CRISPR-enabled control of gene expression sets the isotopic composition of microbial methane

The stable isotopic composition of biogenic methane varies substantially in the environment and is routinely used to fingerprint its source. However, the underlying cause of this variation is debated. Here, we experimentally manipulate the growth rate of the model methanogen, Methanosarcina acetivorans , using CRISPR mutagenesis to generate a tunable version of the key and final enzyme in methanogenesis, methyl-coenzyme M reductase (MCR). We demonstrate that the carbon and hydrogen isotopic composition of methane change as a function of MCR expression and growth rate. Using an isotope enabled metabolic model we show that these changes stem from a substrate-independent increase in reversibility of methanogenic enzymes. Overall, these data provide a novel framework for calibrating growth coupled changes in the isotopic composition of biogenic methane.