Widespread distribution of a unique marine protistan lineage

Abstract

Unicellular eukaryotes (protists) are key components of marine food webs, yet knowledge of their diversity, distributions and respective ecologies is limited. We investigated uncultured protists using 18S rRNA gene sequencing, phylogenetic analyses, specific fluorescence in situ hybridization (FISH) probes and other methods. Because few studies have been conducted in warm water systems, we focused on two Atlantic subtropical regions, the Sargasso Sea and the Florida Current. Cold temperate waters were also sampled. Gene sequences comprising a unique eukaryotic lineage, herein termed 'biliphytes', were identified in most samples, whether from high- (30 degrees C) or from low- (5 degrees C) temperature waters. Sequences within this uncultured group have previously been retrieved from high latitudes. Phylogenetic analyses suggest biliphytes are a sister group to the cryptophytes and katablepharids, although the relationship is not statistically supported. Bootstrap-supported subclades were delineated but coherence was not obvious with respect to geography or physicochemical parameters. Unlike results from the initial publication on these organisms (therein 'picobiliphytes'), we could not detect a nucleomorph, either visually, or by targeted primers. Phycobilin-like fluorescence associated with biliphyte-specific FISH-probed cells supports the hypothesis that they are photosynthetic. Our data indicate the biliphytes are nanoplanktonic in size, averaging 4.1 +/- 1.0 x 3.5 +/- 0.8 microm (+/-SD) for one probed group, and 3.5 +/- 0.9 x 3.0 +/- 0.9 microm (+/-SD) for another. We estimate biliphytes contributed 28 (+/-6)% of the phytoplanktonic biomass in tropical eddy-influenced surface waters. Given their broad thermal and geographic distribution, understanding the role these protists play in biogeochemical cycling within different habitats is essential.

Fig. 1

Geographic location, physical and biological parameters of the sample regions and sites. Three regions were investigated during the cruises. In the Sargasso Sea (dark grey stars), two sites were sampled: BATS (bottom star) and the Northern Sargasso (top star). In the continental shelf area (light grey stars), two sites were sampled: the continental edge (bottom star) and the continental slope (top star). In the Florida Straits (white stars), three sites were sampled for each of the three cruises (March 2005, July/August 2005, December 2005): Station 01 (left star), Station 04 (middle star) and Station 14 (right star) (A). Bottom panels show the cross-straits vertical profiles of the north component of current during March (B) and July (C); temperature during March (D) and July (E); and fluorescence during March (E) and July (F). Sampling stations and depths containing biliphytes are indicated with an asterisk. Florida Straits sampling depths where biliphyte sequences were not recovered are shown by black squares in (F) and (G). Tick marks on the upper x-axis indicate stations at which CTD casts were performed to measure environmental parameters. During the March cruise the ship was only equipped with a 600 kHz ADCP capable of measuring shallow currents, hence (B) only shows velocity in the upper 30 m of the water column. SeaWIFS data (A) are derived from http://oceancolor.gsfc.nasa.gov/cgi/browse.pl from the integrated June 2005 sea surface temperature data.

Fig. 2

Phylogenetic analysis of 18S rRNA gene sequences from environmental clone libraries (all non-italic sequence names) and cultured representatives of eukaryotic first rank taxa. Sequences in coloured bars are reported for the first time in this study and are derived from 17 environmental clone libraries, three from the Sargasso Sea (prefixes: OC413BATS_P, Q; OC413NSS), 11 from three stations in the Florida Straits (prefixes: FS01, FS04, FS14) over multiple dates and depths (see Table 1), as well as three from continental shelf-edge and slope waters (prefixes: EN351). Biliphyte sequences from previous work are shown in coloured font. Bar and font colours correspond to five temperature ranges as indicated on the figure. Other Sargasso Sea and Pacific Ocean environmental sequences (non-italic, black font sequence names without coloured bars) are included to illustrate the distinct nature of biliphyte sequences relative to other uncultured eukaryotes retrieved from the same waters. The tree shown was inferred by maximum likelihood (ML) methods using the model TrN + I +Γ (I = 0.2332; Γ = 0.4748) with global rearrangements, randomized sequence input, 10 jumbles, Transition/Transversion = 1.9433 and six rate categories. Node values reflect bootstrap support in the order maximum likelihood/neighbour-joining distance/parsimony as percentages of 100/1000/100 replicates. Bootstrap support for terminal nodes (*) is indicated only if over 90% by all three methods. Two radiolarian sequences (126033309 and 126033226) served as an outgroup (not shown). The scale bar indicates the estimated number of nucleotide substitutions per site.