Development of Thermotoga maritima strains that surpassed the physiological limit of H2 production.

What is it about?

Biorenewable energy sources are of growing interest to mitigate climate change and H2 serve as a clean fuel. H2 is generated as a metabolic product during fermentation by a fermentative bacterium. However, the overall yield of H2 does not increase from 4 mol H2/ mol glu because of thermodynamic constraints and the preference of the fermentative bacterium to make more biomass over H2. In this study we created the metabolic stress on Thermotoga maritima that resulted in a slower sugar (maltose) uptake due to mutations in the malK3 sub-unit of the maltose transporter. We transferred the mutated malK3 into wild type and observed the similar phenomenon of a slower maltose uptake and excess H2 production in the resultant strain. The overall carbon recovery over 90% confirmed that that consumed carbon primarily formed acetate, lactate, CO2 and biomass. Furthermore, the oxidation-reduction (O-R) balance of oxidized and reduced products close to the theoretical value of 1.0 shows that the products were in balance and were accurately determined without any indication of new products present in significant quantities. In summary, the slower uptake of maltose changed the energy metabolism of Thermotoga maritima and carbon was rerouted through the pentose phosphate pathway leading to H2 yield beyond the previously predicted hypothetical H2 production limit.

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Photo by Mladen Borisov on Unsplash

Photo by Mladen Borisov on Unsplash

Why is it important?

For almost 40 years the biological limit of H2 has been thought to be 4 mol/ mol glucose because during this process 4 mol ATP can be generated to sustain the growth of a fermentative bacterium. However, bacterial growth is possible under limiting conditions of ATP production suggesting that the H2 yield can be further improved. The best organisms to test this hypothesis are hyperthermophiles because certain thermodynamic constrains are relieved and the higher growth temperature inhibits growth of H2 consumers present in raw feedstocks. Thermotoga maritima is the only organism that approaches to the predicted biological limit of H2 production though the Gibbs free energy at its growth temperature supports H2 more than 4 mol. A strong growth coupling with H2 and no use of the pentose phosphate pathway (PPP) during H2 production keep H2 to 4 mol/ mol glu. We changed the metabolic flux in this organism and carbon flew through the PPP which yielded additional H2 and the resultant strain surpassed the predicted biological limit that was proposed 40 years ago. H2 production from a fermentative bacterium has remained economically challenging considering the lower yield of H2 and this has encouraged fossil fuels based H2 production. H2 production from the non-renewable fossil fuel generates a significant amount of CO2 contributing to greenhouse gases. A higher H2 yield from a biological system is the perfect solution to produce economic and green hydrogen. This work supports the renewable green hydrogen energy production from fermentative bacteria. The work is of paramount of importance because this strategy could be used to improve the H2 yield in other fermentative bacteria.