Question

In the sulfur cycle, microbes degrade organic sulfur compounds, such as (a) _________________, to release H₂S, which can be oxidized by Acidithiobacillus to (b) _________________. This ion can be assimilated into amino acids by (c) _________________ or reduced by Desulfovibrio to (d) _________________. H2S is used by photoautotrophic bacteria as an electron donor to synthesize (e) _________________. The sulfur-containing by-product of this metabolism is (f) _________________.

Accepted Answer

Step 1: Identify the organic sulfur compound that microbes degrade to release hydrogen sulfide (H₂S). Common organic sulfur compounds include amino acids like cysteine or methionine, which contain sulfur in their structure. Step 2: Understand that Acidithiobacillus oxidizes H₂S to form a specific sulfur-containing ion. This ion is typically sulfate (SO₄²⁻), which is a more oxidized form of sulfur. Step 3: Recognize that the sulfate ion can be assimilated into amino acids by certain microbes. This assimilation process is often carried out by bacteria or plants that incorporate sulfate into sulfur-containing amino acids such as cysteine. Step 4: Know that sulfate can also be reduced by sulfate-reducing bacteria like Desulfovibrio to regenerate H₂S. This reduction is part of the anaerobic respiration process where sulfate acts as a terminal electron acceptor. Step 5: Learn that photoautotrophic bacteria use H₂S as an electron donor to synthesize organic compounds, such as carbohydrates, through photosynthesis. The sulfur-containing by-product of this metabolism is elemental sulfur (S⁰), which can accumulate outside the cells.

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Key Concepts

Microbial Roles in the Sulfur Cycle

Sulfur Compound Transformations

Assimilation and Energy Use of Sulfur