Selective uptake of rare earth elements in marine systems as an indicator 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 of interest to astrobiologists given their prevalence in terrestrial environmental niches, like geothermal hot springs and hydrothermal vents, that are homologs for proposed extraterrestrial targets. Recent research demonstrates 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. Here we present a multi-season time series of rare earth element concentrations from autumn 2018 in the oligotrophic Sargasso Sea, and an initial analysis of molecular markers for aerobic methanotrophy in summer 2019. We observe a repeated depletion of light rare earth elements relative to heavy rare earth elements in the upper water column, between approximately 200 m and 500 m. This feature was collocated with observations of microbial growth on media with methanol as a sole carbon source, and detection of the xoxF5 gene. 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.