Hydrogen production and fermentation in heliobacteria.

Heliomicrobium modesticaldum produces dihydrogen only during N2 fixation, with ~5-fold higher H2 under argon, and engineered hydAEFG expression enables much higher H2.

• Why it matters: Electron disposal limits reduced fermentation in heliobacteria, and the genus lacks a functional hydrogenase; defining when H2 and propanoate form clarifies metabolic constraints and engineering options.
• What they did: The authors measured H2 and screened reduced fermentation products while growing H. modesticaldum chemotrophically, mixotrophically, and phototrophically on pyruvate, then expressed hydAEFG from Acetivibrio thermocellus.
• The result: Propanoate was the main reduced product, reaching several millimolar, inversely correlated with light intensity on pyruvate, absent in phototrophic cultures, and linked to an rTCA mini-cycle; hydAEFG engineering enabled high H2.

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