Distribution of micro-organisms along a transect in the South-East Pacific Ocean (BIOSOPE cruise) using epifluorescence microscopy

Abstract

Abstract. The distribution of selected groups of microorganisms was analyzed along a South-East Pacific Ocean transect sampled during the BIOSOPE cruise in 2004. The transect could be divided into four regions of contrasted trophic status: a High Nutrient Low Chlorophyll (HNLC) region (mesotrophic) near the equator, the South-East Pacific Ocean gyre (hyper-oligotrophic), a transition region between the gyre and the coast of South America (moderately oligotrophic), and the Chile upwelling (eutrophic). The abundance of phycoerythrin containing picocyanobacteria (PE picocyanobacteria), autotrophic and heterotrophic eukaryotes (classified into different size ranges), dinoflagellates, and ciliates was determined by epifluorescence microscopy after DAPI staining. Despite some apparent loss of cells due to sample storage, distribution patterns were broadly similar to those obtained by flow cytometry for PE picocyanobacteria and picoeukaryotes. All populations reached a maximum in the Chile upwelling and a minimum near the centre of the gyre. The maximum abundance of PE picocyanobacteria was 70 10 3 cell mL −1 . Abundance of autotrophic eukaryotes and dinoflagellates reached 24.5 10 3 and 20 cell mL −1 , respectively. We observed a shift in the size distribution of autotrophic eukaryotes from 2-5 µm in eutrophic and mesotrophic regions to less than 2 µm in the central region. The contribution of autotrophic eukaryotes to total eukaryotes was the lowest in the central gyre. Maximum concentration of ciliates (18 cell mL −1 ) also occurred in the Chile upwelling, but, in contrast to the other groups, their abundance was very low in the HNLC zone and near the Marquesas Islands. Two key findings of this work that could not have been observed with other techniques are the high percentage of PE picocyanobacteria forming colonies in the HLNC region and the observation of numerous dinoflagellates with bright green autofluorescence.

Distribution of nano-sized Cyanobacteria in the western and central Pacific Ocean

“…The occurrence of nano-sized Cyanobacteria had not been reported from a previous survey in the eastern region of the SPSG (Masquelier & Vaulot 2008), suggesting that nanocyanobacteria are constantly scarce in this region.…”

Threats to Ultraoligotrophic Marine Ecosystems

“…In the clear waters of the gyre centre autotrophic eukaryotes shift to smaller cells (<2 μm) compared to more eutrophic conditions (Masquelier & Vaulot, 2008). Flow cytometry sorting carried out in the most oligotrophic areas of the gyre revealed several novel lineages of photosynthetic picoeukaryotes such as a clade of prasinophytes.…”

Biological oceanography, biogeochemical cycles, and pelagic ecosystem functioning of the east central South Pacific Gyre: focus on Easter Island and Salas y Gomez Island

“…In functional diversity, the dominant eukaryotic microbes from the surface until the DCM were heterotrophic nano-and pico-eukaryotes (Masquelier & Vaulot, 2008). Between 150-200 m, heterotrophs represented only 20-40% of eukaryotic microbes, but below 250 m, they increased to 100%.…”

“…Throughout the gyre between 0-50 m depths, heterotrophs were up to 75-80% of all eukaryotic cells when measured directly by epifluorescence microscopy (Masquelier & Vaulot, 2008). In deeper euphotic zone communities, where photosynthetic cells dominated, dot-blot, clone library, and molecular characterization of cells sorted by flow cytometry found that phylogenetic groups that are usually mixotrophic, such as Haptophytes (Prymnesiophytes) or dinoflagellates, or that might contain mixotrophs (such as the Chrysophyceae among photosynthetic Heterokonts/Stramenopiles) were important or dominant among chlorophyll-containing cells (Lepère et al, 2009;Shi et al, 2009Shi et al, , 2011.…”