The ability of Vibrio fischeri to produce bioluminescence chemicals only after reaching a threshold population density exemplifies which ecological/microbial regulatory phenomenon?

Introduction / Context:
Bioluminescence in Vibrio fischeri is a classic example of cells coordinating gene expression in response to population density. This regulation underlies microbial behaviors such as virulence, biofilm formation, competence, and secondary metabolism in many species.

Given Data / Assumptions:

• Cells secrete small signaling molecules (autoinducers).
• Signal concentration increases with cell density.
• Above a threshold, regulatory circuits activate target genes (for example, luciferase operon).
• Feedback loops can amplify the response.

Concept / Approach:
Quorum sensing links cell density to gene expression via diffusible signals (for example, acyl-homoserine lactones in Gram-negatives). In V. fischeri, accumulation of the autoinducer binds a regulator (LuxR), activating luminescence genes (luxICDABE). This ensures light production is energetically invested only when enough cells are present to make it ecologically meaningful (for example, symbiosis with squid hosts).

Step-by-Step Solution:
Identify the trigger: increased cell density raises autoinducer levels. Recognize the regulatory switch (LuxR/autoinducer complex). Connect to phenotype: activation of bioluminescence genes. Choose “quorum sensing.”

Verification / Alternative check:
Exogenous addition of autoinducer to dilute cultures induces luminescence, confirming density-dependent regulation.

Why Other Options Are Wrong:

• Liebig's law / Shelford's law: Resource limitation and tolerance ecology, not cell-to-cell signaling.
• Heisenberg's principle: A physics concept unrelated to microbial regulation.

Common Pitfalls:
Assuming quorum sensing always controls virulence; it regulates diverse traits depending on species and environment.

Final Answer:
quorum sensing.