In soil microbiology, which organism is primarily involved in the reduction of sulfates (SO4^2-) to hydrogen sulfide (H2S) under anaerobic conditions?

Introduction:
Sulfate reduction is a key anaerobic respiratory process in waterlogged soils and sediments. This question tests your ability to identify the classic sulfate-reducing bacterium that converts sulfate (SO4^2-) to hydrogen sulfide (H2S), a gas recognized by its characteristic rotten egg odor and important in biogeochemical sulfur cycling.

Given Data / Assumptions:

• Sulfate reduction occurs under anoxic or strictly anaerobic conditions.
• Hydrogen sulfide is the terminal product of microbial dissimilatory sulfate reduction.
• Different bacterial groups either oxidize reduced sulfur or reduce oxidized sulfur; do not confuse the two.

Concept / Approach:
Members of the genus Desulfotomaculum are endospore-forming, obligately anaerobic, Gram-positive sulfate-reducing bacteria. They use sulfate as a terminal electron acceptor, reducing it to H2S while oxidizing organic substrates or H2. In contrast, Thiobacillus species are sulfur oxidizers that convert reduced sulfur compounds to sulfate, while the purple non-sulfur phototrophs Rhodospirillum and Rhodomicrobium are not canonical sulfate reducers.

Step-by-Step Solution:
Identify the redox direction: the process asked is reduction of sulfate to sulfide. Recall canonical sulfate reducers: genera include Desulfovibrio, Desulfobacter, and Desulfotomaculum. Match to options: only Desulfotomaculum is a sulfate reducer listed. Exclude sulfur oxidizers and phototrophs that do not reduce sulfate to H2S.

Verification / Alternative check:
Culture-based tests show blackened iron sulfide precipitates in the presence of SRB like Desulfotomaculum, confirming H2S generation under anaerobiosis.

Why Other Options Are Wrong:

• Thiobacillus thiooxidans: Sulfur oxidizer; converts sulfide/sulfur to sulfate, not the reverse.
• Rhodospirillum: Purple non-sulfur phototroph; not a classical sulfate reducer.
• Rhodomicrobium: Phototrophic bacterium; not involved in dissimilatory sulfate reduction.

Common Pitfalls:
Learners often confuse sulfur oxidizers with sulfate reducers because both participate in the sulfur cycle. Always check whether the process is oxidation to sulfate or reduction to sulfide.

Final Answer:
Desulfotomaculum reduces sulfate to H2S under anaerobic conditions.