Mangroves are important constituents of the coastal wetlands. They could store atmospheric CO₂ as organic matter, so-called “blue carbon”, inhabiting approximately 0.5% of the coast and contributing 10–15% to the global carbon storage. However, mangrove sediment carbon does not remain stored in perpetuity. Some of organic matter are transformed to CH₄ and returned to the atmosphere, which has the potential to partially offset blue carbon storage in mangrove sediments. Most pristine mangroves showed low CH₄ efflux rate, while mangroves with human disturbances showed significantly higher CH₄ efflux rate. Futian Mangrove Nature Reserve (FT) is located in the urban hinterland of Shenzhen Special Economic Zone, China (Figure 1). FT is a restricted area since establish of Futian Natural Reserve in 1980s. It is dominated by native, true mangrove flora and is one of the largest mangrove habitats along the southeastern coast of mainland China. It has been reported that methane emission rates in FT range from 242 μmol m⁻² day⁻¹ to 124 mmol m⁻² day⁻¹.

Methane is the second most important greenhouse gas after CO₂. Methanogenesis is conducted by methanogens that thrive in strictly anoxic habitats. Methanogens are considered to play important roles in the global carbon cycle and climate change. Methanogens from the phylum Euryarchaeota are currently classified into one class (Methanofastidiosa) and seven orders (Methanococcales, Methanopyrales, Methanobacteriales, Methanomicrobiales, Methanocellales, Methanosarcinales, and Methanomassiliicoccales).

How methane is produced in FT mangrove ecosystems. Previous studies using 16S rRNA gene and genes encoding the methyl-coenzyme M reductase alpha subunit (mcrA) demonstrated that multiple methanogens are widely spread across mangrove sediments. However, the metabolic activity and relative contributions to methane production of diverse methanogens in mangroves remain unclear.

We combined metagenomic and metatranscriptomic analyses to investigate the metabolic activity and relative contributions of diverse methanogens to methane production in a vertical sediment profile in mangrove ecosystem. We found that Methanomassiliicoccales, Methanofastidiosa, Methanosarcinales and Methanomicrobiales were the four dominant methanogens in FT mangroves. Methanomicrobiales were the most abundant methanogens and Methanomassiliicoccales were the most active methanogens in the analyzed sediment profile. Methanomicrobiales are hydrogenotrophs that utilize H₂ and CO₂ to produce methane. Methanomicrobiales could consume H₂ and cooperate with syntrophic microbes to degrade short-chain fatty acids. Methanomassiliicoccales are methylotrophic methanogens using H₂ reduce methyl-compounds for methane production. Methyl-compounds such as trimethylamine (TMA) contribute 35-90% of the methane production in coastal sediments, which could explain why methylotrophic methanogens play an important role in methane production in mangroves. The presented findings imply that Methanomicrobiales and Methanomassiliicoccales play a vital role in methane production in mangroves (Figure 2).

To see the full story, check out the publications in Microbiome. Paper link: https://microbiomejournal.biomedcentral.com/articles/10.1186/s40168-020-00876-z