During a fermentation run, the measured dissolved oxygen concentration in the broth decreases from the saturation level (about 7 ppm) to 1 ppm. What conclusion is most appropriate?

Introduction / Context:
Dissolved oxygen (DO) is a primary constraint in aerobic fermentations. When DO drops substantially below the saturation concentration C*, oxygen transfer cannot keep up with cellular demand, leading to oxygen limitation and altered metabolism (e.g., overflow products, reduced productivities).

Given Data / Assumptions:

• Initial DO ≈ saturation (about 7 ppm for the given conditions).
• DO subsequently measured at ≈ 1 ppm.
• Temperature, pressure, and salinity are assumed roughly constant during the observation.

Concept / Approach:
Oxygen transfer rate OTR = kLa * (C* − CL). When CL (the bulk DO) is far below C*, the driving force is large, yet if CL remains low, the cellular oxygen uptake rate OUR is meeting or exceeding the maximum OTR, signalling oxygen limitation on the culture side. Consequently, growth and production are constrained by O2 availability.

Step-by-Step Solution:

Verification / Alternative check:
Step tests increasing agitation or aeration that restore DO confirm oxygen transfer limitation; off-gas analysis (O2, CO2) corroborates high OUR.

Why Other Options Are Wrong:

• Not oxygen limited: contradicts the definition of limitation when CL is well below C*.
• Experimental error: possible but not the most reasonable default without evidence.
• No conclusion: the conclusion of limitation is standard when DO is stably low.
• Lower oxygen means higher growth: opposite to aerobic physiology.

Common Pitfalls:
Confusing a transient dip with sustained limitation; ensure the DO drop persists despite reasonable control actions before concluding chronic limitation.

Final Answer:
Microbial growth is oxygen limited since the oxygen concentration is lower than the saturation concentration