The “dark ocean” – the water column below the photic zone - constitutes 90% of the ocean’s volume and contains three quarters of its microbial biomass. The dark ocean microbiome controls the aphotic end of the ocean’s carbon biological pump, and play a vital role in biogeochemical cycling. It is characterized by elevated taxonomic diversity, pronounced vertical stratification and complex interactions among heterotrophs and chemoautotrophs. There is an urgent need to improve our understanding of the metabolic processes in light of the emerging use of the dark ocean and its seafloor for fishing, carbon sequestration, and industrial mining. However, due to the challenges of conducting research at such depths, relatively little is known about the identity, ecology, evolution, activity and resilience of microorganisms inhabiting the dark ocean. The goal of this session is to assemble scientific presentations that cover deep-sea technology development and cutting-edge science in a broad spectrum of dark ocean microbiology-related research areas.
Lead Organizer: Ramunas Stepanauskas, Bigelow Laboratory for Ocean Sciences (rstepanauskas@bigelow.org)
Co-organizers:
Maria Pachiadaki, Woods Hole Oceanographic Institution (mpachiadaki@whoi.edu)
Presentations
02:30 PM
SELFISH UPTAKE OF POLYSACCHARIDES BY MARINE HETEROTROPHIC BACTERIA AT ELEVATED HYDROSTATIC PRESSURE (9105)
Primary Presenter: Chad Lloyd, University of North Carolina at Chapel Hill (cchadlloyd@gmail.com)
Heterotrophic bacteria turnover half of the organic matter that is produced by phytoplankton in the ocean. Much of this organic matter is in the form of polysaccharides, complex high-molecular weight sugars. Previously, it was thought that bacteria had to hydrolyze this organic matter to small sizes outside the cell, releasing hydrolysis products to the environment. Within the last decade, however, an alternative mechanism of uptake— ‘selfish uptake’—has been discovered. Using selfish uptake, bacteria bind, partially hydrolyze, and transport large fragments of polysaccharides into the cell with little to no loss of hydrolysis products. To date, selfish uptake has been investigated in both the surface and deep ocean, but potential impacts of hydrostatic pressure have not been investigated. Therefore, in three field expeditions, we measured the extent of selfish uptake in surface and deep waters, under pressures ranging from atmospheric up to ~50 MPa (equivalent to 5,000 m depth). Preliminary data suggests differences in the extent of selfish uptake in surface waters between samples that were pressurized and those that were kept under atmospheric pressure. We also explored the extent of selfish uptake from the deep ocean using an in-situ incubator. Results from these experiments will help us quantify the extent to which pressure affects the mechanisms by which heterotrophic bacteria process polysaccharides throughout the global ocean.
02:45 PM
SPATIOTEMPORAL VARIABILITY AND VERTICAL TRANSPORT OF GIANT VIRUSES IN THE NORTH PACIFIC SUBTROPICAL GYRE (9392)
Primary Presenter: Md Moinuddin Sheam, University of North Carolina at Charlotte (msheam@uncc.edu)
Nucleocytoplasmic large DNA viruses (giant viruses) are widespread in marine environments, infecting a broad range of eukaryotes and regulating microbial population dynamics and biogeochemical cycling. This study is the first to integrate metagenomic datasets from both the water column and seafloor sediment to assess the vertical transport of giant viruses and their association with carbon export. We analyzed metagenomic samples from the Hawaii Ocean Time-series (HOT), focusing on long-term metagenomic datasets (n=861) collected over six years across 15 depths from 5-4000m, with sediment trap samples also collected at 4000m seafloor. We report for the first time the vertical transport of several giant virus species, including algae infecting Phaeocystis globosa virus and Chlorella virus XW01, and their correlation with carbon export from the water column to abyssal depth. Furthermore, we postulate depth-specific adaptations in giant viruses to influence host cell metabolic processes, such as photosynthesis, carbon, sulfur, and nitrogen metabolism. Phylogenetic analysis of 38 metagenome-assembled genomes revealed the presence of three different families of giant viruses, dominated by the protist-infecting Mimiviridae. Spatiotemporal analysis of these genomes uncovered distinct distributions unique to their taxonomic group. Overall, our findings identified key giant viruses associated with vertical transport and carbon export flux in the oligotrophic ocean. Additionally, we observed notable metabolic capacities and a unique spatiotemporal distribution of these viruses.
03:00 PM
Microbial Community Structure and Diversity Across Whale Fall, Submarine Canyon, and Submarine Fan Sites in Deep Sea Marine Sediments (9358)
Primary Presenter: Yu-Chen Ling, Marine Biological Laboratory (lchacol@gmail.com)
Deep-sea ecosystems are essential to global carbon cycling, with microbes driving key biogeochemical processes. In this study, we investigated microbial community structure and diversity across whale fall and submarine canyon sites in Monterey Bay, California, and bathypelagic sediments sites. Twenty sediment cores (extending 15 to 18 cm below seafloor, cmbsf) were collected from five sites at depths ranging from 633 to 3,562 meters. Our results indicate that biological dissimilarity increases with geographical distance, even over small spatial scales (2–50 cm), with rare microbial populations significantly contributing to dissimilarity among replicated samples. Bacterial community dissimilarities were greater across 15 cm vertical depth of the core(s) than over 150 km horizontally. Several key taxa exhibited significant variation (Mann-Whitney U Test, p < 0.01) in relative abundance between surface (0-6 cmbsf) and deeper (>6 cmbsf) horizons. Sediment surface horizons were dominated by Bacteroidota, Gammaproteobacteria, and Verrucomicrobiota, while deeper horizons were characterized by Chloroflexi, Acidobacteriota, Zixibacteria, Spirochaetota, and Caldatribacteriota. Notably, whale-fall sites, where a detrital whale was deposited approximately a decade or more ago, showed that the biosphere characteristic of deeper sediments depths occurs at shallower depths than in non-whale-fall sites located 90 m away, indicating enhanced microbial development. These findings provide insights into microbial diversity in deep-sea sediments.
03:15 PM
Bacterial reworking of dissolved organic matter in the deep Arctic Ocean: influence of its chemical composition (9321)
Primary Presenter: Celine Gueguen, Universite de Sherbrooke (celine.gueguen@usherbrooke.ca)
The microbial transformation of dissolved organic matter (DOM) is crucial for a better understanding of its dynamics in the ocean. In controlled laboratory incubations (20 days), we assessed the optical and molecular changes in the DOM pool to discern how the same bacterial community processes DOM collected in four distinct water masses of the Canada Basin, Arctic Ocean (i.e, Pacific Winter Water, Atlantic Halocline, Atlantic Water - Barents Sea Strait branch, and deep temperature minimum). While all water masses contained the same fluorescent components, they underwent unique processing when in contact with the microbial community. These results suggest that the makeup of a particular fluorescent component probably varied between different water layers. The molecular characteristics measured with a trapped ion mobility spectrometer coupled with a time-of-flight mass spectrometer (TIMS-TOF) and an electrospray ionization source (positive ionization mode) revealed distinct molecular features in the solid-phase extracts of DOM between the water masses, and the number of those features increased by the end of the incubation period. The number of common molecular features was also greater in contiguous water masses, likely a result of water mass mixing. Spearman's rank correlations between fluorescence characteristics and TIMS-TOF features were used to establish the connections between DOM and molecular formulas. 34% of the common TIMS TOF features were associated with at least one PARAFAC component. Overall, these findings indicate that the refractory character of deep DOM is contingent upon its intrinsic assemblage.
03:30 PM
Key functional groups of planktonic protists contribute to particle export fluxes in the upwelling system off Peru (9612)
Primary Presenter: Susanne Neuer, Arizona State University (susanne.neuer@asu.edu)
Sinking of biogenic particles drive the biological carbon pump, which transports photosynthetically fixed CO2 from the ocean’s sunlit surface to the deep sea. In particular in productive ecosystems such as the Eastern Boundary Upwelling Systems vertical fluxes of biomass are high, sustaining extensive microbial respiration at depth and contributing to the formation of oxygen minimum zones. However, little is known on ecological drivers of particle export fluxes and the relative contribution of individual groups of planktonic protists. Here we show results from two campaigns to the Eastern Tropical South Pacific onboard the RV Meteor (April and June 2017), where we collected sinking particles at 6-8 depths in the range 40-600m using surface tethered drifting particle interceptor traps. We analyzed the taxonomic composition of the photoautotrophic community based on the pigment composition and the total protists community using amplicon sequencing of the 18S rRNA gene. We relate the community composition to environmental data and biogeochemical fluxes to identify drivers and controls of export fluxes. Our study provides new insights into the role of functional groups of planktonic protists as key actors in the biological carbon pump.
SS10B - Microbial processes of the dark ocean
Description
Time: 2:30 PM
Date: 28/3/2025
Room: W201CD