GATE 2024 BT Q.15: solution and answer

Official answer: DSource: Official answer key, GATE 2024 BT (IISc Bengaluru)

1. The blank describes a relationship in which two different species exchange nutrients and both gain growth benefit. So the term must mean a mutually beneficial, nutrient-sharing partnership.

2. Option D, syntrophism: one species consumes the metabolic products of the other, such as hydrogen, formate or acetate. This keeps those products at low levels, which makes the partner's reaction thermodynamically favourable. The two species grow together, each relying on the other, so this matches the description.

3. Option A, antagonism: one species inhibits or harms another, for example by producing antibiotics or bacteriocins. Nothing is exchanged for mutual benefit, so it is wrong.

4. Option B, commensalism: one species benefits and the other is neither helped nor harmed. The benefit is one-sided, not mutual, so it is wrong.

5. Option C, parasitism: the parasite benefits at the host's expense. It is harmful to one partner, so it is wrong.

6. Only syntrophism combines nutrient exchange between species with mutual growth benefit, so the answer is (D).