Microbial metabolism and carbon assimilation When acetate is provided as the sole source of carbon, which specialized metabolic cycle do many microorganisms use to assimilate carbon efficiently without losing it as carbon dioxide?

Introduction / Context:
Some bacteria, yeasts, and plants can grow when acetate or other two-carbon (C2) compounds are the only carbon source. To conserve carbon for biosynthesis, they avoid losing carbon as CO2 in the tricarboxylic acid (TCA) cycle by engaging a variant called the glyoxylate pathway (glyoxylate shunt). This question tests recognition of that specific cycle and why it is used.

Given Data / Assumptions:

• Carbon source available: acetate (C2), no sugars provided.
• Organism must assimilate acetate into cell material while generating some reducing power and precursors.
• Objective: identify the pathway that enables net carbon conservation from C2 units.

Concept / Approach:

The classic TCA cycle decarboxylates isocitrate and alpha-ketoglutarate, causing carbon loss as CO2. The glyoxylate shunt bypasses these steps via two hallmark enzymes, isocitrate lyase and malate synthase, allowing two molecules of acetyl-CoA to be converted into malate and succinate without decarboxylation. This conserves carbon skeletons for gluconeogenesis and anabolism.

Step-by-Step Solution:

Verification / Alternative check:

Mutants lacking isocitrate lyase or malate synthase fail to grow on acetate as the only carbon source, confirming the essential role of the glyoxylate pathway for C2 growth.

Why Other Options Are Wrong:

• Pentose phosphate pathway: Generates NADPH and pentoses; not sufficient for C2 assimilation.
• Glycolysis: Catabolizes hexoses; not applicable with acetate alone.
• Oxaloacetate pathway: Not a standard, named cycle for C2 growth.
• Entner–Doudoroff pathway: An alternative to glycolysis for sugars, not acetate assimilation.

Common Pitfalls:

• Assuming any central pathway works for all substrates; acetate requires bypass of TCA decarboxylation.
• Confusing “glyoxylate cycle” with “anaplerotic reactions”; they are related but distinct concepts.

Final Answer:

glyoxylate pathway (glyoxylate shunt)