Continuous Culture Control — What is the name of an open system that maintains approximately constant cell density by continuously adding fresh medium and removing culture based on turbidity feedback?

Introduction:
Open culture systems maintain microbial populations by continuous inflow and outflow. Two widely taught designs are the chemostat and the turbidostat. Distinguishing them is essential for selecting the right strategy to control growth rate and biomass concentration.

Given Data / Assumptions:

• Objective is to keep cell density essentially constant in time.
• Measurements of turbidity or optical density are available for feedback control.
• Media addition and removal rates can be adjusted dynamically.

Concept / Approach:

A turbidostat uses real-time turbidity as a control signal to adjust the dilution rate and maintain a set biomass concentration. In contrast, a chemostat uses a fixed dilution rate and a limiting nutrient to set the steady-state growth rate, with cell density determined by substrate balance rather than feedback on turbidity.

Step-by-Step Solution:

Verification / Alternative check:

Chemostats achieve approximate constancy of biomass only indirectly via nutrient limitation and fixed D. Turbidostats explicitly control density via feedback, matching the question statement.

Why Other Options Are Wrong:

A and D are not standard terms for this feedback-controlled approach. B controls growth rate via fixed D rather than direct turbidity feedback. E is a closed system between feeds and does not remove outflow continuously.

Common Pitfalls:

Assuming any open continuous system that holds density constant must be a chemostat; the key difference is feedback on turbidity for a turbidostat.

Final Answer:

Turbidostat