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

International audienceThe distribution of selected groups of micro-organisms 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), the transition region between the gyre and the coast of South America (moderately oligotrophic), and the Chile upwelling (eutrophic). The abundance of phycoerythrin containing picocyanobacteria, autotrophic and heterotrophic eukaryotes in different size ranges, dinoflagellates, and ciliates was determined by epifluorescence microscopy after DAPI staining. All populations reached a maximum in the Chile upwelling and a minimum near the centre of the gyre. Picocyanobacteria reached a maximum abundance of 70×10³ cell mL-1. In the HNLC zone, up to 50% of picocyanobacteria formed colonies. Autotrophic eukaryote and dinoflagellate abundance reached 24.5×10³ and 200 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

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

International audienceThe distribution of selected groups of micro-organisms 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 70 103 cell mL-1. Abundance of autotrophic eukaryotes and dinoflagellates reached 24.5 103 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

Introduction to the special section bio-optical and biogeochemical conditions in the South East Pacific in late 2004: the BIOSOPE program

International audienceThe objectives of the BIOSOPE (BIogeochemistry and Optics SOuth Pacific Experiment) project was to study, during the austral summer, the biological, biogeochemical and bio-optical properties of different trophic regimes in the South East Pacific: the eutrophic zone associated with the upwelling regime off the Chilean coast, the mesotrophic area associated with the plume of the Marquises Islands in the HNLC (High Nutrient Low Chlorophyll) waters of this subequatorial area, and the extremely oligotrophic area associated with the central part of the South Pacific Gyre (SPG). At the end of 2004, a 55-day international cruise with 32 scientists on board took place between Tahiti and Chile, crossing the SPG along a North-West South-East transect. This paper describes in detail the objectives of the BIOSOPE project, the implementation plan of the cruise, the main hydrological entities encountered along the ~8000 km South East Pacific transect, and ends with a general overview of the 32 other papers published in this special issue

Prochlorococcus and Synechococcus: A comparative study of their optical properties in relation to their size and pigmentation

Three unialgal strains of Prochlorococcus and four of Synechococcus were grown in batch culture at low irradiances. The spectral values of light absorption, scattering and backscattering of intact cells in suspension were determined, together with cell counts, size distribution and pigment composition (via HPLC). The spectral efficiency factors Qa, Qb, Qbb for light absorption, scattering and backscattering respectively, were derived, as well as the corresponding chlorophyll-specific coefficients a*, b* and bb*. The pigment used when normalizing is “true” chlorophyll a for Synechococcus, and divinyl-chlorophyll a for Prochlorococcus. In correspondence with small sizes (0.6 μm, on average) Prochlorococcus exhibits Qb values below those of Synechococcus (size about 0.9 μm, on average). In contrast, Qa is higher for Prochlorococcus than for Synechococcus, in response to high internal divinyl-chlorophyll content. In the blue part of the spectrum the probability for photons of being absorbed by a Prochlorococcus cell exceeds that of being scattered. Such a combination has never been found before for other algal cells, consistently more efficient as scatterers than as absorbers. The magnitude of the three efficiency Q-factors, as well as their spectral variations, can be understood and reconstructed in the frame of the Mie theory. The impact of these small organisms, dominant in oligotrophic environment, upon the optical properties of such waters are discussed on the basis of their chlorophyll-specific optical coefficients. Their absorption capabilities (per unit of chlorophyll) are not far from being maximum, to the extent that the package effect is rather reduced. With respect to scattering, Prochlorococcus cells have a minute signature compared to that of Synechococcus. This point is illustrated using vertical profiles of fluorescence, attenuation coefficient, cell number, Chl a and divinyl-Chl a concentrations, as observed in an oligotrophic tropical situation. Even if the backscattering-to-scattering ratio is, as theoretically expected, higher for Prochlorococcus than for all other algae (including Synechococcus), their light backscattering capacity definitely remains negligible

Correspondence

Effects of high light on transcripts of stress-associated genes for the cyanobacteri

Groups without cultured representatives dominate eukaryotic picophytoplankton in the oligotrophic South East Pacific Ocean

Background: Photosynthetic picoeukaryotes (PPE) with a cell size less than 3 µm play a critical role in oceanic primary production. In recent years, the composition of marine picoeukaryote communities has been intensively investigated by molecular approaches, but their photosynthetic fraction remains poorly characterized. This is largely because the classical approach that relies on constructing 18S rRNA gene clone libraries from filtered seawater samples using universal eukaryotic primers is heavily biased toward heterotrophs, especially alveolates and stramenopiles, despite the fact that autotrophic cells in general outnumber heterotrophic ones in the euphotic zone. Methodology/Principal Findings: In order to better assess the composition of the eukaryotic picophytoplankton in the South East Pacific Ocean, encompassing the most oligotrophic oceanic regions on earth, we used a novel approach based on flow cytometry sorting followed by construction of 18S rRNA gene clone libraries. This strategy dramatically increased the recovery of sequences from putative autotrophic groups. The composition of the PPE community appeared highly variable both vertically down the water column and horizontally across the South East Pacific Ocean. In the central gyre, uncultivated lineages dominated: a recently discovered clade of Prasinophyceae (IX), clades of marine Chrysophyceae and Haptophyta, the latter division containing a potentially new class besides Prymnesiophyceae and Pavlophyceae. In contrast, on the edge of the gyre and in the coastal Chilean upwelling, groups with cultivated representatives (Prasinophyceae clade VII and Mamiellales) dominated. Conclusions/Significance: Our data demonstrate that a very large fraction of the eukaryotic picophytoplankton still escapes cultivation. The use of flow cytometry sorting should prove very useful to better characterize specific plankton populations by molecular approaches such as gene cloning or metagenomics, and also to obtain into culture strains representative of these novel groups

Bolidophyceae, a Sister Picoplanktonic Group of Diatoms – A Review

Pico- and nano-phytoplankton (respectively, 0.2–2 and 2–20 μm in cell size) play a key role in many marine ecosystems. In this size range, Bolidophyceae is a group of eukaryotes that contains species with cells surrounded by 5 or 8 silica plates (Parmales) as well as naked flagellated species (formerly Bolidomonadales). Bolidophyceae share a common ancestor with diatoms, one of the most successful groups of phytoplankton. This review summarizes the current information on taxonomy, phylogeny, ecology, and physiology obtained by recent studies using a range of approaches including metabarcoding. Despite their rather small contribution to the phytoplankton communities (on average less than 0.1%), Bolidophyceae are very widespread throughout marine systems from the tropics to the pole. This review concludes by discussing similarities and differences between Bolidophyceae and diatoms

Explaining the causes of cell death in cyanobacteria: what role for asymmetric division?

Cyanobacteria contribute a significant fraction of global primary production and are therefore of great ecological significance. An individual cyanobacteria cell has four potential fates: to divide, perhaps after a dormant period, to be eaten, to undergo viral lysis, or to undergo cell death. In some studies, cyanobacteria cell death has been classified as programmed cell death, borrowing a concept more widely known in metazoan cells, and there are various biochemical parallels to support such a categorisation. However, at the same time there is a growing awareness of asymmetric division as a fundamental process in bacterial division which can result in non-equal daughter cells with differing fitness. Thanks to recent theoretical and experimental advances it is now possible to explore cyanobacteria cell death in the light of asymmetric division and to test hypotheses on the ultimate causes of cyanobacterial cell death. Assessing the degree of protein damage within individual cells during population growth is a sensible initial research target as is the application of techniques which allow the tracking of cell lineages. The existence of asymmetric division in cyanobacteria is likely given its suggested ubiquity across the bacterial domain of life. It will be technically difficult to test the interaction of asymmetric division with environmental variability, and how that leads to individual cell death via differing susceptibilities to environmental stress. However, testing such ideas could confirm asymmetric division as the ultimate cause of cell death in cyanobacteria and thereby allow a better understanding of the patterns of cell death in natural populations