Interactions between Cyanobacteria and Methane Processing Microbes Mitigate Methane Emissions from Rice Soils

Cyanobacteria play a relevant role in rice soils due to their contribution to soil fertility through nitrogen (N₂) fixation and as a promising strategy to mitigate methane (CH₄) emissions from these systems. However, information is still limited regarding the mechanisms of cyanobacterial modulation of CH₄ cycling in rice soils. Here, we focused on the response of methane cycling microbial communities to inoculation with cyanobacteria in rice soils. We performed a microcosm study comprising rice soil inoculated with either of two cyanobacterial isolates (<i>Calothrix</i> sp. and <i>Nostoc</i> sp.) obtained from a rice paddy. Our results demonstrate that cyanobacterial inoculation reduced CH₄ emissions by 20 times. Yet, the effect on CH₄ cycling microbes differed for the cyanobacterial strains. Type Ia methanotrophs were stimulated by <i>Calothrix</i> sp. in the surface layer, while <i>Nostoc</i> sp. had the opposite effect. The overall <i>pmoA</i> transcripts of Type Ib methanotrophs were stimulated by <i>Nostoc</i>. Methanogens were not affected in the surface layer, while their abundance was reduced in the sub surface layer by the presence of <i>Nostoc</i> sp. Our results indicate that mitigation of methane emission from rice soils based on cyanobacterial inoculants depends on the proper pairing of cyanobacteria–methanotrophs and their respective traits.