Vertical distribution of picoeukaryotic diversity in the Sargasso Sea

Abstract

Eukaryotic molecular diversity within the picoplanktonic size-fraction has primarily been studied in marine surface waters. Here, the vertical distribution of picoeukaryotic diversity was investigated in the Sargasso Sea from euphotic to abyssal waters, using size-fractionated samples (< 2 microm). 18S rRNA gene clone libraries were used to generate sequences from euphotic zone samples (deep chlorophyll maximum to the surface); the permanent thermocline (500 m); and the pelagic deep-sea (3000 m). Euphotic zone and deep-sea data contrasted strongly, the former displaying greater diversity at the first-rank taxon level, based on 232 nearly full-length sequences. Deep-sea sequences belonged almost exclusively to the Alveolata and Radiolaria, while surface samples also contained known and putative photosynthetic groups, such as unique Chlorarachniophyta and Chrysophyceae sequences. Phylogenetic analyses placed most Alveolata and Stramenopile sequences within previously reported 'environmental' clades, i.e. clades within the Novel Alveolate groups I and II (NAI and NAII), or the novel Marine Stramenopiles (MAST). However, some deep-sea NAII formed distinct, bootstrap supported clades. Stramenopiles were recovered from the euphotic zone only, although many MAST are reportedly heterotrophic, making the observed distribution a point for further investigation. An unexpectedly high proportion of radiolarian sequences were recovered. From these, five environmental radiolarian clades, RAD-I to RAD-V, were identified. RAD-IV and RAD-V were composed of Taxopodida-like sequences, with the former solely containing Sargasso Sea sequences, although from all depth zones sampled. Our findings highlight the vast diversity of these protists, most of which remain uncultured and of unknown ecological function.

Targeted metagenomics and ecology of globally important uncultured eukaryotic phytoplankton

“…Lower levels of group 3 were detected in the Station 04 sort (7 clones, 99-100% identity), along with sequences at the tip of the tree (e.g., group 15; SI Materials and Methods, Section 5). Group 8 was also seen in the Sargasso Sea (14) and on multiple dates in the Florida Straits (Table S2). Group 8 had only 93% 18S rDNA identity to any cultured organism (to Chrysochromulina), and phylogenetic placement was unresolved (no bootstrap support).…”

“…Samples were from the North, Equatorial, and South Atlantic, the North, Equatorial, and West Pacific, and the Indian Ocean(s). 18S rDNA clone libraries (14) were constructed from subtropical Atlantic and North West Pacific size-fractionated samples, or from cells sorted by flow cytometry.…”

“…Six picoplanktonic prymnesiophytes exist in culture (6, 7) but prymnesiophyte 18S rDNA sequences from <2-3-μm size-fractioned environmental samples typically belong to uncultured taxa (6,(13)(14)(15). As a whole, this lineage reportedly diverged from other major eukaryotic lineages early on, 1.2 billion years ago (16), and their overall placement among eukaryotes is uncertain (4,16).…”

Ecology and Diversity of Picoeukaryotes

“…At this site, in the Sargasso Sea, 65 percent of sequences recovered from the euphotic zone belonged to the NA (51 out of 78), representing 55 percent of the OTUs using a threshold of 98 percent sequence identity. Similarly, 67 percent of the rDNA sequences fell within the alveolates at 500 m depth and 68 percent at 3000 m depth (Not et al 2007a). These NA form two main groups, NAI and NAII, each of which is composed of many clades, some clades having sequences from a broad range of environments (e.g., anoxic, deep-sea, and open-ocean waters), while others appear to be of a more limited environmental range (Groisillier et al 2006).…”

“…6.4) (Moon-Van Der Staay et al 2000;Not et al 2007a). Flow-cytometric sorting (see below) from one of these oligotrophic sites allowed picophytoeukaryotes to be separated and viewed under the microscope, revealing cells approximately 1.5 mm  2 mm with a characteristic prymnesiophyte chloroplast arrangement (Worden, unpublished data).…”