pH preferences — An organism that grows optimally when hydrogen ion concentration is very high (i.e., at low pH) is called a(n)…

Introduction / Context:
Microbes are adapted to specific pH ranges. Acidophiles thrive at low pH (high hydrogen ion concentration), neutrophiles near pH 7, and alkaliphiles at high pH. Correct labeling helps predict contamination risks, design fermentation conditions, and select preservation hurdles for foods and pharmaceuticals.

Given Data / Assumptions:

• “Very high hydrogen ion concentration” equates to low pH values.
• We compare terms describing osmotic tolerance, pH preference, and oxygen tolerance.
• The task is to match the description to the correct term.

Concept / Approach:
Acidophiles possess membrane and protein adaptations that maintain cytoplasmic pH homeostasis despite acidic surroundings. Proton pumps, impermeable membranes, and acid-stable enzymes enable growth where most organisms are inhibited. Osmotolerant describes solute/osmotic resistance; aerotolerant refers to oxygen tolerance without use; neutrophile denotes preference for near-neutral pH, not low pH.

Step-by-Step Solution:

Verification / Alternative check:
Examples include Acidithiobacillus in acid mine drainage and certain fungi that colonize acidic foods, demonstrating viable growth at low pH.

Why Other Options Are Wrong:

• Osmotolerant: salt/solute tolerance, not pH.
• Neutrophile: grows best near pH 7.
• Aerotolerant anaerobe: oxygen relationship, not pH.

Common Pitfalls:
Confusing survival with growth; many neutrophiles survive transient acidity but do not grow optimally there.

Final Answer:
acidophile