Continuous culture control: Which device uses a photocell to monitor culture turbidity (absorbance) and automatically regulates the inflow of fresh medium to maintain a set cell density?

Introduction / Context:
Continuous culture systems keep microbial populations at steady states for physiological and bioprocess studies. Two classic designs are the chemostat and the turbidostat. Knowing how each maintains control is vital for experimental design and interpretation of growth kinetics.

Given Data / Assumptions:

• The question mentions a photocell measuring absorbance (optical density).
• The device regulates fresh medium flow based on measured turbidity.
• Goal: maintain a target biomass concentration.

Concept / Approach:
A turbidostat uses real-time optical feedback. When turbidity rises above the setpoint, medium inflow increases (and culture overflows), diluting the culture back to the set optical density. In contrast, a chemostat fixes the dilution rate and achieves steady state through nutrient limitation rather than direct optical feedback. The other listed items are for counting or are unrelated to culture control.

Step-by-Step Solution:
Identify the hallmark: photocell-based turbidity control. Map this to the turbidostat’s control logic. Exclude chemostat (no optical feedback) and counting-only devices. Select “Turbidostat.”

Verification / Alternative check:
In practice, turbidostats are preferred for studying growth at maximal rates without nutrient limitation, while chemostats are used to study responses to defined limitations (e.g., carbon, nitrogen).

Why Other Options Are Wrong:
Coulter counter and Petroff–Hausser give counts, not automatic control; hemostat is unrelated; chemostat does not use a photocell feedback loop.

Common Pitfalls:
Assuming any continuous culture with measuring capability is a chemostat; feedback control based on OD uniquely describes a turbidostat.

Final Answer:
Turbidostat (photocell-regulated continuous culture).