Shiller, and 4 co-authors, reported in 2017 that rare-earth elements have generally not been thought to have a biological role. However, recent work has demonstrated that the light REEs (LREEs: La, Ce, Pr, and Nd) are essential for at least some methanotrophs, being co-factors in the XoxF type of methanol dehydrogenase (MDH). The authors demonstrated that dissolved LREEs were signifcantly removed in a submerged plume of methane-rich water during the Deepwater Horizon (DWH) well blowout. Furthermore, incubation experiments conducted with naturally methane-enriched waters from hydrocarbon seeps in the vicinity of the DWH wellhead also showed LREE removal concurrent with methane consumption.
Metagenomic sequencing of incubation samples revealed that LREE-containing MDHs were present. The authors' feld and laboratory observations provide further insight into the biochemical pathways of methanotrophy during the DWH blowout. Additionally, their results are the frst observations of direct biological alteration of REE distributions in oceanic systems. In view of the ubiquity of LREE-containing MDHs in oceanic systems, their results suggest that biological uptake of LREEs is an overlooked aspect of the oceanic geochemistry of this group of elements previously thought to be biologically inactive and an unresolved factor in the fux of methane, a potent greenhouse gas, from the ocean.
The authors commented that other localized regions of high methane input include pore waters in continental margin sediments as well as waters near cold seeps. Indeed, Bayon et al. in 2011 observed slight depletion of dissolved Nd and enrichment in particulate Nd in methane-enriched plumes above cold seeps on the Niger Delta margin. They suggested this could be the result of REE scavenging on Fe/Mn oxides, but our results suggest biological uptake as an alternative explanation. Whether methanotrophy is of importance to REE distributions in low-methane open ocean waters is less clear. On the one hand, reported rates of methane oxidation in the open ocean are quite low (e.g., ref. 48) relative to both the afermath of the DWH blowout and our incubation experiments.
However, a consistent LREE depletion at the depth of the deep chlorophyll maximum (DCM) has been observed in sections across both the North and South Atlantic. Given that the DCM is ofen the locus of a methane maximum (e.g., ref. 51), investigation of the possible role of methanotrophy in overall oceanic REE cycling seems warranted. Furthermore, other biological stimulatory efects of REEs have previously been noted; thus, methanotrophy could well be only part of the story of the biogeochemistry of lanthanides.
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