Arsenic is present at nanomolar levels throughout the ocean, and microbes assimilate this potentially toxic element due to its similarity to inorganic phosphorus. Although dissolved arsenic has been a focus of several oceanographic studies, the size and chemical character of the particulate arsenic pool is poorly understood. We measured particulate arsenic in five samples from the open ocean and determined the contribution of arsenic-containing lipids to this pool. Here we show that the accumulation of arsenic into lipids is a widespread phenomenon in the surface ocean. Particulate arsenic concentrations were 15 to 42 pmol L⁻¹ with 7–20% of the particulate arsenic pool within arsenolipids. We found that arsenosugar phospholipids dominated the arsenolipid pools in our samples with a minor component of arsenohydrocarbons and other unidentified lipids. A significant portion of the arsenosugar phospholipids (up to 35%) were present as previously undescribed mixed acyl ether lipids, suggesting a bacterial source.

Surface samples for particulate arsenic speciation and quantification were collected from five locations noted in Table 1 (more details in Table S1). These samples include two from oligotrophic subtropical regions: one from the North Pacific (ALOHA: A Long-Term Oligotrophic Habitat Assessment) and one from the North Atlantic (BATS: Bermuda Atlantic Time Series). Three additional samples were collected from the equatorial upwelling influenced Eastern Tropical North Pacific (ETNP), including two offshore samples (ETNP-PS1 and ETNP-PS2) and one coastal sample (ETNP-PS3). We took quantitative subsamples for total particulate carbon and total particulate arsenic. We extracted samples for arsenolipids which we analyzed by liquid chromatography – inductively coupled plasma mass spectrometry (LC-ICP-MS, for quantification) and liquid chromatography – high resolution electrospray ionization mass spectrometry (LC-HR-ESI-MS, for identification of individual lipids). Details for each step are in Supplemental Methods of the manuscript and preprint on bioRxiv (doi 10.1101/2021.03.22.436501).