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Diatoms of the iron-replete continental margins and North Atlantic are key exporters of organic carbon. In contrast, diatoms of the iron-limited Antarctic Circumpolar Current sequester silicon, but comparatively little carbon, in the underlying deep ocean and sediments. Because the Southern Ocean is the major hub of oceanic nutrient distribution, selective silicon sequestration there limits diatom blooms elsewhere and consequently the biotic carbon sequestration potential of the entire ocean. We investigated this paradox in an in situ iron fertilization experiment by comparing accumulation and sinking of diatom populations inside and outside the iron-fertilized patch over 5 wk. A bloom comprising various thin-and thick-shelled diatom species developed inside the patch despite the presence of large grazer populations. After the third week, most of the thinner-shelled diatom species underwent mass mortality, formed large, mucous aggregates, and sank out en masse (carbon sinkers). In contrast, thicker-shelled species, in particular Fragilariopsis kerguelensis, persisted in the surface layers, sank mainly empty shells continuously, and reduced silicate concentrations to similar levels both inside and outside the patch (silica sinkers). These patterns imply that thick-shelled, hence grazer-protected, diatom species evolved in response to heavy copepod grazing pressure in the presence of an abundant silicate supply. The ecology of these silica-sinking species decouples silicon and carbon cycles in the iron-limited Southern Ocean, whereas carbon-sinking species, when stimulated by iron fertilization, export more carbon per silicon. Our results suggest that large-scale iron fertilization of the silicate-rich Southern Ocean will not change silicon sequestration but will add carbon to the sinking silica flux.evolutionary arms race | top-down control | geo-engineering

Among eukaryotes, four major phytoplankton lineages are responsible for marine photosynthesis; prymnesiophytes, alveolates, stramenopiles, and prasinophytes. Contributions by individual taxa, however, are not well known, and genomes have been analyzed from only the latter two lineages. Tiny "picoplanktonic" members of the prymnesiophyte lineage have long been inferred to be ecologically important but remain poorly characterized. Here, we examine pico-prymnesiophyte evolutionary history and ecology using cultivation-independent methods. 18S rRNA gene analysis showed picoprymnesiophytes belonged to broadly distributed uncultivated taxa. Therefore, we used targeted metagenomics to analyze uncultured pico-prymnesiophytes sorted by flow cytometry from subtropical North Atlantic waters. The data reveal a composite nuclear-encoded gene repertoire with strong green-lineage affiliations, which contrasts with the evolutionary history indicated by the plastid genome. Measured pico-prymnesiophyte growth rates were rapid in this region, resulting in primary production contributions similar to the cyanobacterium Prochlorococcus. On average, picoprymnesiophytes formed 25% of global picophytoplankton biomass, with differing contributions in five biogeographical provinces spanning tropical to subpolar systems. Elements likely contributing to success include high gene density and genes potentially involved in defense and nutrient uptake. Our findings have implications reaching beyond pico-prymnesiophytes, to the prasinophytes and stramenopiles. For example, prevalence of putative Ni-containing superoxide dismutases (SODs), instead of Fe-containing SODs, seems to be a common adaptation among eukaryotic phytoplankton for reducing Fe quotas in low-Fe modern oceans. Moreover, highly mosaic gene repertoires, although compositionally distinct for each major eukaryotic lineage, now seem to be an underlying facet of successful marine phytoplankton.comparative genomics | primary production | prymnesiophytes | marine photosynthesis | haptophytes G lobal primary production is partitioned equally among terrestrial and marine ecosystems, each accounting for ≈50 gigatons of carbon per year (1). The phytoplankton responsible for marine primary production include the cyanobacteria, Prochlorococcus and Synechococcus, and a multitude of eukaryotic phytoplankton, such as diatoms, dinoflagellates, prasinophytes, and prymnesiophytes (2-4). Most oceanic phytoplankton are "picoplanktonic" (<2-3 μm diameter) and have high surface area to volume ratios, an advantage in open-ocean low-nutrient conditions (5-8). Despite the importance of eukaryotic phytoplankton to carbon cycling only six genomes have been sequenced and analyzed comparatively, all being from diatoms and prasinophytes. These revealed greater differentiation than anticipated on the basis of 18S rRNA gene analyses (9-11). The observed genomic divergence is associated with major differences in physiology and niche adaptation (10).Pigment-based estimates indicate that prymnesiophytes, also know...

Analysis of marine cyanobacteria and proteobacteria genomes has provided a profound understanding of the life strategies of these organisms and their ecotype differentiation and metabolisms. However, a comparable analysis of the Bacteroidetes, the third major bacterioplankton group, is still lacking. In the present paper, we report on the genome of Polaribacter sp. strain MED152. On the one hand, MED152 contains a substantial number of genes for attachment to surfaces or particles, gliding motility, and polymer degradation. This agrees with the currently assumed life strategy of marine Bacteroidetes. On the other hand, it contains the proteorhodopsin gene, together with a remarkable suite of genes to sense and respond to light, which may provide a survival advantage in the nutrient-poor sun-lit ocean surface when in search of fresh particles to colonize. Furthermore, an increase in CO2 fixation in the light suggests that the limited central metabolism is complemented by anaplerotic inorganic carbon fixation. This is mediated by a unique combination of membrane transporters and carboxylases. This suggests a dual life strategy that, if confirmed experimentally, would be notably different from what is known of the two other main bacterial groups (the autotrophic cyanobacteria and the heterotrophic proteobacteria) in the surface oceans. The Polaribacter genome provides insights into the physiological capabilities of proteorhodopsin-containing bacteria. The genome will serve as a model to study the cellular and molecular processes in bacteria that express proteorhodopsin, their adaptation to the oceanic environment, and their role in carbon-cycling.Bacteroidetes ͉ marine bacteria ͉ whole-genome analysis ͉ heterotrophic CO2 fixation

The class Prasinophyceae (Chlorophyta) contains several photosynthetic picoeukaryotic species described from cultured isolates. The ecology of these organisms and their contributions to the picoeukaryotic community in aquatic ecosystems have received little consideration. We have designed and tested eight new 18S ribosomal DNA oligonucleotide probes specific for different Prasinophyceae clades, genera, and species. Using fluorescent in situ hybridization associated with tyramide signal amplification, these probes, along with more general probes, have been applied to samples from a marine coastal site off Roscoff (France) collected every 2 weeks between July 2000 and September 2001. The abundance of eukaryotic picoplankton remained high (>10 3 cells ml ؊1 ) during the sampling period, with maxima in summer (up to 2 ؋ 10 4 cells ml ؊1 ), and a single green algal species, Micromonas pusilla (Prasinophyceae), dominated the community all year round. Members of the order Prasinococcales and the species Bathycoccus prasinos (Mamiellales) displayed sporadic occurrences, while the abundances of all other Prasinophyceae groups targeted remained negligible.Several studies have demonstrated the importance of eukaryotic picoplankton (cell size, 0.2-to 3-m) in terms of biomass and productivity in the euphotic zone of oceanic oligotrophic waters (15), as well as in coastal waters (10). To date, only ϳ40 species belonging to nine algal classes (Chlorophyceae, Prasinophyceae, Trebouxiophyceae, Prymnesiophyceae, Bolidophyceae, Eustigmatophyceae, Pinguiophyceae, Bacillariophyceae, and Pelagophyceae) of photosynthetic picoplanktonic eukaryotes have been formerly described (41). However, phylogenetic analyses of sequences retrieved from natural samples in different oceanic regions have demonstrated much higher diversity, since many of these sequences do not correspond to any described taxa (19). The contributions of the different taxonomic groups to the picoplanktonic biomass, diversity, and ecology are poorly known because simple and reliable methods to detect and quantify such organisms in natural samples are lacking. Pigment signatures, scanning electron microscopy, and serial dilution cultures suggest that the classes Prasinophyceae (division Chlorophyta), Pelagophyceae (division Heterokontophyta), and Prymnesiophyceae are major components of the picoplankton biomass in different marine systems (20,35).Among these, the class Prasinophyceae contains several photosynthetic picoeukaryote species. This class is considered to be the most primitive in the green lineage and to have given rise to all other green algal classes, as well as to the land plants (34). Members are known to be common in temperate and cold regions and can occur as prominent constituents of marine picoplankton (38). Within these organisms, genera such as Ostreococcus, Bathycoccus, and Micromonas have been described in coastal waters (4b, 6). Micromonas pusilla (the only described species in the genus Micromonas) has been identified as a major component of the pic...

We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in cold, temperate, and warm waters of the world's oceans. Our data set covers all combinations of temperature and resource availability, thus allowing us to disentangle their effects. The partitioning of biomass between different size classes is independent of temperature, but depends strongly on the rate of resource use as reflected in the rate of primary production. Temperature and primary production explained 2% and 62%, respectively, of the variability in the contribution of microphytoplankton to total biomass. This contribution increases rapidly with total biomass and productivity, reaching values . 80% when chlorophyll a concentration is . 2 mg L 21 or primary production is . 100 mg C L 21 d 21 , irrespective of water temperature. Conversely, picophytoplankton contribution is substantial (. 40%), at all temperatures, only when chlorophyll a concentration is , 1 mg L 21 or primary production is , 50 mg C L 21 d 21 . The temperature-size rule cannot explain these changes, which instead reflect fundamental reorganizations in the species composition of the assemblage, arising from taxon-and sizedependent differences in resource acquisition and use. Given that resource availability, rather than temperature per se, is the key factor explaining the relative success of different algal size classes, there will be no single, universal effect of global warming on phytoplankton size structure.

Dynamics of biological processes on the deep-sea floor are traditionally thought to be controlled by vertical sinking of particles from the euphotic zone at a seasonal scale. However, little is known about the influence of lateral particle transport from continental margins to deep-sea ecosystems. To address this question, we report here how the formation of dense shelf waters and their subsequent downslope cascade, a climate induced phenomenon, affects the population of the deep-sea shrimp Aristeus antennatus. We found evidence that strong currents associated with intense cascading events correlates with the disappearance of this species from its fishing grounds, producing a temporary fishery collapse. Despite this initial negative effect, landings increase between 3 and 5 years after these major events, preceded by an increase of juveniles. The transport of particulate organic matter associated with cascading appears to enhance the recruitment of this deep-sea living resource, apparently mitigating the general trend of overexploitation. Because cascade of dense water from continental shelves is a global phenomenon, we anticipate that its influence on deep-sea ecosystems and fisheries worldwide should be larger than previously thought.

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