Gas fermentation is gaining momentum. More and more bioprocesses rely on this technology, and PHA production makes no exemption. Indeed, several PHA producer start-ups and firms are testing the conversion of gases into polymers. However, C1 biological fixation faces a paradox: while gases like CO₂ and CH₄ are extremely abundant and polluting, thereby representing ideal feedstocks, their mass transportation within bioreactors and biomass is highly challenging. 

Given the enormous potential of gas fermentation, we invited three high-profile experts to shed some light on the limitations of this technology and, moreover, on potential strategies to overcome them.

Professor Raul Munoz Torre, from the University of Valladolid (Spain) pointed out that there is a great availability of biogas and biomethane. However, its use to produce electricity is not economically competitive, given the lower costs of other renewables. Instead, its conversion into PHA might represent a feasible alternative. Technically speaking, the biggest challenge is represented by the scarce solubility of CH₄ and O₂ (the other substrate needed) in water. Torre’s team proposed the use of a bubble column reactor equipped with a system enabling the recirculation of the gases flowing from the outlet.  In their experiments, gas mix composition, temperature, and nitrogen levels were optimized and air (cheaper than pure oxygen) adopted. The promising results, in terms of CH₄ consumption and PHA production, overwent a Techno-Economic Analysis. The analysis highlighted that revenues from PHA sales are much larger than for biogas, although PHA production requires more electricity. For this reason, a full conversion of biogas into PHA is more economically feasible where the cost of electricity is lower, while a hybrid strategy targeting the mixed production of biogas and PHA is more convenient where the cost of electricity is higher. 

Professor Cristian Torri from the University of Bologna (Italy) suggested how the ideal system to produce PHA is from VFAs in a liquid broth. Biological fixation of gases like CO₂, H₂, and CO can be adopted to convert them in VFAs. Despite being a simple and low-cost methodology, biological fixation, compared to chemical fixation, is very slow. Additionally, gases like hydrogen are poorly soluble in water. Torri’s group of research proposed the use of biochar (derived from the pyrolysis of biomass) to solve this problem. Biochar has high conductivity and surface, which favor the electrons travelling. Additionally, microbes can be embedded on biochar in the form of biofilms to create a direct interspecies electron transfer (DIET). In the char-based biofilm sparged reactor (CBSR) H₂ reducing bacteria provide the electrons to acetic acid producers. Despite H₂ reducing bacteria are more sensitive to acetic acid, the biochar itself shields them from it. Torri’s team tested successfully this technology also adopting syngas as initial substrate, reaching significative acetic acid productivity levels that shall be converted into PHA.

Finally, Professor Marina Kalyuzhnaya from the San Diego State University (United States) vouched for a mixed strategy where electrocatalysis and bioconversion are combined. Inside an electro catalytical cell, CO₂ is chemically converted into methanol in a very efficient manner, to overcome slower biological kinetics. Afterwards, methanol is biologically converted into PHA, also to avoid the high costs to methanol distillation. A selected bacterium, Methylotuvimicrobium alcaliphilum can metabolize methanol. This strain shows great growth rate and is a robust species. Because it is not naturally able to accumulate PHA, the bacterium was genetically engineered to express PHA synthase operon. Interestingly, the accumulation of long-branched mcl-PHAs from electro catalytical-derived methanol was observed.

To conclude, several strategies can be adopted to solve mass transfer limitations of gases in liquid systems where PHA biotechnological synthesis occurs. Overall, one has to consider the costs derived from the selected methodology and the kinetic advantage it brings to the bioprocess, without forgetting the impact of scale.