Maximizing biomass productivity — Saccharomyces cerevisiae shows high biomass yields at low glucose and high dissolved oxygen. Which reactor strategy best supports maximum biomass productivity?

Introduction:
Saccharomyces cerevisiae displays high biomass yields when glucose is low (to avoid the Crabtree effect) and oxygen is abundant. The objective is to choose a reactor mode that sustains these conditions continuously for maximal productivity (g biomass per L per h).

Given Data / Assumptions:

• Crabtree effect: high glucose drives fermentative overflow, lowering Yx/s.
• High dissolved oxygen supports fully respiratory metabolism with higher ATP yield per glucose.
• Productivity depends on both yield and growth rate at steady state.

Concept / Approach:
A chemostat allows tight control of residual glucose at low levels by setting dilution rate and feed concentration, maintaining respiratory metabolism and high biomass yield. Batch with high initial glucose transiently experiences high substrate, inviting ethanol formation and yield loss. Fed-batch can work if residual glucose is kept very low, but without control it often produces spikes; continuous offers the most stable low-glucose environment for maximum sustained biomass productivity.

Step-by-Step Solution:
Identify desired state: low residual glucose, high DO, steady respiration.Map states to modes: chemostat maintains constant limitation and DO control.Reject modes that cause glucose spikes (batch high initial, uncontrolled fed-batch).Conclude that continuous low-glucose aerobic operation maximizes biomass productivity.

Verification / Alternative check:
Chemostat studies show higher Yx/s and stable high qO2 under glucose limitation; ethanol is minimized, and productivity mu*X can be optimized by selecting D below washout threshold.

Why Other Options Are Wrong:

• (a) High initial glucose triggers overflow and reduces yield.
• (c) Without residual glucose control, fed-batch often experiences transient high glucose.
• (d) Modes are not equivalent.
• (e) Anaerobic operation produces ethanol with low biomass yield.

Common Pitfalls:
Confusing titer with productivity; neglecting oxygen transfer limits at high cell densities; ignoring washout limits when setting D.

Final Answer:
Continuous fermenter operated with low glucose (substrate limitation) and high dissolved oxygen.