Ubiquitous healthy diatoms in the deep sea confirm deep carbon injection by the biological pump

Abstract

The role of the ocean as a sink for CO2 is partially dependent on the downward transport of phytoplankton cells packaged within fast-sinking particles. However, whether such fast-sinking mechanisms deliver fresh organic carbon down to the deep bathypelagic sea and whether this mechanism is prevalent across the ocean requires confirmation. Here we report the ubiquitous presence of healthy photosynthetic cells, dominated by diatoms, down to 4,000 m in the deep dark ocean. Decay experiments with surface phytoplankton suggested that the large proportion (18%) of healthy photosynthetic cells observed, on average, in the dark ocean, requires transport times from a few days to a few weeks, corresponding to sinking rates (124–732 m d−1) comparable to those of fast-sinking aggregates and faecal pellets. These results confirm the expectation that fast-sinking mechanisms inject fresh organic carbon into the deep sea and that this is a prevalent process operating across the global oligotrophic ocean.

Aphotic N<sub>2</sub> fixation along an oligotrophic to ultraoligotrophic transect in the Western Tropical South Pacific Ocean

“…Members of Cluster 3 have also been recovered in the mesopelagic WTSP (Benavides et al, 2015), although this study reports longitudinal variation as a primary driver of Cluster 3 phylotype diversity and relative abundance. The cyanobacterial subcluster 1B including Trichodesmium, UCYN-A, and Richelia was observed at very low relative abundance throughout all stations and depths (average 0.5% of total community), in agreement with the findings of Caffin et al (2017), who detected those phylotypes in sediment traps deployed during the OUTPACE cruise at 150 and 325 m. Dead Trichodesmium colonies are thought to be mainly degraded in the photic zone (Letelier and Karl, 1998), although the detection 5 of Trichodesmium in sediment traps and seawater samples obtained from the mesopelagic zone (Agustí et al, 2015;Chen et al, 2003) suggests that decayed dense blooms likely sink fast down the water column. The detection of cyanobacterial diazotroph nifH sequences in the mesopelagic zone questions whether the N 2 fixation rates measured are solely attributable to non-cyanobacterial diazotrophs .…”

“…However, recent studies have detected photosynthetically active diatoms at depths overpassing the mesopelagic zone (Agustí et al, 2015), indicating that dead cell packages can be exported vertically at high speed. If cyanobacterial diazotrophs remain active when they reach the aphotic layer, or if they die or shut down N 2 fixation on the way, remains an open question.…”

Large variability of bathypelagic microbial eukaryotic communities across the world’s oceans

“…Although sequences from Bacillariophyta, Bolidophyta, Dictyochophyta, Prasinophyceae, Prymnesiophyceae, Raphidophyta and Eustigmatophyceae were retrieved in our survey, together they only represented 0.14% of the pyrotags and 2% of the OTUs. The phototrophic signal in bathypelagic waters is likely due to sinking particles (Agusti et al, 2015), although we do not totally dismiss the idea of some species being facultative heterotrophs. The absence of photosynthetic groups explains the low contribution of Stramenopiles in the global deep community, whereas Alveolate and Rhizaria were represented with percentages similar as in surface (Moon-van der Staay et al, 2001;Not et al, 2007;Massana, 2011).…”

Ubiquitous marine bacterium inhibits diatom cell division

“…Diatoms are an ecologically important group of phytoplankton that have a major role in supporting fisheries and driving the marine carbon cycle (Armbrust, 2009;Agusti et al, 2015). Specific bacterial phyla, mainly the Proteobacteria and Bacteroidetes (Amin et al, 2012), are commonly associated with diatoms.…”