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).

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...

It has long been recognized that a suite of proteins exists in coral skeletons that is critical for the oriented precipitation of calcium carbonate crystals, yet these proteins remain poorly characterized. Using liquid chromatography-tandem mass spectrometry analysis of proteins extracted from the cell-free skeleton of the hermatypic coral, Stylophora pistillata , combined with a draft genome assembly from the cnidarian host cells of the same species, we identified 36 coral skeletal organic matrix proteins. The proteome of the coral skeleton contains an assemblage of adhesion and structural proteins as well as two highly acidic proteins that may constitute a unique coral skeletal organic matrix protein subfamily. We compared the 36 skeletal organic matrix protein sequences to genome and transcriptome data from three other corals, three additional invertebrates, one vertebrate, and three single-celled organisms. This work represents a unique extensive proteomic analysis of biomineralization-related proteins in corals from which we identify a biomineralization “toolkit,” an organic scaffold upon which aragonite crystals can be deposited in specific orientations to form a phenotypically identifiable structure.

algal blooms ͉ programmed cell death