Nitrogen cycling — Assimilatory (assimilative) nitrate reduction to ammonia and incorporation into biomass is carried out by which organisms?

Introduction / Context:Nitrate and nitrite can be reduced by two distinct pathways: assimilatory reduction, which converts them to ammonia for biosynthesis, and dissimilatory reduction (denitrification), which uses them as terminal electron acceptors and releases gaseous products. The question asks about assimilatory reduction, a growth-linked process found across kingdoms.

Given Data / Assumptions:

• Assimilatory reduction results in NH3 incorporation into amino acids.
• Occurs under nitrogen limitation to support growth.
• Pathway enzymes exist in plants, fungi, and many bacteria/archaea.

Concept / Approach:Plants, fungi, and prokaryotes possess nitrate and nitrite reductases that channel reduced nitrogen into glutamine/glutamate via GS-GOGAT or GDH pathways. Therefore, assimilatory reduction is not restricted to a single domain; it is widespread among phototrophs and heterotrophs alike when nitrate is a nitrogen source.

Step-by-Step Solution:

Verification / Alternative check:Textbook nitrogen metabolism depicts assimilatory nitrate reductase and nitrite reductase across taxa, distinct from denitrification enzymes used for respiration.

Why Other Options Are Wrong:

• Single-group answers omit taxa that clearly perform assimilatory reduction.

Common Pitfalls:Confusing assimilatory nitrate reduction with denitrification and assuming only bacteria do it; plants rely heavily on the assimilatory route.

Final Answer:all of these