Abstract
The Arctic Ocean currently receives a large supply of global river discharge and terrestrial dissolved organic matter. Moreover, an increase in freshwater runoff and riverine transport of organic matter to the Arctic Ocean is a predicted consequence of thawing permafrost and increased precipitation. The fate of the terrestrial humic-rich organic material and its impact on the marine carbon cycle are largely unknown. Here, a metagenomic survey of the Canada Basin in the Western Arctic Ocean showed that pelagic Chloroflexi from the Arctic Ocean are replete with aromatic compound degradation genes, acquired in part by lateral transfer from terrestrial bacteria. Our results imply marine Chloroflexi have the capacity to use terrestrial organic matter and that their role in the carbon cycle may increase with the changing hydrological cycle.
Highlights
The Arctic Ocean currently receives a large supply of global river discharge and terrestrial dissolved organic matter
A metagenomic co-assembly was generated from samples originating from the surface layer (5–7 m), the subsurface chlorophyll maximum (25–79 m) and a layer corresponding to the terrestrially-derived DOM fluorescence (FDOM) maximum previously described within the cold Canada Basin halocline comprised of Pacific-origin waters (177–213 m)[20]
Fragment recruitment of 21 TARA Ocean metagenomic datasets spanning epipelagic to mesopelagic waters at 7 locations and 4 separate bathypelagic metagenomes indicated that the Canada Basin Chloroflexi metagenome-assembled genomes (MAGs) were not widely distributed in the oceans (Fig. 2c, Supplementary Data 1)
Summary
The Arctic Ocean currently receives a large supply of global river discharge and terrestrial dissolved organic matter. Our results imply marine Chloroflexi have the capacity to use terrestrial organic matter and that their role in the carbon cycle may increase with the changing hydrological cycle. With thawing permafrost and increased precipitation occurring across the Arctic[4], increases in freshwater runoff and riverine transport of organic matter to the Arctic Ocean are predicted, which will increase tDOM fluxes and loadings[5,6]. The additional tDOM may represent new carbon and energy sources for the Arctic Ocean microbial community and contribute to increased respiration, which would result in the Arctic being a source of dissolved inorganic carbon to the ocean. Chloroflexi, including those in the SAR202 clade, are present in the upper layers of the Arctic Ocean[19], leading to the hypothesis that recalcitrant organic compounds present in high Arctic tDOM could be utilized by this group
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