Current sampling of genomic sequence data from eukaryotes is relatively poor, biased, and inadequate to address important questions about their biology, evolution, and ecology; this Community Page describes a resource of 700 transcriptomes from marine microbial eukaryotes to help understand their role in the world's oceans.

Marine viruses that infect phytoplankton are recognized as a major ecological and evolutionary driving force, shaping community structure and nutrient cycling in the marine environment. Little is known about the signal transduction pathways mediating viral infection. We show that viral glycosphingolipids regulate infection of Emiliania huxleyi, a cosmopolitan coccolithophore that plays a major role in the global carbon cycle. These sphingolipids derive from an unprecedented cluster of biosynthetic genes in Coccolithovirus genomes, are synthesized de novo during lytic infection, and are enriched in virion membranes. Purified glycosphingolipids induced biochemical hallmarks of programmed cell death in an uninfected host. These lipids were detected in coccolithophore populations in the North Atlantic, which highlights their potential as biomarkers for viral infection in the oceans.

We present experimental laboratory evidence and field observations of an autocatalyzed, programmed cell death (PCD) pathway in the nitrogen-fixing cyanobacterium Trichodesmium spp., which forms massive blooms in the subtropical and tropical oceans. The PCD pathway was induced in response to phosphorus and iron starvation as well as high irradiance and oxidative stress. Transmission electron microscopy revealed morphological degradation of internal components including thylakoids, carboxysomes, and gas vesicles, whereas the plasma membranes remained intact. Physiologically stressed cells displayed significantly elevated endonuclease activity and terminal d-UTP nick-end labeling. Nucleic acid degradation was concordant with increased immunoreactivity to human caspase-3 polyclonal antisera and enhanced cleavage of a caspase-specific substrate, DEVD. Caspase activity was positively correlated with mortality and was inhibited by the irreversible caspase inhibitor Z-VAD-FMK. A search of the Trichodesmium erythraeum genome identified several protein sequences containing a conserved caspase domain structure, including the histidine-and cysteine-containing catalytic diad found in true caspases, paracaspases, and metacaspases. Induction of PCD by caspase-like proteases in a bacterial photoautotroph with an ancient evolutionary history requires a reassessment about the origins and roles of cell death cascades. This process is a previously unappriciated mortality mechanism that can lead to the termination of natural Trichodesmium blooms and that can influence the fluxes of organic matter in the ocean.Planktonic marine cyanobacteria of the genus Trichodesmium form extensive blooms in the oligotrophic tropical and subtropical oceans, where they make significant contributions to global nitrogen fixation (Capone et al. 1997). Natural blooms and laboratory cultures of Trichodesmium often terminate abruptly, with cell lysis and biomass degradation occurring within 1-2 d (Ohki 1999). The mechanisms controlling the dramatic and abrupt termination of Trichodesmium blooms are not well understood, even though this termination drives nutrient flow and biogeochemical cycling of organic and inorganic matter produced by these organisms, including the redistribution of fixed nitrogen in the upper ocean and the flow of organic matter through ecosystem pathways like the grazer food chain, the microbial loop, and vertical sinking flux (Azam 1998).

letters to nature 508 NATURE | VOL 397 | 11 FEBRUARY 1999 | www.nature.com As shown in Fig. 3a, ®lms (III) and (IV) produced a small, negative g e that remains largely constant throughout the spectral region, supporting the assumption that the emission of Exalite 428 is characterized by a single dipole moment parallel to the long molecular axis. In sharp contrast to the typical CPPL behaviour outside the resonance region, ®lms (I) and (II) show emission in the resonance region, resulting in a large g e that undergoes handedness reversal in a single-handed chiral-nematic host. This is an observation inexplicable by CPPL theory, in contrast to emission outside the resonance region 13,15 . The effect of thickness on g e for ®lm (I) is assessed in Fig. 3b, revealing the general cross-over behaviour for all thicknesses. We found nearly pure right-handed photoluminescence, manifested as g e approaching -2, in the 400±420 nm wavelength range at a thickness of 35 mm. In addition, the single-photon counting technique was employed to determine the excited-state lifetime, t ex . For emission outside the resonance region, a constant t ex 0:66 6 0:02 ns across the emission spectrum was found. For emission inside the resonance region, we found t ex 0:67 6 0:02 ns. The lack of a prolonged t ex inside the resonance region alone seems to suggest the absence of microcavity processes.Well aligned solid ®lms with superior morphological stability hold promise for practical applications. Speci®c examples include colour image projection 10 and stereoscopic displays 11 that have been proposed on the basis of non-emitting chiral-nematic ®lms. Furthermore, embedding laser dyes in vitri®ed chiral-nematic ®lms has the potential to realize low-threshold distributed feedback lasers 17 , with a cost advantage over inorganic semiconductor lasers.M

Marine viruses are major evolutionary and biogeochemical drivers in marine microbial foodwebs. However, an in-depth understanding of the cellular mechanisms and the signal transduction pathways mediating host-virus interactions during natural bloom dynamics has remained elusive. We used field-based mesocosms to examine the "arms race" between natural populations of the coccolithophore Emiliania huxleyi and its double-stranded DNA-containing coccolithoviruses (EhVs). Specifically, we examined the dynamics of EhV infection and its regulation of cell fate over the course of bloom development and demise using a diverse suite of molecular tools and in situ fluorescent staining to target different levels of subcellular resolution. We demonstrate the concomitant induction of reactive oxygen species, caspase-specific activity, metacaspase expression, and programmed cell death in response to the accumulation of virus-derived glycosphingolipids upon infection of natural E. huxleyi populations. These subcellular responses to viral infection simultaneously resulted in the enhanced production of transparent exopolymer particles, which can facilitate aggregation and stimulate carbon flux. Our results not only corroborate the critical role for glycosphingolipids and programmed cell death in regulating E. huxleyi-EhV interactions, but also elucidate promising molecular biomarkers and lipid-based proxies for phytoplankton host-virus interactions in natural systems. P hytoplankton are the basis of marine foodwebs and are responsible for nearly half the global primary production (1). Although they grow rapidly and form massive blooms in ocean surface waters, phytoplankton cell fate is regulated by a suite of abiotic (e.g., nutrients and light availability) and biotic (e.g., grazers and viruses) interactions. The coccolithophore Emiliania huxleyi (Prymnesiophyceae, haptophyte) is a cosmopolitan unicellular photoautotroph that plays a prominent role in the marine carbon cycle. Its intricate calcite coccoliths account for ∼1/3 of the total marine CaCO 3 production (2), and it is a key producer of dimethylsulfide, a bioactive gas that plays a significant role in climate regulation by enhancing cloud formation (3).E. huxleyi forms massive annual spring blooms in the North Atlantic that have been shown to be routinely terminated by lytic, giant, double-stranded, DNA-containing (dsDNA) coccolithoviruses (EhVs) (4, 5). Viruses are the most abundant biological entities in aquatic environments and turn over more than a quarter of the photosynthetically fixed carbon, thereby fueling microbial foodwebs and short-circuiting carbon transfer to higher trophic levels and export to the deep sea (6, 7). However, very little is known about the molecular mechanisms mediating phytoplankton host-virus interactions and the associated regulation of cell fate. Consequently, we lack cellular biomarkers to constrain and quantify active viral infection, and this hinders our understanding of the role of viruses and virusmediated processes in the oceans.Coccol...

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algal blooms ͉ programmed cell death

International audienceDiatoms play a major role in carbon export from surface waters, but their role in the transport of carbon to the deep sea has been questioned by global analyses of sediment trap fluxes which suggest that organic carbon fluxes and transfer efficiencies through the mesopelagic are tightly correlated with CaCO3 ( Klaas and Archer, 2002; Franc, ois et al., 2002). Here we explore the role of diatoms in the biological pump through a study of Si and C interactions from the molecular to the global scale. Recent findings on molecular interactions between Si and C are reviewed. The roles of bacteria, grazers and aggregation are explored and combined, to account for the extent of Si and C decoupling between surface waters and 1000 m, observed to be very homogeneous in different biogeochemical provinces of the ocean. It is suggested that the mesopelagic food web plays a crucial role in this homogeneity: Sites of high export are also sites where diatom C is being either remineralized or channeled toward the long-lived carbon pool most efficiently in the mesopelagic zone. The amount of carbon participating in the biological pump but not collected in sediment traps remains to be explored. It is also demonstrated that statistical analyses performed at global scales hide spatial variability in carrying coefficients, indicating a clear need to understand the mechanisms that control spatial and temporal variations in the relative importance of ballast minerals and other export mechanisms such as particle dynamics