Light-driven carbon dioxide reduction to methane by Methanosarcina barkeri in an electric syntrophic coculture

Abstract

The direct conversion of CO₂ to value-added chemical commodities, thereby storing solar energy, offers a promising option for alleviating both the current energy crisis and global warming. Semiconductor-biological hybrid systems are novel approaches. However, the inherent defects of photocorrosion, photodegradation, and the toxicity of the semiconductor limit the application of these biohybrid systems. We report here that Rhodopseudomonas palustris was able to directly act as a living photosensitizer to drive CO₂ to CH₄ conversion by Methanosarcina barkeri under illumination after coculturing. Specifically, R. palustris formed a direct electric syntrophic coculture with M. barkeri. Here, R. palustris harvested solar energy, performed anoxygenic photosynthesis using sodium thiosulfate as an electron donor, and transferred electrons extracellularly to M. barkeri to drive methane generation. The methanogenesis of M. barkeri in coculture was a light-dependent process with a production rate of 4.73 ± 0.23 μM/h under light, which is slightly higher than that of typical semiconductor-biohybrid systems (approximately 4.36 μM/h). Mechanistic and transcriptomic analyses showed that electrons were transferred either directly or indirectly (via electron shuttles), subsequently driving CH₄ production. Our study suggests that R. palustris acts as a natural photosensitizer that, in coculture with M. barkeri, results in a new way to harvest solar energy that could potentially replace semiconductors in biohybrid systems.

Fig. 1. Methanogenesis in R. palustris (R.p) and M. barkeri (M.b) cocultures.

Methane accumulation after initial inoculation (A) and at the sixth transfer after being transferred five times (B). C Methane production by R. palustris and M. barkeri coculture during continuous light-dark cycling.

Fig. 2. R. palustris and M. barkeri coculture aggregates.

A Phase-contrast microscopy image and B fluorescence in situ hybridization (FISH) image. R. palustris was detected by a green probe, and M. barkeri was labeled by a red probe.

Fig. 3. Direct electrosyntrophic methanogenesis between R. palustris and M. barkeri.

A Linear sweep voltammetry analysis. R. palustris generated anodic current in a potential range of greater than −0.62 V, and M. barkeri produced a cathodic current in a potential range of less than 0.57 V. B Current generation of photo-MFCs with an external resistance of 1 MΩ, showing direct electron transfer from the R. palustris anode to the M. barkeri cathode. The current generation of monocultures of R. palustris in the anode chamber and M. barkeri in the cathode chamber and of the two species in the separated chambers were measured. C Corresponding methane production in the photo-MFCs, showing that electrons produced by R. palustris could directly drive methanogenesis of M. barkeri. D Current generation of photo-MFCs during alternate light-dark cycles, showing light-driven electron transfer between R. palustris and M. barkeri. The test was performed after a steady current was generated.

Fig. 4. Indirect interspecies electron transfer between R. palustris and M. barkeri.

A Methane production by physically separated R. palustris and/or M. barkeri cultures, indicating the possibility of indirect electron transfer between these two species. B Baseline subtracted differential pulse voltammogram of the cell-free culture supernatant from physically separated culture cells, showing the presence of at least two redox-active compounds that could act as electron shuttles to facilitate electron transfer between R. palustris and M. barkeri. C Three-dimensional fluorescence spectrum of the culture supernatants from physically separated R. palustris and M. barkeri cultures. The two fluorescence peaks (I and II) correspond to the fluorescence of humic-like compounds.

Fig. 5. The mechanism of light-driven CO₂-to-CH₄ conversion in R. palustris and M. barkeri coculture as revealed by metatranscriptomics.

Genes involved in the processes include those that encode (1) Sox proteins, (2) proteins involved in cyclic photophosphorylation, (3) NADH-quinone oxidoreductase, (4) Ribulose-1,5-bisphosphate carboxylase/oxygenase, (5) and (23) ATP synthase, (6) enzymes participating in the Calvin–Benson–Bassham cycle, (7) PioABC, (8) nitrogenases, (9) c-type cytochromes, (10) flagellar proteins, (11) pilus proteins, (12) F₄₂₀H₂ dehydrogenases Fpo, (13) and (16) proteins involved in methanophenazine biosynthesis, (14) EchA-F hydrogenases, (15) MP-reducing hydrogenases VhtACG, (17) heterodisulfide reductases HdrDE, (18) heterodisulfide reductases HdrABC, (19) Formylmethanofuran dehydrogenases, (20) F420 hydrogenases, (21) Methenyl cyclohydrolases, (22) Methyl-CoM reductases, (24) methyltransferases MtrA-H, (25) Na⁺/H⁺ antiporter Nha, (26) acyl-CoA synthetases. Inset heat maps show the average transcript abundance of genes from triplicate independent cocultures presented as log₂ FPKM values. The median log₂ FPKM was 6.6 for R. palustris and 7.1 for M. barkeri. The blue and red lines indicate the speculated electron flux and CO₂ fixation pathway, respectively, during light-driven methanogenesis in the coculture.