Abstract
AbstractSuspended marine particles constitute most of the particulate organic matter pool in the oceans, thereby providing substantial substrates for heterotrophs, especially in the mesopelagic. Conversely, sinking particles are major contributors to carbon fluxes defining the strength of the biological carbon pump (BCP). This study is the first to investigate the differential influence of eukaryotic communities to suspended and sinking particles, using 18S rRNA gene sequencing on particles collected with a marine snow catcher in the mixed layer and upper mesopelagic of the Scotia Sea, Southern Ocean. In the upper mesopelagic, most eukaryotic phytoplankton sequences belonged to chain‐forming diatoms in sinking particles and to prymnesiophytes in suspended particles. This suggests that diatom‐enriched particles are more efficient in carbon transfer to the upper mesopelagic than those enriched in prymnesiophytes in the Scotia Sea, the latter more easily disintegrating into suspended particles. In the upper mesopelagic, copepods appeared most influential on sinking particles whereas soft‐tissue metazoan sequences contributed more to suspended particles. Heterotrophic protists and fungi communities were distinct between mixed layer and upper mesopelagic, implying that few protists ride along sinking particles. Furthermore, differences between predatory flagellates and radiolarians between suspended and sinking particles implied different ecological conditions between the two particles pools, and roles in the BCP. Molecular analyses of sinking and suspended particles constitute powerful diagnostic tools to study the eukaryotic influence on the BCP in a more holistic manner compared to classic carbon export studies focusing on sinking particles.
Highlights
After sequence trimming, pairing and merging, and separation from metazoan sequences, a total of 1,533,266 protist sequences remained with an average length of 450 bp (29,852 Æ 22,428 sequences/library) (Fig. S2). At both depths, there was a higher proportion of Metazoa affiliated sequences in sinking particles ≥ 10 μm (SK10) and suspended particles ≥ 100 μm; (SS100), except at ICE, while a higher proportion of eukaryotic phytoplankton sequences was observed in SS0.22 and suspended particles 10–100 μm (SS10) in the mixed layer (Table S1)
particulate organic carbon (POC) concentrations were higher in suspended particles (Table S2), and within the mixed layer depth compared to the upper mesopelagic (Belcher et al 2016b)
This study provides the first insights into eukaryotic contribution to suspended and sinking particle pools in the mixed layer and the upper mesopelagic of the Scotia Sea
Summary
Some allochthonous POM, such as POM originating from river runoffs, can constitute sinking particles The nature of these sinking particles depends largely on the structure of phytoplankton community (Korb et al 2012) and heterotrophic community members, including mesozooplankton (> 200 μm) (Steinberg and Landry 2017) and heterotrophic microbes (prokaryotes and unicellular eukaryotes or protists) (Worden et al 2015). Examples include decapods (Pakhomov et al 2018), copepods (Cavan et al 2015; Yebra et al 2018), and gelatinous zooplankton (Alldredge 1976; Wilson et al 2008) They can release DOM and break sinking particles into smaller pieces that remain in suspension in the water column (suspended particles) (De La Rocha and Passow 2007) via sloppy feeding (Strom et al 1997). The proportion of suspended particles originating from the disaggregation of sinking particles is currently unknown (Lam and Marchal 2015), as it is governed by complex abiotic and biotic processes such as those mentioned above, as well as feeding behaviors of micro- and nanozooplankton (protists 20–200 and 2–20 μm, respectively) (Ploug and Grossart 2000; Poulsen and Iversen 2008)
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