Directional Selection of Microbial Community Reduces Propionate Accumulation in Glycerol and Glucose Anaerobic Bioconversion Under Elevated pCO2

Abstract

Volatile fatty acid accumulation is a sign of digester perturbation. Previous work showed the thermodynamic limitations of hydrogen and CO2 in syntrophic propionate oxidation under elevated partial pressure of CO2 (pCO2). Here we study the effect of directional selection under increasing substrate load as a strategy to restructure the microbial community and induce cross-protection mechanisms to improve glucose and glycerol conversion performance under elevated pCO2. After an adaptive laboratory evolution (ALE) process, viable cell density increased and predominant microbial groups were modified: an increase in Methanosaeta and syntrophic propionate oxidizing bacteria (SPOB) associated with the Smithella genus was found with glycerol as the substrate. A modest increase in SPOB along with a shift in the predominance of Methanobacterium toward Methanosaeta was observed with glucose as the substrate. The evolved inoculum showed affected diversity within archaeal spp. under 5 bar initial pCO2; however, higher CH4 yield resulted from enhanced propionate conversion linked to the community shifts and biomass adaptation during the ALE process. Moreover, the evolved inoculum attained increased cell viability with glucose and a marginal decrease with glycerol as the substrate. Results showed differences in terms of carbon flux distribution using the evolved inoculum under elevated pCO2: glucose conversion resulted in a higher cell density and viability, whereas glycerol conversion led to higher propionate production whose enabled conversion reflected in increased CH4 yield. Our results highlight that limited propionate conversion at elevated pCO2 resulted from decreased cell viability and low abundance of syntrophic partners. This limitation can be mitigated by promoting alternative and more resilient SPOB and building up biomass adaptation to environmental conditions via directional selection of microbial community.

Steering the product spectrum in high-pressure anaerobic processes: CO2 partial pressure as a novel tool in biorefinery concepts

“…Higher CH 4 production occurred in GG_N2 than in GG_CO2 during phases II-A and II-B (Fig. 2 A, B), which aligns with results previously presented for glucose and glycerol conversion with non-adapted inoculum at 5 bar pN 2 /pCO 2 [ 55 ]. Our previous work [ 51 ] reported a reduction (53 to 85%) in the methane yields when reactor headspace was pressurized with CO 2 rather than N 2 .…”

“…However, an increasing S/X ratio may impair bioconversions in non-adapted biomass, due to kinetic limitations in the production and utilization of intermediates [ 35 ]. Substrate specificity and biomass adaptation at increasing substrate concentrations has been pivotal in selecting a microbial community that is more resilient to the detrimental effects of elevated pCO 2 [ 36 ] and, most likely, also fluctuations in the S/X ratio.…”

Pre‐incubation conditions determine the fermentation pattern and microbial community structure in fermenters at mild hydrostatic pressure

“…Rombouts et al (2020) andHoelzle et al (2021) have proposed that complex medium and high glucose concentrations favor lactate production over butyrate, propionate, and acetate, which are typically observed in mixed culture conversions at low substrate concentration and even occur at elevated operational pressure(Ceron-Chafla et al, 2021).We did not observe significant differences in the product spectrum of the different incubations due to MHP. However, otherF I G U R E 7 Graphicalsummary of foreseen effects of incubation temperature (T) (left) and T + mild hydrostatic pressure (MHP; right) on kinetic and bioenergetic parameters.…”

“…High substrate concentration, high biomass load, complex medium, and sequencing batch incubations can be considered the process parameters with the most substantial influence on the product spectrum. Rombouts et al (2020) and Hoelzle et al (2021) have proposed that complex medium and high glucose concentrations favor lactate production over butyrate, propionate, and acetate, which are typically observed in mixed culture conversions at low substrate concentration and even occur at elevated operational pressure (Ceron‐Chafla et al, 2021).…”

“…Due to reactor availability, MHP experiments at different temperatures were carried out in a sequential incubation. The increment of hydrostatic pressure to 5 and 8 MPa, to achieve CO 2 partial pressures comparable to our previous work (Ceron‐Chafla et al, 2020, 2021), was done by pumping distilled water inside the reactors using a manual hydraulic pump. One syringe from each replicate and negative control of the enrichment was incubated in parallel under atmospheric conditions (atmospheric controls; ACs) at a pressure of 0.1 MPa (Figure 1).…”