Nitrogen metabolism — nitrate reduction capability: Which organisms can carry out biological nitrate reduction (nitrate → nitrite) under physiological conditions?

Introduction / Context:
Nitrate reduction (NO3− → NO2−) is a key step in the nitrogen cycle, enabling assimilation and energy generation in diverse organisms. Understanding who performs this reduction informs agriculture, ecology, and environmental biotech.

Given Data / Assumptions:

• Plants possess assimilatory nitrate reductase in the cytosol.
• Many bacteria and some fungi carry assimilatory or dissimilatory nitrate reductases.
• Animals generally cannot reduce nitrate to nitrite enzymatically for assimilation.

Concept / Approach:
Identify taxa with nitrate reductase enzymes. Plants reduce nitrate for amino acid synthesis; microorganisms do so for assimilation or anaerobic respiration (denitrification pathways).

Step-by-Step Solution:
Check plants: assimilatory nitrate reductase reduces nitrate for nitrogen assimilation.Check microbes: bacteria (and some archaea/fungi) perform assimilatory/dissimilatory reduction.Animals lack this pathway for assimilation; thus eliminate options including animals only.Select “Plants and microorganisms.”

Verification / Alternative check:
Crop physiology texts and microbial metabolism sources consistently list plant nitrate reductase and bacterial nitrate reductases as canonical examples.

Why Other Options Are Wrong:
“Only microorganisms” ignores plant nitrate assimilation; “Only plants” omits microbes; “None” is incorrect; “Animals and fungi only” is false.

Common Pitfalls:
Confusing dietary nitrate handling in animals with enzymatic nitrate reduction for assimilation.

Final Answer:
Plants and microorganisms.