Saccharomyces cerevisiae at high glucose concentration exhibits the Crabtree effect. Under such conditions, how does dissolved oxygen level influence its metabolic activity?

Introduction / Context:
Saccharomyces cerevisiae is a Crabtree-positive yeast. At sufficiently high glucose, it prefers fermentative metabolism and produces ethanol even when oxygen is present—a phenomenon sometimes called aerobic fermentation. This question evaluates recognition of that behavior and its process implications.

Given Data / Assumptions:

• High extracellular glucose concentration (above the Crabtree threshold).
• Aerated bioreactor with measurable dissolved oxygen.
• Standard laboratory or industrial yeast strains.

Concept / Approach:
In Crabtree-positive organisms, overflow metabolism is triggered by high glucose uptake rates: glycolytic flux exceeds respiratory capacity, diverting pyruvate to ethanol even under aerobic conditions. Thus, DO is not the controlling switch; glucose level and catabolite repression are. Oxygen can still support biomass formation and sterol synthesis, but ethanol production persists at high glucose regardless of DO, unless feed control reduces glucose or cells adapt to respiratory metabolism.

Step-by-Step Solution:

Verification / Alternative check:
Chemostat and fed-batch studies show ethanol formation above a critical specific glucose uptake rate; reducing glucose (e.g., via feed control) shifts metabolism to respiration at the same aeration.

Why Other Options Are Wrong:

• High DO essential: incorrect—ethanol still forms at high glucose.
• Low DO essential: ethanol also forms at high DO under Crabtree conditions.
• DO must be zero or respiration dominates: contradicts observed aerobic fermentation.
• Requirement for sodium salts is irrelevant to core metabolism.

Common Pitfalls:
Confusing Pasteur effect (oxygen suppresses fermentation) with Crabtree effect (glucose induces fermentation despite oxygen).

Final Answer:
Metabolic pattern is largely independent of dissolved oxygen (fermentation proceeds even with O2).