Abstract
Rivers connect the carbon cycle in land with that in aquatic ecosystems by transporting and transforming terrestrial organic matter (TeOM). The Amazon River receives huge loads of TeOM from the surrounding rainforest, promoting a substantial microbial heterotrophic activity and consequently, CO2 outgassing. In the Amazon River, microbes degrade up to 55% of the lignin present in the TeOM. Yet, the main microbial genomes involved in TeOM degradation were unknown. Here, we characterize 51 population genomes (PGs) representing some of the most abundant microbes in the Amazon River deriving from 106 metagenomes. The 51 reconstructed PGs are among the most abundant microbes in the Amazon River, and 53% of them are not able to degrade TeOM. Among the PGs capable of degrading TeOM, 20% were exclusively cellulolytic, while the others could also oxidize lignin. The transport and consumption of lignin oxidation byproducts seemed to be decoupled from the oxidation process, being apparently performed by different groups of microorganisms. By connecting the genomic features of abundant microbes in the Amazon River with the degradation machinery of TeOM, we suggest that a complex microbial consortium could explain the quick turnover of TeOM previously observed in this ecosystem.
Highlights
Rivers connect land and ocean ecosystems, carrying about 1.9 Pg of organic carbon per year and performing carbon transformations in their course (Cole et al, 2007)
Degradation, as isolates are absent, we do acknowledge the fact that some of those lacking known genes related to terrestrial organic matter (TeOM) degradation may perform it (Fig. 3)
Among the protein families involved in TeOM degradation, laccases seemed to be present as single copy genes in almost all genomes, except in Bacteroidetes
Summary
Rivers connect land and ocean ecosystems, carrying about 1.9 Pg of organic carbon per year and performing carbon transformations in their course (Cole et al, 2007). Incubation experiments with sets of organic matter (Ward et al, 2016) and measurements of microbial respiration rates (Farjalla, 2014) suggest that confluence river sections are hotspots of bacterial production with CO2 levels higher than in other regions It indicates that the turbulence, diversity of substrates and microbes contribute to a priming effect in the Amazon River waters contributing to accelerate the degradation of TeOM. The AMnrGC revealed the main biochemical machinery used by microbes to degrade plant-derived organic matter, which consisted mainly in glycosyl-hydrolases and laccases Based on this catalogue, we proposed that a microbial consortium could work for the Amazon River (Santos-Júnior et al, 2020), where two interacting populations (a ligninolytic and a cellulolytic one) work synergistically for the TeOM degradation. /cellulose degradation decoupled? Is the biochemical machinery of lignin-oxidation coupled to the one used for processing lignin-derived aromatic monomers and dimers?
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