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...

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It is generally agreed that the origin and initial diversification of Eucarya occurred in the late Archaean or Proterozoic Eons when atmospheric oxygen levels were low and the risk of DNA damage due to ultraviolet radiation was high. Because deep water provides refuge against ultraviolet radiation and early eukaryotes may have been aerotolerant anaerobes, deep-water dysoxic environments are likely settings for primeval eukaryotic diversification. Fossil evidence shows that deep-sea microbial mats, possibly of sulphur bacteria similar to Beggiatoa, existed during that time. Here we report on the eukaryotic community of a modern analogue, the Santa Barbara Basin (California, USA). The Beggiatoa mats of these severely dysoxic and sulphidic sediments support a surprisingly abundant protistan and metazoan meiofaunal community, most members of which harbour prokaryotic symbionts. Many of these taxa are new to science, and both microaerophilic and anaerobic taxa appear to be represented. Compared with nearby aerated sites, the Santa Barbara Basin is a 'symbiosis oasis' offering a new source of organisms for testing symbiosis hypotheses of eukaryogenesis.

Deep-sea Bathymodiolus mussels, depending on species and location, have the capacity to host sulfuroxidizing (thiotrophic) and methanotrophic eubacteria in gill bacteriocytes, although little is known about the mussels' mode of symbiont acquisition. Previous studies of Bathymodiolus host and symbiont relationships have been based on collections of nonoverlapping species across wide-ranging geographic settings, creating an apparent model for vertical transmission. We present genetic and cytological evidence for the environmental acquisition of thiotrophic endosymbionts by vent mussels from the Mid-Atlantic Ridge. Open pit structures in cell membranes of the gill surface revealed likely sites for endocytosis of free-living bacteria. A population genetic analysis of the thiotrophic symbionts exploited a hybrid zone where two Bathymodiolus species intergrade. Northern Bathymodiolus azoricus and southern Bathymodiolus puteoserpentis possess species-specific DNA sequences that identify both their symbiont strains (internal transcribed spacer regions) and their mitochondria (ND4). However, the northern and southern symbiont-mitochondrial pairs were decoupled in the hybrid zone. Such decoupling of symbiont-mitochondrial pairs would not occur if the two elements were transmitted strictly vertically through the germ line. Taken together, these findings are consistent with an environmental source of thiotrophic symbionts in Bathymodiolus mussels, although an environmentally "leaky" system of vertical transmission could not be excluded.

Oceanic dimethylsulfide (DMS) emissions to the atmosphere are potentially important to the Earth's radiative balance. Since these emissions are driven by the surface seawater concentration of DMS, it is important to understand the processes controlling the cycling of sulfur in surface seawater. During the third Pacific Sulfur/Stratus Investigation (PSI-3, April 1991) we measured the major sulfur reservoirs (total organic sulfur, total low molecular weight organic sulfur, ester sulfate, protein sulfur, dimethylsulfoniopropionate (DMSP), DMS, dimethylsulfoxide) and quantified many of the processes that cycle sulfur through the upper water column (sulfate assimilation, DMSP consumption, DMS production and consumption, air-sea exchange of DMS, loss of organic sulfur by particulate sinking). Under conditions of low plankton biomass (<0.4 gg/L chlorophyll a) and high nutrient concentrations (>8 tam nitrate), 250 km off the Washington State coast, DMSP and DMS were 22% and 0.9%, respectively, of the total particulate organic sulfur pool. DMS production from the enzymatic cleavage of DMSP accounted for 29% of the total sulfate assimilation. However, only 0.3% of sulfate-S assimilated was released to the atmosphere. From these data it is evident that air-sea exchange is currently only a minor sink in the seawater sulfur cycle and thus there is the potential for much higher DMS emissions under different climatic conditions. Introduction Oceanic dimethylsulfide (DMS) is currently thought to be the major natural source of sulfur to the atmosphere [Bates et al., 1992; Spiro et al., 1992]. Once in the atmosphere, DMS is oxidized to produce aerosol particles which affect the acidbase chemistry of the atmosphere [Charlson and Rodhe, 1982] and the radiative properties of marine stratus clouds [Charlson et al., 1987; Falkowski et al., 1992]. This latter effect is calculated to have a major impact on the Earth's radiative balance and hence its climate [Charlson et al., 1987]. The starting point in the marine atmospheric sulfur cycle is the air-sea exchange of DMS which is a function of the gas i NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington. transfer velocity and surface seawater DMS concentration. The gas transfer velocity is controlled primarily by surface turbulence, seawater temperature and gas diffusivity and can be modeled as a function of wind speed for various trace gases [Liss and Merlivat, 1986; Wanninkhof, 1992]. The different T. S. Bates, NOAA/Pacific Marine