Combining climate mitigation and eutrophication combatting, rapid onset of Fe-AOM in Fe(II)Cl2 treated sediments

Aquatic ecosystems are the largest natural contributors to total global methane (CH4) emissions. Eutrophication, a global threat to aquatic ecosystems, is known to significantly enhance CH4 emissions. However, little is known about how measures mitigating eutrophication affect CH4 emission, and the microbiome responsible for CH4 production and breakdown. Here we show the effect of the P-binding bioremediation agent Fe(II)Cl2 on microbial CH4 cycling, in a field experiment and batch incubations. Fe(II)Cl2 application significantly lowered CH4 emissions in the field, and also led to lower net CH4-production potential, while sediment aerobic CH4-oxidation potentials remained similar. However, Fe(II)Cl2 application stimulated iron-dependent anaerobic methane oxidation (Fe-AOM), and in Fe(II)Cl2 treated sediment, a substantial part of the reduction of net CH4-production potential could be attributed to Fe-AOM activity. This is supported by 16S rRNA sequencing data, which shows the presence of anaerobic methane-oxidizing archaea and iron-oxidizing bacteria in both tested sediments. In conclusion, we find strong evidence for a rapid onset of Fe-AOM in FeIICl2-treated sediments, which contributes to lowering methane emissions from eutrophic pond sediments, without substantially changing the relative abundance of the native microbial community in systems where iron-oxidizing communities are already established.