Award: OCE-1235169

This project generated new insights into the composition and activity of microbial communities in the deep-sea and deep-sea particles they are associated with. The abundance of a diverse group of eukaryotic microbes was determined in the North Atlantic through the entire water column to a maximum depth of 7000 meters, ranging from the subtropics, to the north-temperate, and Arctic oceans. Eukaryotes were enumerated by applying traditional fluorescent nucleic-acid dyes and fluorescence in-situ hybridization with newly developed probes that allowed us to distinguish specific target groups (i.e., fungi, labyrinthulomycetes, kinetoplastids, alveolates, marine stramenopiles, and diplonemids). On average, eukaryotic microbial abundances decreased with depth at a greater rate than viruses and prokaryotes (bacteria and archaea). Most of the remaining deep-sea eukaryotic microbes were composed of heterotrophic flagellates that feed on prokaryotes. While our laboratory experiments indicated that average concentrations of bacteria in the deep sea were too low and flagellates would starve, deep-sea marine snow particles were greatly populated with microbial prey. In fact, eukaryotic microbes comprised a large fraction of the biomass on marine snow particles. Biomass on deep-sea marine snow was dominated by fungi and labyrinthulomycetes. Both groups are known to degrade refractory carbon and thus their abundance on deep-sea marine snow indicates the presence of hard to digest organic material. We concluded that that the contrast between marine snow and the ambient water in terms of biological activity and species composition is even greater in the deep sea than in shallow water. Indeed, marine snow particles appear to be oases of life in a desert of overall extremely low biological activity. Using a holographic microscope designed and built in our laboratory, and capable to being deployed to depths of up to 6000 m, we found that particles in a size range from approximately 400 micrometers to several millimeters are disproportionally enriched in bathypelagic and abyssopelagic environments (i.e., 2000 -6000 m). Detailed optical analysis of these particles, revealed that the morphology is different from those particles that make the bulk of sediment trap fluxes. A large percentage of particles in this size range have very little apparent ballast and consist of large amounts of transparent exopolymer material, a substance that is either neutrally or even slightly positive buoyant. Our conclusion that a large fraction of deep-sea marine snow is made of slowly sinking or neutrally buoyant particles affects the usefulness of optically derived particle inventories to estimate sinking fluxes unless morphological parameters of particles are taken into account. With our holographic microscope intact phytoplankton have frequently been observed at thousands of meters of water depth. Using decay rates from the literature it was difficult to reconcile the presence of these intact and possibly live cells without invoking unrealistically high sinking rates. To measure actual decay rates, we designed and built a deep-sea incubator which yielded important insights into the remineralization rates of fresh phytoplankton arriving at depth. With this new instrument and supplemental laboratory incubations, we were able to demonstrate that phytoplankton survive long-term exposure to cold dark and high-pressure environments, and that remineralization rates of live phytoplankton introduced into the deep sea are extremely low. As a result, intact cells arrive at great depths even when traveling at the typical sinking velocities recorded for marine particles. This is especially relevant for highly productive systems, for instance, during spring phytoplankton blooms in the North Atlantic, and in upwelling regions dominated by diatoms. We also determined surprisingly high survival rates of surface flagellates when exposed to 5000 m of water pressure and 2 degrees Celsius which therefore contribute to the connectivity between surface and deep-ocean layers. All these results point towards the notion that the biological pump is more efficient in transferring organic carbon from the surface ocean to the deep sea than previously believed. This project supported three graduate students (2 MS and 1 PhD) who participated in international deep-sea expeditions, and five undergraduate research internships. We published our results in seven peer-reviewed publications so far with more in preparation, and disseminated our results at conferences as contributing and invited speakers. Members of our lab participated in the annual Blue Crab Bowl which is Virginia's regional competition of the National Ocean Sciences Bowl showcasing some of the Commonwealth's most talented students in their mastery of oceanic knowledge. Our outreach efforts included open houses for the general public as well as participation in the "Mentoring Young Scientists" program by the Virginia Aquarium for middle school children and their mentors. Last Modified: 10/13/2018 Submitted by: Alexander B Bochdansky