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

Hydrogenotrophic methanogens are of high environmental and biotechnological relevance, as they convert CO2 with H2 into CH4. Despite their common metabolism, variations among these methanogens likely exist. This study therefore determined 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 ranged from 0.51 +/- 0.28 gDCW / mol CH4) for Methanococcus maripaludis to 5.28 +/- 1.25 gDCW / mol CH4. In addition, ATP gains, estimated from both the H2 threshold and growth yield, correlated reasonably well, suggesting that these variations are due to differences in energy conservation strategies. Our results fitted with the classification previously proposed by Thauer, dividing hydrogenotrophic methanogens into two distinct groups: methanogens using cytochromes for energy conservation had a high H2 threshold (> 20 Pa) and high growth yield (> 4.0 gDCW / (mol CH4), whereas all methanogens without cytochromes had a low H2 threshold (< 8 Pa) and low growth yield (< 1.7 gDCW / (mol CH4). Moreover, our H2 thresholds showed significant additional variations within both groups. Overall, this study found strong variations between hydrogenotrophic methanogens, which are important to understand their environmental prevalence and biotechnological applicability.