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Single‐celled microbial eukaryotes inhabit deep‐sea hydrothermal vent environments and play critical ecological roles in the vent‐associated microbial food web. 18S rRNA amplicon sequencing of diffuse venting fluids from four geographically‐ and geochemically‐distinct hydrothermal vent fields was applied to investigate community diversity patterns among protistan assemblages. The four vent fields include Axial Seamount at the Juan de Fuca Ridge, Sea Cliff and Apollo at the Gorda Ridge, all in the NE Pacific Ocean, and Piccard and Von Damm at the Mid‐Cayman Rise in the Caribbean Sea. We describe species diversity patterns with respect to hydrothermal vent field and sample type, identify putative vent endemic microbial eukaryotes, and test how vent fluid geochemistry may influence microbial community diversity. At a semi‐global scale, microbial eukaryotic communities at deep‐sea vents were composed of similar proportions of dinoflagellates, ciliates, Rhizaria, and stramenopiles. Individual vent fields supported distinct and highly diverse assemblages of protists that included potentially endemic or novel vent‐associated strains. These findings represent a census of deep‐sea hydrothermal vent protistan communities. Protistan diversity, which is shaped by the hydrothermal vent environment at a local scale, ultimately influences the vent‐associated microbial food web and the broader deep‐sea carbon cycle.

 

The transport and transformation of carbon in subseafloor environments is a significant component of past, present, and future global fluxes. Seawater’s dissolved organic matter (DOM) enters the subseafloor and undergoes complex reactions including microbial processing, interactions with the rock matrix, and thermal restructuring and remineralization to carbon dioxide. Large shifts in concentrations, isotopic compositions, and molecular abundances provide a rich source of information about the environments through which fluids have circulated. Broad patterns linking geological settings to the fate of organic molecules can now be drawn, including the wide-scale removal of seawater DOM in ridge-flank systems, and large additions of abiotically synthesized compounds into fluids that interact with mantle rocks. Outstanding questions remain concerning the role of hydrothermal circulation as a source of refractory organic matter and its impact on the isotopic signature of deep oceanic DOM. 

In January – February 2020, RV Atlantis cruise AT42-22 collected water, volatile, and fluid samples with ROV Jason from hydrothermal vent fields on the mid-Cayman rise. The expedition carried out 4 dives at the Von Damm field and 5 at the Piccard field. The first file is the sampling logs and fluid geochemistry from the Hydrothermal Organic Geochemistry (HOG) sampler. It includes sampling locations, depths, heading, volumes, the highest temperature recorded during sampling, the average fluid temperature recorded during sampling, and pH.  The second file is the measured geochemistry of the fluids, including concentrations of hydrogen sulfide, dissolved inorganic carbon, formate, phosphate, nitrate, nitrite, ammonia, and the stable isotope composition (d13C) of dissolved inorganic carbon.