Diatoms are one of the major primary producers in the ocean, responsible annually for ~20% of photosynthetically fixed CO2 on Earth. In oceanic models, they are typically represented as large (>20 µm) microphytoplankton. However, many diatoms belong to the nanophytoplankton (2-20 µm) and a few species even overlap with the picoplanktonic size-class (<2 µm). Due to their minute size and difficulty of detection they are poorly characterized. Here we describe a massive spring bloom of the smallest known diatom (Minidiscus) in the northwestern Mediterranean Sea. Analysis of Tara Oceans data, together with literature review, reveal a general oversight of the significance of these small diatoms at the global scale. We further evidence that they can reach the seafloor at high sinking rates, implying the need to revise our classical binary vision of pico- and nanoplanktonic cells fueling the microbial loop, while only microphytoplankton sustain secondary trophic levels and carbon export.

https://hal.archives-ouvertes.fr/hal-02006275/document

Nanoplanktonic diatoms are globally overlooked but play a role in spring blooms and carbon export.

Background: Photosynthetic picoeukaryotes (PPE) with a cell size less than 3 mm 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.

/pdf/groups-without-cultured-representatives-dominate-eukaryotic-4et1m62off.pdf

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

A small siliceous species of marine phytoplankton, order Parmales (Heterokonta), was isolated and characterized for the first time with the aid of a fluorescent silicon tracer 2-(4-pyridyl)-5-([4-(2-dimethylaminoethylaminocarbamoyl)-methoxy]phenyl)oxazole (PDMPO). This dye was easily detected by clear fluorescence in newly produced silica cell plates. Our isolate was surrounded by eight smooth plates without any ornamentation, suggesting a similarity to Triparma laevis B. C. Booth. TEM observation showed the typical ultrastructure of photosynthetic heterokontophytes; with two chloroplast endoplasmic reticulate membranes, a girdle lamella, three thylakoid lamellae, and mitochondrion with tubular cristae. Molecular phylogenetic analyses of SSU rDNA and rbcL genes showed that the parmalean alga was within the bolidophycean clade of autotrophic naked flagellates and a sister group of diatoms. HPLC analysis detected chl a, c1  + c2 , and c3 ; fucoxanthin; and diadinoxanthin as major photosynthetic pigments, and a composition that is shared with Bolidophyceae and diatoms. Together, these data indicate a close evolutionary relationship between Parmales, Bolidophyceae, and diatoms. The PDMPO-staining procedure should accelerate isolation of other Parmales species, helping to establish their diversity and aiding quantitative study of their role in oceanic processes.

Isolation and characterization of parmales (heterokonta/heterokontophyta/stramenopiles) from the oyashio region, western north pacific(1).

This paper serves as an introduction to the Bering Sea, preceding the 14 contributed papers contained in this volume. Several parts of the paper are allocated for current reviews of the Bering Sea. Included in the review topics are geomorphology, surface water circulation, and the importance of the formation of intermediate waters. Representative results of previous paleoceanographic studies are also presented, in particular focusing on spatial and temporal distributions of biosiliceous microfossils as well as isotopic analyses of carbon and nitrogen concerning changes in productivity and nutrients. Motivations behind the scientific expeditions conducted, which resulted in the many papers included here, are briefly discussed in order to illustrate the recent trend in paleoceanography in the Bering Sea and the adjacent regions. Brief summaries or highlights of each of the contributed papers are given in order to provide an overview of the volume.

The Bering Sea and paleoceanography

Modern distribution of dinocysts from the North Pacific Ocean (37-64°N, 144°E-148°W) in relation to hydrographic conditions, sea-ice and productivity

The microplankton communities on filter samples from three shallow (0–300 m) hydrocasts, conducted in August 1999, January 2000, and May 2000 at Station KNOT in the Western Subarctic Gyre of the NW Pacific, have been analyzed by scanning electron microscopy (SEM). The results clearly show that small (o10mm) phytoplankton, such as coccolithophorids, Parmales, and the pennate diatom Fragilariopsis pseudonana, play important roles in seasonal productivity. The Parmales were the most dominant mineralized phytoplankton group in May and August, and their yearround presence in scaly form suggests they are not cysts as previously thought. As the Parmales are rare or absent in the underlying sediment traps or surface sediments, their siliceous scales are undoubtably being efficiently recycled in the euphotic zone where silica levels are never depleted. The phytoplankton community structure observed by SEM is different to those reported previously, in which more traditional light microscopical techniques were used, with the consequence of missing the smaller phytoplankton. This study, regardless of its biogeographic location, highlights the potential problem of enumerating marine phytoplankton assemblages purely by light microscopy. An inability to determine the contribution made by these small phytoplankton can lead to an overestimation of the importance of large phytoplankton, and an underestimation of total productivity. r 2005 Elsevier Ltd. All rights reserved.

Microplankton assemblages at Station KNOT in the subarctic western Pacific, 1999–2000

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Microplankton assemblages at Station KNOT in the subarctic western Pacific, 1999–2000