Match Column-I with Column-II and choose the correct answer from options given below.

List - I	List - II
(a) Nitrococcus	(i) Denitrification
(b) Rhizobium	(ii) Conversion of ammonia to nitrite
(c) Thiobacillus	(iii) Conversion of nitrite to nitrate
(d) Nitrobacter	(iv) Conversion of atmospheric nitrogen to ammonia

The correct matching is: (a) Nitrococcus → (ii) Conversion of ammonia to nitrite, (b) Rhizobium → (iv) Conversion of atmospheric nitrogen to ammonia, (c) Thiobacillus → (i) Denitrification, (d) Nitrobacter → (iii) Conversion of nitrite to nitrate.

To understand why each organism pairs with its respective nitrogen‑cycle step, we examine the metabolic role of each bacterium.

1. Nitrococcus – conversion of ammonia to nitrite (ii)

Nitrococcus is a chemolithoautotrophic ammonia‑oxidizing bacterium. It oxidises NH₃ (ammonia) to NO₂⁻ (nitrite) as its primary energy‑generating reaction:

$NH\_3\; +\; 1.5\; O\_2\; \to \; NO\_2^-\; +\; H^+\; +\; H\_2O$

Therefore it directly carries out the step “conversion of ammonia to nitrite”.

2. Rhizobium – conversion of atmospheric nitrogen to ammonia (iv)

Rhizobium species form symbiotic nodules on legume roots. Inside these nodules they possess the nitrogenase enzyme complex, which reduces N₂ (gaseous atmospheric nitrogen) to NH₃ (ammonia):

$N\_2\; +\; 8\; H^+\; +\; 8\; e^-\; +\; 16\; ATP\; \to \; 2\; NH\_3\; +\; H\_2\; +\; 16\; ADP\; +\; 16\; P\_i$

This process is called biological nitrogen fixation, matching option (iv).

3. Thiobacillus – denitrification (i)

Thiobacillus is a genus of chemolithotrophic bacteria that can use reduced sulphur compounds as electron donors. Certain Thiobacillus species are also capable of anaerobic respiration using nitrate as a terminal electron acceptor, reducing nitrate stepwise to nitrogen gases (N₂), a process known as denitrification:

$NO\_3^-\; \to \; NO\_2^-\; \to \; NO\; \to \; N\_2O\; \to \; N\_2$

Hence, Thiobacillus is associated with the denitrification pathway (i).

4. Nitrobacter – conversion of nitrite to nitrate (iii)

Nitrobacter are chemolithoautotrophic nitrite‑oxidising bacteria. They oxidise NO₂⁻ (nitrite) to NO₃⁻ (nitrate) to obtain energy:

$NO\_2^-\; +\; 0.5\; O\_2\; \to \; NO\_3^-$

This reaction corresponds exactly to the “conversion of nitrite to nitrate” step (iii).

Putting all four matches together yields the mapping “a – ii, b – iv, c – i, d – iii”, which matches the provided correct option.