerobic respiration typically yields less ATP per molecule of glucose.
• Diverse Microorganisms: It is carried out by prokaryotes that inhabit anaerobic environments like sediments, wetlands, deep subsurface environments, and even certain host-microbe interactions.
• Ecological Importance: Anaerobic respiration plays crucial roles in global biogeochemical cycles, particularly the nitrogen, sulfur, and carbon cycles, by transforming various compounds. It's also vital in bioremediation and wastewater treatment.

Steps involved:

1. Glycolysis: Glucose is broken down into pyruvate, producing a small amount of ATP and NADH. This step is common to both aerobic and anaerobic respiration.
2. Krebs Cycle: Pyruvate is further oxidized, generating more NADH and FADH₂ (electron carriers).
3. Electron Transport Chain: The electrons from NADH and FADH₂ are passed down an electron transport chain. Instead of oxygen, the alternative inorganic electron acceptor is reduced at the end of chain.
4. Chemiosmosis: The movement of electrons through the electron transport chain pumps protons across a membrane, creating an electrochemical gradient or proton motive force. This gradient is then used by ATP synthase to produce ATP from ADP and inorganic phosphate.

Dissimilatory Nitrate Reduction (DNR)

Some bacteria can use nitrate as the electron acceptor at the end of their electron transport chain and produce ATP. This process is called dissimilatory nitrate reduction. This is termed "dissimilatory" because the nitrate is reduced for energy generation, not for assimilation into cellular biomass (In assimilatory nitrate reduction, nitrate is incorporated into organic molecules for growth).   Nitrate may be reduced to nitrite by nitrate reductase.

However, since a nitrate molecule will accept only two electrons, reduction of nitrate to nitrite is not a particularly effective way of making ATP, and thus a large amount of nitrate is required for growth.   Also the nitrite formed is quite toxic.

So nitrate is often is further reduced to nitrogen gas, and this process is known as denitrification. In this case, each nitrate will then accept five electrons, and the product will be nontoxic too.

Denitrification is a multistep process with four enzymes: nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase with stepwise reduction of nitrate (NO₃^−​) to nitrite (NO₂⁻​), nitric oxide (NO), nitrous oxide (N₂​O), and finally to dinitrogen gas (N₂​). This process releases gaseous nitrogen compounds into the atmosphere.  The four enzymes use electrons from coenzyme Q and c-type cytochromes to reduce nitrate and generate PMF.

• Nitrate Reductase: Reduces NO₃⁻​ to NO₂⁻​.   
• Nitrite Reductase: Reduces NO₂⁻​ to NO.  This is present in periplasmic space in gram-negative bacteria.
• Nitric Oxide Reductase: Reduces NO to N₂​O.  Nitric oxide reductase catalyzes the formation of nitrous oxide from NO and is a membrane-bound cytochrome bc complex.
• Nitrous Oxide Reductase: Reduces N₂​O to N₂​.  This enzymes is also periplasmic.

Two types of bacterial nitrite reductases catalyze the formation of NO in bacteria. One contains cytochromes c and d1 (e.g., Paracoccus and Pseudomonas aeruginosa), and the other is a copper protein (e.g., Alcaligenes).

Key Characteristics of Denitrification:

• The ultimate product, N2​ gas, is unreactive and returns to the atmosphere, effectively removing fixed nitrogen from the ecosystem.
• It is a crucial component of the global nitrogen cycle, returning fixed nitrogen to the atmosphere.
• Many denitrifying bacteria are facultative anaerobes, they can switch between aerobic respiration (when oxygen is available) and denitrification (when oxygen is absent or low). Examples are Pseudomonas denitrificans and Paracoccus denitrificans.
• Intermediate products like N₂​O (nitrous oxide) are potent greenhouse gases and can contribute to ozone depletion.
• Denitrification in anaerobic soil results in the loss of soil nitrogen and adversely affects soil fertility.

Dissimilatory Nitrate Reduction to Ammonium (DNRA) or Nitrate Ammonification

This is the reduction of nitrate (NO₃^−​) or nitrite (NO₂⁻​) directly to ammonium (NH₄⁺​). This process conserves bioavailable nitrogen within the ecosystem.

Here, the final product is a soluble, bioavailable form that can be readily utilized by plants and microorganisms.  DNRA and denitrification often compete for the same substrates (NO₃⁻and NO₂⁻​) in anaerobic environments.  Certain bacteria like Beggiatoa, Thioploca, and Shewanella species are known to perform DNRA.

 

Other mechanisms of anaerobic respiration employed by obligate anaerobes are as follows.

·       Methanogens use CO₂ or carbonate as a terminal electron acceptor and they reduce CO₂ to methane.   This is a significant process in anaerobic environments like wetlands and the guts of ruminants. 

·       Desulfovibrio use Sulfate as  the final acceptor and get reduced to sulfide (S^2_ or H₂S).