NDA (II) 2016 General Ability Test · Question 68 of 150

Correct — B, an aerobic environment. Methanogens are strict anaerobes — oxygen is not merely unnecessary to them, it is poisonous, because their energy-yielding chemistry depends on enzymes that oxygen destroys. So wherever air has free access, methanogens cannot live, and an aerobic environment is precisely the setting from which they are absent. Check that against the other three options and each turns out to be a classic methanogen habitat. The rumen of cattle is a sealed fermentation chamber where these microbes help break down cellulose, which is why cattle belch methane. Anaerobic sludge in a sewage works is deliberately kept air-free so that they can digest the waste. Cattle dung teems with them, which is exactly what a gobar gas plant exploits when it seals dung into a pit and collects the gas that comes off.

• (a)Rumen of cattle — One of the best-known homes of methanogens. The rumen is an oxygen-free fermentation vat in which microbes digest cellulose for the animal, and the methane the cattle release is their by-product — a significant agricultural source of greenhouse gas.
• (c)Anaerobic sludge — The name of the option gives it away: anaerobic means without oxygen, which is the one condition methanogens require. Sewage treatment plants run anaerobic digesters specifically to let these organisms work and to capture the methane they produce.
• (d)Excreta of cattle — Cattle dung carries methanogens out of the rumen with it, which is why dung is the standard charge for a biogas plant. Seal it away from air and it goes on producing methane.

Methanogens are microbes that obtain energy by reducing carbon dioxide or simple compounds to methane, and they can do so only in the complete absence of oxygen. They are the final link in an anaerobic food chain in which other microbes first break complex organic matter down to simple acids and hydrogen. Their habitats are all sealed or waterlogged: the rumen, marshes and paddy fields, the guts of termites, the bottom mud of ponds, sewage digesters and dung pits.

The stem asks where they are NOT present, so read carefully and then look for the option that names a condition rather than a place. Three options give locations, all of them oxygen-free by nature, while option (b) gives an environment defined by the presence of oxygen — that asymmetry alone points to the answer. One point of vocabulary is worth noting honestly: the paper calls methanogens 'methane producing bacteria', which follows the older textbook usage; in modern classification they are archaea, a domain separate from true bacteria. The naming does not change the answer.

• Methanogens are strict anaerobes, and oxygen inactivates the enzymes on which their metabolism depends.
• Their habitats include the rumen of cattle, cattle dung, anaerobic sewage sludge, waterlogged paddy fields, marshes and the guts of termites.
• Methane is the chief combustible component of biogas, which is why gobar gas plants seal dung away from the air.
• Livestock and flooded paddy fields are among the largest sources of methane, a greenhouse gas far more potent than carbon dioxide molecule for molecule.
• Methanogens are classified today as archaea rather than as true bacteria, though older texts group them with bacteria as the question's wording does.

• Missing the negative in the stem and marking a habitat where methanogens are abundant.
• Assuming every microbe needs oxygen; for these organisms oxygen is lethal.

Microbes in human use are a steady NDA GAT theme — biogas, sewage treatment, fermentation and antibiotics — and the questions usually turn on whether the process needs air or excludes it.

• practice — not a real PYQ

  The main combustible constituent of biogas is

  • (a)carbon dioxide
  • (b)methane
  • (c)hydrogen sulphide
  • (d)nitrogen

  Answer(b) methane — it makes up the larger share of biogas and supplies its heating value.

• practice — not a real PYQ

  Organisms for which oxygen is toxic and which can grow only in its absence are called

  • (a)obligate aerobes
  • (b)facultative aerobes
  • (c)obligate anaerobes
  • (d)autotrophs

  Answer(c) obligate anaerobes — methanogens are the standard example.