In soil ecosystems, most protozoa are flagellates or amoebae. What is their dominant mode of acquiring nitrogen?

Introduction:
Protozoa are key predators in soil food webs, regulating bacterial populations and releasing mineral nitrogen back into the environment. This question focuses on the principal trophic interaction by which soil protozoa obtain nitrogen for growth and metabolism.

Given Data / Assumptions:

• Soil protozoa include flagellates, amoebae, and ciliates.
• Bacterial biomass is abundant in the rhizosphere and bulk soil.
• Predation leads to nitrogen mineralization, benefiting plants.

Concept / Approach:
Flagellates and amoebae predominantly graze on bacteria. Through ingestion and digestion, they convert bacterial organic nitrogen into protozoan biomass and excrete excess nitrogen as ammonium, enhancing nitrogen availability to plants. Although some protozoa can ingest fungal spores or hyphae fragments, bacterial predation is the dominant and most frequent pathway.

Step-by-Step Solution:
Identify the most abundant and accessible prey in soil microhabitats. Relate protozoan grazing to nitrogen cycling via ammonium release. Compare alternatives: fungal ingestion happens but is less dominant for most groups. Choose bacterial ingestion as the primary nitrogen acquisition route.

Verification / Alternative check:
Microscopic observations and isotope tracing confirm that bacterivory is central to protozoan nutrition and nitrogen turnover in soils.

Why Other Options Are Wrong:

• Ingestion of mold / fungi: Possible for some species but not the dominant mode across soil protozoa.
• All of these: Overstates fungal predation as equally common; bacterivory dominates.

Common Pitfalls:
Assuming visible fungal biomass implies it is the main protozoan prey; microbial scale and encounter rates favor bacteria.

Final Answer:
ingestion of bacteria is the dominant route for nitrogen acquisition by soil protozoa.