Get salty with me : acetate production through homoacetogenesis in (hyper)saline conditions

Homoacetogenesis has become a hot topic, with industry interest, as a bio-process that can convert waste CO2 directly to a valorizable product (organic acids). However, the microorganisms catalyzing this reaction have remained little explored outside of their original discovery and later in the pioneering work on a few isolates in elucidation of the Wood-Ljungdahl pathway. We are exploring homoacetogenesis in saline environments for two main reasons: i) application in microbial electrosynthesis (MES) reactors under saline conditions, where salinity decreases ohmic resistance and inhibits competing methanogens by salt stress; and ii) fundamental microbiological interest. Motivating questions include: Can we find and enrich for novel salt-tolerant homoacetogens from un- or underexplored environments? Where is homoacetogenesis most active in the depth/redox profile of saline soil and sediment? We collected samples from North Atlantic coastal marine sediment (10 m.b.s.l.) and from high-altitude (3921 m.a.s.l.) shallow salt lakes of the Argentinian Andes. Serum-flask enrichments of North Sea samples in a homoacetogen seawater medium (35 g/L salinity) using a headspace of H2/CO2 (80/20 vol%) showed an initial period of acetate production (1-2 g/L in two weeks) from the top 10 cm of sediment, followed by a switch to methanogenesis. Samples collected from a salt lake in Argentina ranged in salinity from ~10 to 90 g/L total salts. We therefore explored the correlation between environmental salinity and acetate-production capacity in culture media of 20, 60, and 100 g/L salts, also with a H2/CO2 headspace. Acetate concentrations in the 20-, 60-, and 100 g/L media reached 8500, 3000, and 73 mg/L, respectively, after 150 days. Furthermore, methane production was only observed in one of the 20 g/L incubations. Results from 16S rRNA amplicon sequencing, targeted RNA-based qPCR and flow cytometry of environmental samples and enriched cultures will also be presented. Finally, we have begun testing the performance of selected homoacetogenic enrichment cultures in cathode-driven MES reactors.