Concept and option‑wise explanation

• Desulfovibrio desulfuricans is an anaerobic sulfate‑reducing bacterium known primarily as a mercury methylator, converting inorganic Hg²⁺ (or elemental Hg⁰) into methylmercury (MeHg), a more mobile and toxic organomercurial.

• Many Pseudomonas strains carry the mer operon and act as demethylating bacteria, converting methylmercury back to inorganic mercury (Hg²⁺) and/or Hg⁰, which can be further immobilized or volatilized.

With that in mind:

1. A converts methyl mercury to mercuric ion, B converts mercury to methyl mercury – incorrect

• This reverses their main roles: D. desulfuricans is known for methylation (Hg → MeHg), not demethylation.

2. A converts mercury to methyl mercury, B converts mercury to mercuric ions – incomplete/misleading

• The first half is fine (A methylates Hg), but the second half says Pseudomonas converts “mercury to mercuric ions,” which does not distinguish demethylation; mercury is already “mercury” in ionic form.

3. A converts mercury to methyl mercury, B converts methyl mercury to mercuric ions – correct

• Matches the typical environmental picture:

  • A (Desulfovibrio): methylates inorganic mercury → methylmercury.

  • B (Pseudomonas): demethylates methylmercury → inorganic Hg²⁺ (and often further to Hg⁰).

4. A converts methyl mercury to mercuric ions, B converts mercury to mercury ions – incorrect

• Again assigns demethylation to A and gives B a vague, incorrect role.

Therefore, the option that correctly represents their predominant roles in mercury cycling and bioremediation is option (3).