Biology subjects

Philips, J.

Publications and source records attributed to Philips, J..

3 recordsLinked to original sources

bioRxiv2025

Variations in H2 thresholds and growth yields among hydrogenotrophic methanogens relate to different energy conservation strategies

Hydrogenotrophic methanogens are of high environmental and biotechnological importance, converting CO2 with H2 into CH4. Despite their common metabolism, variations in the energy metabolism among these methanogens exist, likely affecting their H2 thresholds and growth yields. However, a systematic comparison of these traits for a wide range of hydrogenotrophic methanogens has been lacking. Here, we measured the H2 thresholds and growth yields of nine different hydrogenotrophic methanogens. The H2 threshold, i.e. the H2 partial pressure at which H2 consumption halts, ranged over two orders of magnitude from 1.0 {+/-} 0.5 Pa for Methanobrevibacter arboriphilus to 120 {+/-} 10 Pa for Methanosarcina mazei. Growth yields in our experimental conditions ranged from 0.51 {+/-} 0.28 gDCWx(mol CH4)-1 for Methanococcus maripaludis to 5.28 {+/-} 1.25 gDCWx(mol CH4)-1 for Methanosarcina mazei. The ATP gains, estimated from both H2 thresholds and growth yields, correlated reasonably well, confirming that these variations are due to differences in energy conservation strategies. Our results strongly differentiated the two previously proposed groups of hydrogenotrophic methanogens: methanogens with cytochromes had a high H2 threshold ([≥] 21 Pa) and high growth yield (> 4.0 gDCWx(mol CH4)-1), whereas methanogens without cytochromes had lower H2 threshold ([≤] 7 Pa) and low growth yield (< 1.7 gDCWx(mol CH4)-1). Moreover, our H2 thresholds indicated that additional variations in energy metabolism exist within both groups. Overall, this study found strong variations between hydrogenotrophic methanogens, which are important to understand their environmental prevalence and biotechnological applicability. ImportanceHydrogenotrophic methanogens play key roles in natural ecosystems and biotechnological processes. Even though all hydrogenotrophic methanogens convert CO2 with H2 into CH4, they can differ in their H2 threshold and growth yield, due to variations in their energy metabolism. Here, we found that H2 thresholds of hydrogenotrophic methanogens range over two orders of magnitude, while a ten-fold difference was observed in their growth yields. These strong variations in H2 thresholds and growth yields demonstrate that hydrogenotrophic methanogens are confronted with a trade-off between the capacity to grow at low H2 levels (low H2 threshold) and efficient growth (high growth yield). Curiously, the observed H2 thresholds also correlate with the electroactivity of different hydrogenotrophic methanogens. Overall, by reporting H2 thresholds and growth yields for a wide range of hydrogenotrophic methanogens, this work provides new insights into the bioenergetic diversity of these microbes, as well as their environmental prevalence and biotechnological applicability.

microbiology↗

bioRxiv2025

Let-them-stick: Increasing biofilm formation by the acetogen Sporomusa ovata through adaptive laboratory evolution

Acetogenic bacteria are attractive biocatalysts for the conversion of CO2 with H2 into acetate, as in gas fermentation. Gas fermentation reactors may benefit from biofilm formation, but cell attachment by acetogens is often limited. This study indeed found that the acetogen Sporomusa ovata 2663 was mainly planktonic and aimed to increase its biofilm formation through adaptive laboratory evolution. The adaptation strategy consisted of growing S. ovata on plastic carriers in bottles with a H2:CO2 headspace and transferring few carriers to bottles with fresh carriers over eight serial transfers. This procedure resulted in the evolved S. ovata 2663-BF, which had a consistent increased propensity to attach. In heterotrophic growth conditions, four times more cells attached to the bottom of well plates in comparison to the wild type. Moreover, twice as many cells adhered to carriers when grown on H2:CO2. This improved attachment, however, did not lead to higher acetate production rates in simple trickle bed reactors, as the used experimental setup likely stimulated planktonic growth due to a low trickling frequency. Only after some medium replacements to remove planktonic cells, higher gas consumption rates were recorded for the evolved culture. Interestingly, the evolved S. ovata had a relevant point mutation in the gene galU, encoding UDP-glucose pyrophosphorylase, an enzyme involved in the synthesis of extracellular polysaccharides. Overall, this study demonstrates that cell attachment by S. ovata was increased through adaptive laboratory evolution, offering the prospect of investigating the importance of biofilm formation in biofilm-based gas fermentation reactors. IMPORTANCEAcetogenic bacteria are attractive biocatalysts for the conversion of CO2 with H2 into acetate, as in gas fermentation and microbial electrosynthesis. Both biotechnologies could benefit from biofilm formation, since biofilm formation retains the catalytic activity inside continuously operated reactors. In addition, biofilm-based gas fermentation reactor systems are advantageous to improve the gas to liquid mass transfer of H2 and CO2. Moreover, microbial electrosynthesis benefits from biofilms on cathodes to consume H2 as soon as it gets generated. Acetogenic bacteria are however often found to only form thin or sparse biofilms. In the current study, we demonstrated that adaptive laboratory evolution is a useful strategy to increase the biofilm formation capabilities of the acetogen, Sporomusa ovata. The obtained biofilm improved S. ovata is of interest to deepen our fundamental understanding of biofilm formation by acetogens, as well as to assess the importance of biofilm formation for the performance of biofilm-based bioreactors.

microbiology↗

bioRxiv2024

H2 consumption by acetogenic bacteria follows first-order kinetics

Acetogenic bacteria play an important role in various biotechnological processes, because of their chemolithoautotrophic metabolism converting carbon dioxide with molecular hydrogen (H2) as electron donor into acetate. As the main factor limiting acetogenesis is often H2, insights into the H2 consumption kinetics of acetogens is required to assess their potential in biotechnological processes. In this study, initial H2 consumption rates at a range of different initial H2 concentrations were measured for three different acetogens. Interestingly, for all three strains, H2 consumption was found to follow first-order kinetics, i.e. the H2 consumption rate increased linearly with the dissolved H2 concentration, up to almost saturated H2 levels (600 {micro}M). This is in contrast with Monod kinetics and low half-saturation concentrations, which have commonly been assumed for acetogens. The obtained biomass specific first-order rate coefficients (k1X) were further validated by comparison with values obtained by fitting first-order kinetics on previous time-course experimental results. The latter method was also used to determine the k1X value of five additional acetogens strains. Biomass specific first-order rate coefficients were found to vary up to six-fold, with the highest k1X for Acetobacterium wieringae and the lowest for Sporomusa sphaeroides. Overall, our results demonstrate the importance of the dissolved H2 concentration to understand the rate of acetogenesis in biotechnological systems.

microbiology↗