Assessing environmental gradients in relation to dark CO₂ fixation in estuarine wetland microbiomes

The rising atmospheric concentration of CO₂ is a major concern to society due to its global warming potential. In soils, CO₂-fixing microorganisms are preventing some of the CO₂ from entering the atmosphere. Yet, the controls of dark CO₂ fixation are rarely studied in situ. Here, we examined the gene and transcript abundance of key genes involved in microbial CO₂ fixation along major environmental gradients within estuarine wetlands. A combined multi-omics approach incorporating metabarcoding, deep metagenomic, and metatranscriptomic analyses confirmed that wetland microbiota harbor four out of seven known CO₂ fixation pathways, namely, the Calvin cycle, reverse tricarboxylic acid cycle, Wood-Ljungdahl pathway, and reverse glycine pathway. These pathways are transcribed at high frequencies along several environmental gradients, albeit at different levels depending on the environmental niche. Notably, the transcription of the key genes for the reverse tricarboxylic acid cycle was associated with high nitrate concentration, while the transcription of key genes for the Wood-Ljungdahl pathway was favored by reducing, O₂-poor conditions. The transcript abundance of the Calvin cycle was favored by niches high in organic matter. Taxonomic assignment of transcripts implied that dark CO₂ fixation was mainly linked to a few bacterial phyla, namely, Desulfobacterota, Methylomirabilota, Nitrospirota, Chloroflexota, and Pseudomonadota.

The increasing concentration of atmospheric CO₂ has been identified as the primary driver of climate change and poses a major threat to human society. This work explores the mostly overlooked potential of light-independent CO₂ fixation by soil microbes (a.k.a. dark CO₂ fixation) in climate change mitigation efforts. Applying a combination of molecular microbial tools, our research provides new insights into the ecological niches where CO₂-fixing pathways are most active. By identifying how environmental factors, like oxygen, salinity and organic matter availability, influence these pathways in an estuarine wetland environment, potential strategies for enhancing natural carbon sinks can be developed. The importance of our research is in advancing the understanding of microbial CO₂ fixation and its potential role in the global climate system.