The world's oceans represent by far the largest biome, with great importance for the global ecosystem [1-4]. The vast majority of ocean biomass and biodiversity is composed of microscopic plankton. Recent results from the Tara Oceans metabarcoding study revealed that a significant part of the plankton in the upper sunlit layer of the ocean is represented by an understudied group of heterotrophic excavate flagellates called diplonemids [5, 6]. We have analyzed the diversity and distribution patterns of diplonemid populations on the extended set of Tara Oceans V9 18S rDNA metabarcodes amplified from 850 size- fractionated plankton communities sampled across 123 globally distributed locations, for the first time also including samples from the mesopelagic zone, which spans the depth from about 200 to 1,000 meters. Diplonemids separate into four major clades, with the vast majority falling into the deep-sea pelagic diplonemid clade. Remarkably, diversity of this clade inferred from metabarcoding data surpasses even that of dinoflagellates, metazoans, and rhizarians, qualifying diplonemids as possibly the most diverse group of marine planktonic eukaryotes. Diplonemids display strong vertical separation between the photic and mesopelagic layers, with the majority of their relative abundance and diversity occurring in deeper waters. Globally, diplonemids display no apparent biogeographic structuring, with a few hyperabundant cosmopolitan operational taxonomic units (OTUs) dominating their communities. Our results suggest that the planktonic diplonemids are among the key heterotrophic players in the largest ecosystem of our biosphere, yet their roles in this ecosystem remain unknown.
- MeSH
- Biodiversity * MeSH
- Ecosystem * MeSH
- Euglenozoa classification genetics MeSH
- Oceans and Seas MeSH
- Plankton classification genetics MeSH
- RNA, Protozoan genetics MeSH
- RNA, Ribosomal, 18S genetics MeSH
- Sequence Analysis, RNA MeSH
- DNA Barcoding, Taxonomic MeSH
- Aquatic Organisms physiology MeSH
- Publication type
- Journal Article MeSH
- Geographicals
- Oceans and Seas MeSH
Recent global surveys of marine biodiversity have revealed that a group of organisms known as "marine diplonemids" constitutes one of the most abundant and diverse planktonic lineages [1]. Though discovered over a decade ago [2, 3], their potential importance was unrecognized, and our knowledge remains restricted to a single gene amplified from environmental DNA, the 18S rRNA gene (small subunit [SSU]). Here, we use single-cell genomics (SCG) and microscopy to characterize ten marine diplonemids, isolated from a range of depths in the eastern North Pacific Ocean. Phylogenetic analysis confirms that the isolates reflect the entire range of marine diplonemid diversity, and comparisons to environmental SSU surveys show that sequences from the isolates range from rare to superabundant, including the single most common marine diplonemid known. SCG generated a total of ∼915 Mbp of assembled sequence across all ten cells and ∼4,000 protein-coding genes with homologs in the Kyoto Encyclopedia of Genes and Genomes (KEGG) orthology database, distributed across categories expected for heterotrophic protists. Models of highly conserved genes indicate a high density of non-canonical introns, lacking conventional GT-AG splice sites. Mapping metagenomic datasets [4] to SCG assemblies reveals virtually no overlap, suggesting that nuclear genomic diversity is too great for representative SCG data to provide meaningful phylogenetic context to metagenomic datasets. This work provides an entry point to the future identification, isolation, and cultivation of these elusive yet ecologically important cells. The high density of nonconventional introns, however, also portends difficulty in generating accurate gene models and highlights the need for the establishment of stable cultures and transcriptomic analyses.
- MeSH
- Biodiversity MeSH
- Euglenozoa classification cytology genetics MeSH
- Phylogeny MeSH
- Genome, Protozoan * MeSH
- Metagenomics MeSH
- Plankton classification cytology genetics MeSH
- RNA, Protozoan genetics MeSH
- Amino Acid Sequence MeSH
- Sequence Alignment MeSH
- Publication type
- Journal Article MeSH
- Geographicals
- California MeSH
- Pacific Ocean MeSH
