Composition of the summer photosynthetic pico and nanoplankton communities in the Beaufort Sea assessed by T-RFLP and sequences of the 18S rRNA gene from flow cytometry sorted samples

Abstract

The composition of photosynthetic pico and nanoeukaryotes was investigated in the North East Pacific and the Arctic Ocean with special emphasis on the Beaufort Sea during the MALINA cruise in summer 2009. Photosynthetic populations were sorted using flow cytometry based on their size and pigment fluorescence. Diversity of the sorted photosynthetic eukaryotes was determined using terminal-restriction fragment length polymorphism analysis and cloning/sequencing of the 18S ribosomal RNA gene. Picoplankton was dominated by Mamiellophyceae, a class of small green algae previously included in the prasinophytes: in the North East Pacific, the contribution of an Arctic Micromonas ecotype increased steadily northward becoming the only taxon occurring at most stations throughout the Beaufort Sea. In contrast, nanoplankton was more diverse: North Pacific stations were dominated by Pseudo-nitzschia sp. whereas those in the Beaufort Sea were dominated by two distinct Chaetoceros species as well as by Chrysophyceae, Pelagophyceae and Chrysochromulina spp.. This study confirms the importance of Arctic Micromonas within picoplankton throughout the Beaufort Sea and demonstrates that the photosynthetic picoeukaryote community in the Arctic is much less diverse than at lower latitudes. Moreover, in contrast to what occurs in warmer waters, most of the key pico- and nanoplankton species found in the Beaufort Sea could be successfully established in culture.

Figure 1

MALINA station locations for Legs 1b and 2b. Grey shades correspond to bottom depths.

Figure 2

Diversity of flow cytometry sorted photosynthetic picoeukaryotes and nanoeukaryotes from the surface at stations 320 and 390 assessed by T-RFLP chromatograms of MnlI digests of 18S rDNA. Please note that the identification shown here has been confirmed by T-RFLP chromatograms of HhaI digests. The enzyme Hpy188I, which allows discriminating among the different Micromonas clades (Supplementary Table S7), was also used to validate the identification of the Arctic Micromonas ecotype. The full list of ribotypes identified is shown on Table 2. C., Chaetoceros.

Figure 3

Taxonomic composition of photosynthetic pico and nanoeukaryotes based on T-RFLP on 18S rRNA gene sequences obtained from sorted photosynthetic populations at the different surface stations across the Leg 1b. Please note that while for picoplankton only one Chrysophyceae ribotype has been found (uncultured Chrysophyceae, Table 2), several have been found for nanoplankton. See Figure 1 for station locations.

Figure 4

Taxonomic composition of photosynthetic picoeukaryotes based on T-RFLP on 18S rRNA gene sequences obtained from photosynthetic populations sorted from the surface and the DCM throughout the Beaufort Sea.

Figure 5

Temperature profile, absolute abundance and taxonomic composition of photosynthetic pico and nanoeukaryotes sorted from different depths at station 235.

Figure 6

Taxonomic composition of photosynthetic nanoeukaryotes based on T-RFLP of 18S rRNA gene sequences obtained from photosynthetic populations sorted from the surface and the DCM throughout the Beaufort Sea.

Figure 7

Neighbour joining (NJ) phylogeny of almost full-length 18S rRNA genes from photosynthetic pico and nanoeukaryotes sorted from the stations 320 and 390. A fungal sequence (Hormonema dematioides) was used as outgroup. Sequences corresponding to cultures are indicated by a dot (blue for cultures isolated during MALINA and black for others) whereas environmental sequences are in blue. Details on phylogenetic analyses are given in the Materials and methods Section. 1556 unambiguously aligned positions were considered from an alignment of 115 nucleotide sequences. The percentage of NJ bootstrap (based on 1000 replicates) is shown next to the branches for values ⩾70%.

Figure 8

Overall comparison of composition of photosynthetic pico and nanoeukaryotes assessed by T-RFLP and cloning/sequencing of the 18S rRNA gene. Only ribotypes from which at least three sequences were recovered by cloning/sequencing are represented.