Directly Observing and Quantifying Mechanisms of the Biological Pump Through Particle-resolved Imaging and DNA Sequencing

Colleen A Durkin, Moss Landing Marine Laboratories, Moss Landing, CA, United States, Alyson Santoro, University of California Santa Barbara, Ecology, Evolution, and Marine Biology, Santa Barbara, United States, Ken Buesseler, Woods Hole Oceanographic Institution, Department of Marine Chemistry & Geochemistry, Woods Hole, United States, Ivona Cetinic, NASA Goddard Space Flight Cent, Greenbelt, United States, Margaret L Estapa, Skidmore College, Saratoga Springs, NY, United States and Melissa Omand, University of Rhode Island, Graduate School of Oceanography, Narragansett, United States
This work combines imaging, chemistry, and molecular approaches to identify the phytoplankton that generate the carbon in sinking particles, the ecological mechanisms driving carbon export, and the efficiency with which these various particle types are transported to depth. Sediment traps were deployed at mesopelagic depths in the subtropical and coastal Pacific Ocean and during the EXPORTS cruise at Station P. Sinking particles collected in gel layers at the bottom of trap tubes were individually resolved by microscopy and categorized using machine learning classifiers and manual verification. The contribution of crustacean fecal pellets to POC flux was greatest in the coastal and subarctic locations, but attenuated rapidly with depth. When present, gelatinous zooplankton (salps, pyrosomes) pellets export a large percentage of carbon relative to their abundance. The composition of sinking particles changed with depth, representing a continuum of particle types at various stages of disaggregation and degradation. To identify the organismal source of the sinking carbon contained within these various particle types, ~500 individual particles were isolated from the gel layers, their DNA contents extracted, and the 18S and 16S rRNA gene fragments sequenced. Most of the DNA sequences within particles were assigned to heterotrophic protists and microbes, indicative of the highly processed character of sinking particles. All particles contained DNA from photosynthetic organisms, presumably the original source of the sinking carbon. Diatom, chlorophyte, and cyanobacterial DNA were frequently detected but varied across locations and particle types. Association of different phytoplankton with specific sinking particle types, locations, and depths will be assessed. By identifying the biological source of individually resolved particles in combination with DNA sequencing, we directly observe and quantify mechanisms of the biological pump in a way not previously possible.