Oxygen limitation criterion:\nIn a microbial suspension culture, oxygen will limit growth only under which condition for the dissolved oxygen (DO) level?

Introduction / Context:
Cells exhibit a characteristic critical dissolved oxygen level (Ccrit). Above Ccrit, the specific growth rate μ is essentially independent of DO; below Ccrit, μ declines as oxygen becomes limiting. Recognizing this threshold guides aeration and agitation strategies in bioreactors to avoid mass-transfer-limited performance.

Given Data / Assumptions:

• Critical concentration is organism- and condition-specific.
• DO is measured as concentration or % saturation at the operating temperature and pressure.
• We consider steady-state or quasi-steady operation.

Concept / Approach:
Growth limitation occurs when oxygen supply cannot meet cellular demand. Operationally, when DO < Ccrit, respiration becomes oxygen-limited, lowering growth and product formation rates. Maintaining DO above Ccrit via higher KLa (agitation, aeration, oxygen enrichment) or reduced demand avoids this limitation.

Step-by-Step Solution:

Verification / Alternative check:
Respirometry and DO step tests show μ and OUR plateau above Ccrit and drop below it; many processes control DO at a setpoint above Ccrit (e.g., 20–40% air saturation) for safety margin.

Why Other Options Are Wrong:

• Greater than Ccrit: non-limiting regime.
• Greater than saturation: physically impossible at equilibrium without overpressure/supersaturation, and still not limiting.
• Exactly at saturation: ensures maximum C*; no limitation solely due to oxygen.
• Probe calibration gas: unrelated to physiological limitation.

Common Pitfalls:
Confusing Ccrit with saturation concentration C*; C* is a physicochemical property, Ccrit is biological and varies with strain and conditions.

Final Answer:
Concentration is less than the critical concentration