Wetlands constitute the main natural source of methane on Earth due to their high content of natural organic matter (NOM), but key drivers, such as electron acceptors, supporting methanotrophic activities in these habitats are poorly understood. We performed anoxic incubations using freshly collected sediment, along with water samples harvested from a tropical wetland, amended with 13 C-methane (0.67 atm) to test the capacity of its microbial community to perform anaerobic oxidation of methane (AOM) linked to the reduction of the humic fraction of its NOM. Collected evidence demonstrates that electron-accepting functional groups (e.g., quinones) present in NOM fueled AOM by serving as a terminal electron acceptor. Indeed, while sulfate reduction was the predominant process, accounting for up to 42.5% of the AOM activities, the microbial reduction of NOM concomitantly occurred. Furthermore, enrichment of wetland sediment with external NOM provided a complementary electron-accepting capacity, of which reduction accounted for ϳ100 nmol 13 CH 4 oxidized · cm Ϫ3 · day Ϫ1 . Spectroscopic evidence showed that quinone moieties were heterogeneously distributed in the wetland sediment, and their reduction occurred during the course of AOM. Moreover, an enrichment derived from wetland sediments performing AOM linked to NOM reduction stoichiometrically oxidized methane coupled to the reduction of the humic analogue anthraquinone-2,6-disulfonate. Microbial populations potentially involved in AOM coupled to microbial reduction of NOM were dominated by divergent biota from putative AOMassociated archaea. We estimate that this microbial process potentially contributes to the suppression of up to 114 teragrams (Tg) of CH 4 · year Ϫ1 in coastal wetlands and more than 1,300 Tg · year Ϫ1 , considering the global wetland area.
IMPORTANCEThe identification of key processes governing methane emissions from natural systems is of major importance considering the global warming effects triggered by this greenhouse gas. Anaerobic oxidation of methane (AOM) coupled to the microbial reduction of distinct electron acceptors plays a pivotal role in mitigating methane emissions from ecosystems. Given their high organic content, wetlands constitute the largest natural source of atmospheric methane. Nevertheless, processes controlling methane emissions in these environments are poorly understood. Here, we provide tracer analysis with 13 CH 4 and spectroscopic evidence revealing that AOM linked to the microbial reduction of redox functional groups in natural organic matter (NOM) prevails in a tropical wetland. We suggest that microbial reduction of NOM may largely contribute to the suppression of methane emissions from
In Azotobacter vinelandii the two-component GacS/GacA system is required for synthesis of polyhydroxybutyrate (PHB) and of the exopolysaccharide alginate. The RsmA protein was shown to interact with the alginate biosynthetic algD mRNA, acting as a translational repressor, and GacA was found to activate transcription of the rsmZ1 and rsmZ2 genes that encode small RNAs interacting with RsmA to counteract its repressor activity. The phbBAC operon encodes the enzymes of PHB synthesis and is activated by the transcriptional regulator PhbR. This study shows that GacA is required for transcription of one rsmY and seven rsmZ1-rsmZ7 genes present in the A. vinelandii genome, and that inactivation of rsmA results in increased PHB production. Transcriptional and translational phbR-gusA gene fusions were used to show that the gacA mutation negatively affected the expression of the phbR gene at the translational level. We also demonstrated an in vitro interaction of RsmA with RNAs corresponding to phbB and phbR mRNA leaders, and showed that the stability of phbR and phbB mRNAs is increased in the rsmA mutant. Taken together these results indicate that in A. vinelandii, RsmA post-transcriptionally represses the expression of PhbR.
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