Methane formation across living organisms driven by ROS: new perspectives for understanding of biochemical methane formation and cycling on Earth
Authors: Keppler Frank , Ernst Leonard , Bischofs Ilka 
Published: March 2022

&lt;p&gt;Methane (CH&lt;sub&gt;4&lt;/sub&gt;) is the most abundant hydrocarbon in the atmosphere, largely originating from biogenic sources that recently have been linked to an increasing number of organisms living in both oxic and anoxic environments. Traditionally, biogenic CH&lt;sub&gt;4&lt;/sub&gt; has been regarded as the final product of the anoxic decomposition of organic matter by methanogenic &lt;em&gt;Archaea&lt;/em&gt;. However, plants, fungi, algae, lichens and cyanobacteria have recently been shown to produce CH&lt;sub&gt;4&lt;/sub&gt; in the presence of oxygen. While methanogens produce CH&lt;sub&gt;4 &lt;/sub&gt;enzymatically during anaerobic energy metabolism, the requirements and pathways for CH&lt;sub&gt;4 &lt;/sub&gt;production by &amp;#8220;non-methanogenic&amp;#8221; cells are poorly understood. Here, we present a CH&lt;sub&gt;4&lt;/sub&gt; formation mechanism that most likely occurs in all living organisms (Ernst et al. 2022). Firstly, we show results from two bacterial species (&lt;em&gt;Bacillus subtilis&lt;/em&gt; and &lt;em&gt;Escherichia coli&lt;/em&gt;) demonstrating that CH&lt;sub&gt;4&lt;/sub&gt; formation is triggered by free iron and reactive oxygen species (ROS), which are generated by metabolic activity and enhanced by oxidative stress. ROS-induced methyl radicals, derived from organic compounds containing sulfur- or nitrogen-bonded methyl groups, are key intermediates that ultimately lead to CH&lt;sub&gt;4&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;In a second step, we made numerous experiments and collected data from many other model organisms (over 30 species) from the three domains of life&lt;em&gt; &lt;/em&gt;(&lt;em&gt;Bacteria, Archaea&lt;/em&gt; and &lt;em&gt;Eukarya&lt;/em&gt;), including several human cell lines and a non-methanogenic archaeal species. All of the selected species clearly showed CH&lt;sub&gt;4&lt;/sub&gt; formation under sterile growth conditions. As the mechanism described for CH&lt;sub&gt;4&lt;/sub&gt; formation depends on several factors such as the availability of methylated precursor compounds, free iron, cellular stress factors and antioxidants, production rates can vary by several orders of magnitude. For terrestrial plants and cyanobateria, measured CH&lt;sub&gt;4 &lt;/sub&gt;emission rates have been reported to vary by almost four orders of magnitude. In both cases, rates were measured for many species and under varying environmental conditions and stressors, although the formation mechanism(s) were unknown. Our proposed ROS-driven pathway not only provides a mechanistic explanation for the observed CH&lt;sub&gt;4&lt;/sub&gt; emissions under oxic conditions but also for the large variability of emission rates observed for terrestrial plants, marine and freshwater algae, fungi, lichens and cyanobacteria, which have caused many controversial discussions since their publication. Furthermore, now it is very clear that any global upscaling will be highly challenging given the complex variables that control emissions from specific organisms.&lt;/p&gt;&lt;p&gt;In summary, the observed and experimental validated process of CH&lt;sub&gt;4&lt;/sub&gt; formation across all living organisms is a major step to better understand biological CH&lt;sub&gt;4&lt;/sub&gt; (in addition to the well-described archaeal methanogenesis) formation and cycling on Earth.&lt;/p&gt;&lt;p&gt;Reference:&lt;/p&gt;&lt;p&gt;Ernst, L., Steinfeld, B., Barayeu, U., Klintzsch, T., Kurth, M., Grimm, D., Dick, T.P., Rebelein, J.G., Bischofs, I.B., Keppler, F. (2022). ROS-driven methane formation across living organisms. &lt;em&gt;Nature&lt;/em&gt;,&lt;em&gt; &lt;/em&gt;in press.&lt;/p&gt;