Formate transport, growth inhibition and the membrane protonmotive force in two methylotrophs (T15 and L3)

Utilization of formic acid by extremely thermoacidophilic archaea species

The exploration of novel hosts with the ability to assimilate formic acid, a C1 substrate that can be produced from renewable electrons and CO2, is of great relevance for developing novel and sustainable biomanufacturing platforms. Formatotrophs can use formic acid or formate as a carbon and/or reducing power source. Formatotrophy has typically been studied in neutrophilic microorganisms because formic acid toxicity increases in acidic environments below the pKa of 3.75 (25°C). Because of this toxicity challenge, utilization of formic acid as either a carbon or energy source has been largely unexplored in thermoacidophiles, species that possess the ability to produce a variety of metabolites and enzymes of high biotechnological relevance. Here we investigate the capacity of several thermoacidophilic archaea species from the Sulfolobales order to tolerate and metabolize formic acid. Metallosphaera prunae, Sulfolobus metallicus and Sulfolobus acidocaldarium were found to metabolize and grow with 1-2 mM of formic acid in batch cultivations. Formic acid was co-utilized by this species alongside physiological electron donors, including ferrous iron. To enhance formic acid utilization while maintaining aqueous concentrations below the toxicity threshold, we developed a bioreactor culturing method based on a sequential formic acid feeding strategy. By dosing small amounts of formic acid sequentially and feeding H2 as co-substrate, M. prunae could utilize a total of 16.3 mM of formic acid and grow to higher cell densities than when H2 was supplied as a sole electron donor. These results demonstrate the viability of culturing thermoacidophilic species with formic acid as an auxiliary substrate in bioreactors to obtain higher cell densities than those yielded by conventional autotrophic conditions. Our work underscores the significance of formic acid metabolism in extreme habitats and holds promise for biotechnological applications in the realm of sustainable energy production and environmental remediation.

Heterologous expression of formate dehydrogenase enables photoformatotrophy in the emerging model microalga, Picochlorum renovo

Rising global greenhouse gas emissions and the impacts of resultant climate change necessitate development and deployment of carbon capture and conversion technologies. Amongst the myriad of bio-based conversion approaches under evaluation, a formate bio-economy has recently been proposed, wherein CO2-derived formate serves as a substrate for concurrent carbon and energy delivery to microbial systems. To date, this approach has been explored in chemolithotrophic and heterotrophic organisms via native or engineered formatotrophy. However, utilization of this concept in phototrophic organisms has yet to be reported. Herein, we have taken the first steps to establish formate utilization in Picochlorum renovo, a recently characterized eukaryotic microalga with facile genetic tools and promising applied biotechnology traits. Plastidial heterologous expression of a formate dehydrogenase (FDH) enabled P. renovo growth on formate as a carbon and energy source. Further, FDH expression enhanced cultivation capacity on ambient CO2, underscoring the potential for bypass of conventional CO2 capture and concentration limitations. This work establishes a photoformatotrophic cultivation regime that leverages light energy-driven formate utilization. The resultant photosynthetic formate platform has widespread implications for applied phototrophic cultivation systems and the bio-economy at large.

Effect of short-chain organic acids on macromolecular synthesis in Escherichia coli

Incubating cultures of Escherichia coli with propionic acid (5 mmol/l) or formic acid (10 mmol/l) at pH 5.0 produced bacteriostasis lasting 30 and 120 min respectively. During this time rates of RNA, DNA, protein, lipid and cell wall synthesis were reduced. Growth resumed after continued incubation in the presence of acid, but cells from acid-treated cultures were larger than controls. DNA synthesis was particularly sensitive to the presence of the propionic or formic acid.

Transport of substrates and metabolites and their effect on cell metabolism (in butyric-acid and methylotrophic fermentations)

The two components, delta pH and delta psi, of the membrane protonmotive force (delta p) effect and are affected by the transport of many substrates and metabolites. Because the integrity (or restoration) of the delta p requires the expenditure of metabolic energy, such transport processes affect the overall cell bioenergetics. However, the transport or high concentrations of certain substrates and metabolites can have more serious effects on cell metabolism because they partially or completely abolish either or both the delta pH and delta psi. If the cells cannot eventually restore the collapsed component(s) of the delta p, complete growth inhibition and cell death become inevitable. In the butanol/acetone fermentation of Clostridium acetobutylicum, the transport and the presence of key metabolites (acetic and butyric acids, and butanol) have serious and some necessary effects on the delta p. Acetic and butyric acids act as uncouplers of the delta pH, thereby reducing the internal pH. Using other acid uncouplers (such as acetoacetate, which is metabolized by the cells, or FCCP, which is not metabolized by the cells), we found that a lower pHo combined with the metabolic-energy drain of the uncoupling effect and high internal acid concentrations are implicated in the mechanism(s) of solventogenesis. Thus, the production or presence (or both) of the two acids (acetic and butyric) is beneficial to the initiation of solvent production. The transport mechanisms of CH3OH, CH2O, and HCOOH in obligate CH3OH utilizers (methylotrophs) were also discussed in detail. We showed that CH3OH is actively transported by the cells at the expense of metabolic energy and that its transport significantly affects the dynamics of continuous bioreactors. The accumulation of CH2O was found to be driven by the membrane delta p. Finally, formate was accumulated by the delta pH according to the general transport mechanism of short-chain fatty acids. The inhibition of growth by formate was explained by its uncoupling effect on the cells. Growth inhibition by CH3OH appeared to be related to the severe reduction of the membrane delta pH and cell pHi by relatively low CH3OH concentrations.