Aquatic microbial ecology — defining the microbial loop The following describes a pathway where dissolved organic matter from phytoplankton is metabolized by heterotrophic bacteria, incorporated into particulate matter, and passed up the food web while being remineralized to CO2 and nutrients. This pathway is known as the:

Introduction / Context:
The microbial loop is a central concept in aquatic ecology that explains how dissolved organic carbon (DOC) released by phytoplankton is recovered by bacteria and routed back into higher trophic levels. This closes an important energy and nutrient cycle that would otherwise be lost to classical grazing food chains.

Given Data / Assumptions:

• Phytoplankton exude DOC during photosynthesis or upon cell lysis.
• Heterotrophic bacteria uptake this DOC and convert it into biomass.
• Protistan grazers and zooplankton consume bacterial biomass, transferring carbon and nutrients up the food web while CO2 and mineral nutrients are regenerated.

Concept / Approach:
The microbial loop complements the traditional phytoplankton-zooplankton-fish food chain by incorporating bacteria and protozoa as key intermediaries. It explains high bacterial production in oligotrophic waters and the tight coupling between carbon cycling and nutrient regeneration.

Step-by-Step Solution:
Map the described steps: DOC release → bacterial assimilation → grazing → particulate transfer → remineralization.Recognize this sequence as the microbial loop, not merely a stoichiometric ratio or lab model.Select “Microbial loop.”

Verification / Alternative check:
Conceptual diagrams of pelagic ecosystems consistently label this DOC–bacteria–protozoa pathway as the microbial loop.

Why Other Options Are Wrong:
Winogradsky column is a stratified enrichment device; Redfield ratio is an elemental stoichiometry (C:N:P = 106:16:1); Gibbs free energy is a thermodynamic concept; biological pump refers to carbon export to the deep ocean, not specifically the bacteria-DOC pathway described.

Common Pitfalls:
Equating microbial loop with biological pump. The pump emphasizes carbon sequestration at depth; the loop emphasizes recycling and trophic transfer in surface waters.

Final Answer:
Microbial loop.