Microbial roles in biogeochemical cycles: As a group, bacteria are responsible for which of the following processes in natural ecosystems and engineered systems?

Introduction / Context:
Bacteria drive key transformations in nitrogen and sulfur cycles. Exams often test recognition that multiple, seemingly unrelated transformations are all carried out by diverse bacterial groups in nature and technology (e.g., wastewater plants, biofilters).

Given Data / Assumptions:

• Nitrogen oxidation refers mainly to nitrification (NH3/NH4+ → NO2− → NO3−).
• Sulfur oxidation includes conversion of reduced sulfur compounds (H2S, S^0, thiosulfate) to sulfate.
• Nitrogen fixation is the conversion of N2 to NH3 by nitrogenase-containing microbes.

Concept / Approach:
No single bacterial species performs all steps, but bacterial guilds collectively execute them. Ammonia- and nitrite-oxidizing bacteria (and archaea) carry out nitrification; chemolithotrophic sulfur oxidizers (e.g., Thiobacillus-like genera) oxidize sulfur; diazotrophs (e.g., Azotobacter, Rhizobium, cyanobacteria) fix nitrogen.

Step-by-Step Solution:
Match each listed process to known bacterial groups. Recognize that all the listed processes are indeed bacterial functions. Select the inclusive option “All of these.”

Verification / Alternative check:
Standard biogeochemical cycle diagrams and environmental microbiology texts attribute these transformations to bacteria, sometimes alongside archaea.

Why Other Options Are Wrong:

• Any single process alone is incomplete; bacteria perform all listed functions.
• “None of these” contradicts extensive literature and practice.

Common Pitfalls:
Thinking a single clade does everything; in reality, specialized guilds collaborate to cycle elements.

Final Answer:
All of these processes are carried out by bacteria.