Selective uptake of rare earth elements in marine systems as an indication of and control on aerobic bacterial methanotrophy

Unicellular methanotrophs are a widespread, polyphyletic group of organisms that oxidize methane to assimilate carbon and obtain energy. Methanotrophs are currently a focus of attention of astrobiologists given their prevalence in terrestrial environmental niches, like geothermal hot springs and hydrothermal vents, that are homologs for proposed extraterrestrial targets. In particular, recent research has found that light rare earth elements are required for the catalytic activity of the common xoxF isoform of methanol dehydrogenase, an enzyme in the aerobic methanotrophy pathway whose main isoform, mxa-MeDH, in contrast uses a calcium ion for catalysis. Although rare earth elements exist in picomolar concentrations in the ocean, the gene for xoxF is maintained in marine methanotrophs, pointing to a possible terrestrial origin for aerobic methanotrophy. In this work, data is presented from a multi-season time series from autumn 2018 and summer 2019 in the oligotrophic Sargasso Sea. Here we present water column dissolved rare earth element, nitrous oxide concentrations, and the prevalence of molecular markers for aerobic methanotrophy in the mid-latitude North Atlantic subtropical gyre. This information is used to characterize the environmental conditions that regulate the distribution of marine methanotrophs and the use of each isoform of methanol dehydrogenase. Furthermore, we aim to determine if aerobic methanotrophs generate a reproducible and identifiable signal through the selective uptake of light rare earth elements over heavier REEs that could be recorded in the geologic record indicating past aerobic methanotrophy. Such a signal could be sensed remotely using existing mass spectrometry techniques.