Abstract
Phototrophic microbial eukaryotes dominate primary production over large oceanic regions. Due to their small sizes and slow sinking rates, it is assumed they contribute relatively little to the downward export of organic carbon via the biological pump. Therefore, the community structure of phototrophic cells in the deep ocean has long been overlooked and remains largely unknown. In this study, we used an integrative approach, including epifluorescence microscopy, sequencing of 18S rRNA and photosystem-II psbA gene transcripts, to investigate phototrophic microbial eukaryotes in samples collected from the tropical Western Pacific Ocean. It was found that: (i) pigmented nano-sized eukaryotes (PNEs) are ubiquitous in the deep Western Pacific Ocean down to 5000 m depth; (ii) the PNE community is dominated by cells 2-5 μm in size; (iii) their abundance is significant, averaging 4 ± 1 (± s.e.) cells ml-1 in waters below 1000 m which is comparable to that of heterotrophic nanoflagellates; (iv) the active pigmented microbial eukaryotes in the deep waters are highly diverse and dominated by Haptophyta followed by Chlorophyta and Bacillariophyta; (v) PNEs in deep waters were likely transported from surface ocean by various fast-sinking mechanisms, thus contributing to the biological pump and fuelling the deep-sea communities by supplying fresh organic carbon.
Highlights
Marine phytoplankton encompasses prokaryotic and eukaryotic organisms that are capable of oxygenic photosynthesis and inhabit the photic zone of all oceans (Falkowski et al, 2004; Worden et al, 2004)
The water temperature, salinity, and chlorophyll a (Chl a) at each depth interval are summarized in Supporting Information Fig. S1
Surface (5 m) Chl a concentration was low (0.02 – 0.16 μg L-1) throughout the study area reflecting the oligotrophic conditions in the tropical Western Pacific Ocean
Summary
Marine phytoplankton encompasses prokaryotic and eukaryotic organisms that are capable of oxygenic photosynthesis and inhabit the photic zone of all oceans (Falkowski et al, 2004; Worden et al, 2004) They account for almost half of all photosynthetic activity on Earth and play a fundamental role in marine ecosystems and global biogeochemical processes. In vast areas of the oceans, especially the oligotrophic regions, the majority of primary production is performed by small-sized cells, i.e. the picoplankton (less than 2 – 3 m in diameter) and nanoplankton (2 – 20 m in diameter) (Marañón et al, 2001) Among these minute cells, the abundance of phototrophic microbial eukaryotes is two to three orders of magnitude lower than their prokaryote counterparts, the cyanobacteria. Compared with cyanobacteria that are dominated by just two genera (Prochlorococcus and Synechococcus), photosynthetic microbial eukaryotes are much more diverse and include representatives of almost all major clades of the eukaryotic phylogenetic tree (Massana, 2011; Decelle et al, 2015)
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