title: Global abundance of planktonic heterotrophic protists in the deep ocean.
abstract: The dark ocean is one of the largest biomes on Earth, with critical roles in organic matter remineralization and global carbon sequestration. Despite its recognized importance, little is known about some key microbial players, such as the community of heterotrophic protists (HP), which are likely the main consumers of prokaryotic biomass. To investigate this microbial component at a global scale, we determined their abundance and biomass in deepwater column samples from the Malaspina 2010 circumnavigation using a combination of epifluorescence microscopy and flow cytometry. HP were ubiquitously found at all depths investigated down to 4000 m. HP abundances decreased with depth, from an average of 72±19 cells ml(-1) in mesopelagic waters down to 11±1 cells ml(-1) in bathypelagic waters, whereas their total biomass decreased from 280±46 to 50±14 pg C ml(-1). The parameters that better explained the variance of HP abundance were depth and prokaryote abundance, and to lesser extent oxygen concentration. The generally good correlation with prokaryotic abundance suggested active grazing of HP on prokaryotes. On a finer scale, the prokaryote:HP abundance ratio varied at a regional scale, and sites with the highest ratios exhibited a larger contribution of fungi molecular signal. Our study is a step forward towards determining the relationship between HP and their environment, unveiling their importance as players in the dark ocean's microbial food web.
authors.affiliation: Department of Marine Biology and Oceanography, Institut de Ciències del Mar (CSIC), Barcelona, Catalonia, Spain.
authors.forename: Massimo C
authors.initials: MC
authors.lastname: Pernice
authors.affiliation: Department of Marine Biology and Oceanography, Institut de Ciències del Mar (CSIC), Barcelona, Catalonia, Spain.
authors.forename: Irene
authors.initials: I
authors.lastname: Forn
authors.affiliation: Department of Marine Biology and Oceanography, Institut de Ciències del Mar (CSIC), Barcelona, Catalonia, Spain.
authors.forename: Ana
authors.initials: A
authors.lastname: Gomes
authors.affiliation: Department of Marine Biology and Oceanography, Institut de Ciències del Mar (CSIC), Barcelona, Catalonia, Spain.
authors.forename: Elena
authors.initials: E
authors.lastname: Lara
authors.affiliation: Centro Oceanográfico de Gijón, Instituto Español de Oceanografía, Gijón, Spain.
authors.forename: Laura
authors.initials: L
authors.lastname: Alonso-Sáez
authors.affiliation: Department of Global Change Research, Mediterranean Institute of Advanced Studies (CSIC-UiB), Esporles, Spain.
authors.forename: Jesus M
authors.initials: JM
authors.lastname: Arrieta
authors.affiliation: Centro Oceanográfico de Gijón, Instituto Español de Oceanografía, Gijón, Spain.
authors.forename: Francisca
authors.initials: F
authors.lastname: del Carmen Garcia
authors.affiliation: Department of Ecology and Animal biology, University of Vigo, Vigo, Spain.
authors.forename: Victor
authors.initials: V
authors.lastname: Hernando-Morales
authors.affiliation: Department of Marine Biology and Oceanography, Institut de Ciències del Mar (CSIC), Barcelona, Catalonia, Spain.
authors.forename: Roy
authors.initials: R
authors.lastname: MacKenzie
authors.affiliation: Department of Marine Biology and Oceanography, Institut de Ciències del Mar (CSIC), Barcelona, Catalonia, Spain.
authors.forename: Mireia
authors.initials: M
authors.lastname: Mestre
authors.affiliation: Department of Marine Biology, University of Vienna, Vienna, Austria.
authors.forename: Eva
authors.initials: E
authors.lastname: Sintes
authors.affiliation: Department of Ecology and Animal biology, University of Vigo, Vigo, Spain.
authors.forename: Eva
authors.initials: E
authors.lastname: Teira
authors.affiliation: Centro Oceanográfico de A Coruña, Instituto Español de Oceanografía, A Coruña, Spain.
authors.forename: Joaquin
authors.initials: J
authors.lastname: Valencia
authors.affiliation: Centro Oceanográfico de Vigo, Instituto Español de Oceanografía, Vigo, Spain.
authors.forename: Marta M
authors.initials: MM
authors.lastname: Varela
authors.affiliation: Department of Marine Biology and Oceanography, Institut de Ciències del Mar (CSIC), Barcelona, Catalonia, Spain.
authors.forename: Dolors
authors.initials: D
authors.lastname: Vaqué
authors.affiliation: 1] Department of Global Change Research, Mediterranean Institute of Advanced Studies (CSIC-UiB), Esporles, Spain [2] The UWA Oceans Institute and School of Plant Biology, The University of Western Australia Oceans Institute, Perth, WA, Australia.
authors.forename: Carlos M
authors.initials: CM
authors.lastname: Duarte
authors.affiliation: Department of Marine Biology and Oceanography, Institut de Ciències del Mar (CSIC), Barcelona, Catalonia, Spain.
authors.forename: Josep M
authors.initials: JM
authors.lastname: Gasol
authors.affiliation: Department of Marine Biology and Oceanography, Institut de Ciències del Mar (CSIC), Barcelona, Catalonia, Spain.
authors.forename: Ramon
authors.initials: R
authors.lastname: Massana
mesh_terms: D018533:Biomass; D056890:Eukaryota / Q000302:isolation & purification; D052836:Heterotrophic Processes; D009792:Oceans and Seas; D010933:Plankton / Q000166:cytology / Q000302:isolation & purification*; D012623:Seawater / Q000382:microbiology