Then you find out that the lack of an ingredient results in the microbes producing the product lol.
Certain algae for example, need to be "starved" in a way that results in them switching from conventional photosynthesis that produces glucose to photosynthesis that functions to generate Hydrogen and Oxygen alone. The Oxygen is used for respiration and the Hydrogen is essentially dumped as a waste product. They can only sustain this for relatively short periods before their stockpile of carbohydrates is sufficiently depleted. Which means these bioreactors need to be cycled through a fattening phase where the algae stockpile fuel and a starvation phase where they exhaust that stockpile while producing Hydrogen.
Ive been trying to keep tabs on the literature in biochemistry, chemistry and other fields for decades. I inevitably come accross things like this from time to time. These particular articles I just googled because I dont have the bookmark for the original paper discussing Hydrogen production back in 2000.