Justin E. Ossolinski

Viral Glycosphingolipids Induce Lytic Infection and Cell Death in Marine Phytoplankton

The Death of Cocco Emiliania huxleyi is a coccolithophore, a class of unicellular phytoplankton that forms vast blooms mediating the oceanic carbon cycle through shedding of its calcium carbonate scales. E. huxleyi is routinely infected and killed by lytic viruses that can abruptly halt a bloom. Vardi et al. (p. 861) have found that in E. huxleyi strains that are sensitive or resistant to infection, a sphingolipid-based “arms race” appears to regulate cell fate during host-virus interactions. The lipid also serves as a biomarker for active infection that may help to quantify the role and activity of viruses and virus-mediated processes in the oceans. This information will help in assessing the biogeochemical impact of these plankton species. A specific virus encodes membrane components that broadcast cell death and population demise of its coccolithophore host. 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.

The multiple fates of sinking particles in the North Atlantic Ocean

The direct respiration of sinking organic matter by attached bacteria is often invoked as the dominant sink for settling particles in the mesopelagic ocean. However, other processes, such as enzymatic solubilization and mechanical disaggregation, also contribute to particle flux attenuation by transferring organic matter to the water column. Here we use observations from the North Atlantic Ocean, coupled to sensitivity analyses of a simple model, to assess the relative importance of particle-attached microbial respiration compared to the other processes that can degrade sinking particles. The observed carbon fluxes, bacterial production rates, and respiration by water column and particle-attached microbial communities each spanned more than an order of magnitude. Rates of substrate-specific respiration on sinking particle material ranged from 0.007 ± 0.003 to 0.173 ± 0.105 day−1. A comparison of these substrate-specific respiration rates with model results suggested sinking particle material was transferred to the water column by various biological and mechanical processes nearly 3.5 times as fast as it was directly respired. This finding, coupled with strong metabolic demand imposed by measurements of water column respiration (729.3 ± 266.0 mg C m−2 d−1, on average, over the 50 to 150 m depth interval), suggested a large fraction of the organic matter evolved from sinking particles ultimately met its fate through subsequent remineralization in the water column. At three sites, we also measured very low bacterial growth efficiencies and large discrepancies between depth-integrated mesopelagic respiration and carbon inputs.

Zooplankton May Serve as Transmission Vectors for Viruses Infecting Algal Blooms in the Ocean

Marine viruses are recognized as a major driving force regulating phytoplankton community composition and nutrient cycling in the oceans. Yet, little is known about mechanisms that influence viral dispersal in aquatic systems, other than physical processes, and that lead to the rapid demise of large-scale algal blooms in the oceans. Here, we show that copepods, abundant migrating crustaceans that graze on phytoplankton, as well as other zooplankton can accumulate and mediate the transmission of viruses infecting Emiliania huxleyi, a bloom-forming coccolithophore that plays an important role in the carbon cycle. We detected by PCR that >80% of copepods collected during a North Atlantic E. huxleyi bloom carried E. huxleyi virus (EhV) DNA. We demonstrated by isolating a new infectious EhV strain from a copepod microbiome that these viruses are infectious. We further showed that EhVs can accumulate in high titers within zooplankton guts during feeding or can be adsorbed to their surface. Subsequently, EhV can be dispersed by detachment or via viral-dense fecal pellets over a period of 1 day postfeeding on EhV-infected algal cells, readily infecting new host populations. Intriguingly, the passage through zooplankton guts prolonged EhVs half-life of infectivity by 35%, relative to free virions in seawater, potentially enhancing viral transmission. We propose that zooplankton, swimming through topographically adjacent phytoplankton micropatches and migrating daily over large areas across physically separated water masses, can serve as viral vectors, boosting host-virus contact rates and potentially accelerating the demise of large-scale phytoplankton blooms.

Quantitative exploration of the contribution of settlement, growth, dispersal and grazing to the accumulation of natural marine biofilms on antifouling and fouling-release coatings

The accumulation of microbial biofilms on ships’ hulls negatively affects ship performance and efficiency while also playing a role in the establishment of even more detrimental hard-fouling communities. However, there is little quantitative information on how the accumulation rate of microbial biofilms is impacted by the balance of the rates of cell settlement, in situ production (ie growth), dispersal to surrounding waters and mortality induced by grazers. These rates were quantified on test panels coated with copper-based antifouling (AF) or polymer-based fouling-release (FR) coatings by using phospholipids as molecular proxies for microbial biomass. The results confirmed the accepted modes of efficacy of these two types of coatings. In a more extensive set of experiments with only the FR coatings, it was found that seasonally averaged cellular production rates were 1.5 ± 0.5 times greater than settlement and the dispersal rates were 2.7 ± 0.8 greater than grazing. The results of this study quantitatively describe the dynamic balance of processes leading to the accumulation of microbial biofilm on coatings designed for ships’ hulls.

Quorum Sensing Plays a Complex Role in Regulating the Enzyme Hydrolysis Activity of Microbes Associated with Sinking Particles in the Ocean

The concentration of atmospheric carbon dioxide is directly linked to the sinking of photosynthetically derived particulate organic carbon (POC) from surface waters to deep waters. This process, known as the marine biological carbon pump, removes carbon from exchange with the atmosphere, thus regulating global climate. Recent evidence suggests that microbial chemical communication systems (e.g. quorum sensing) amongst heterotrophic bacteria associated with sinking POC, significantly influences their hydrolytic enzyme activity and, as such, may affect the efficiency of the biological carbon pump. Here, we present data showing that a class of quorum sensing molecules, acylated homeserine lactones (AHLs) substantially impact hydrolytic phosphatase, aminopeptidase, and lipase activity in samples of sinking particles collected from the Atlantic and Pacific Ocean. Incubations of sinking particles amended with exogenous AHLs showed both stimulated and inhibited rates of activity after 24 hours of incubation, suggesting a critical link between bacterial AHL signaling mechanisms and the rate of POC degradation. Further experiments reveal that hydrolytic pathways could be affected within a few hours of amendment with AHLs, suggesting that microbial communities are able to dynamically modify their metabolic pathways in response to perceived quorum sensing. Finally, the concentration of the AHL amendment also affected hydrolytic activity. Our results indicated that quorum sensing has the potential to influence hydrolytic activity in a range of oceanic environments, but response also vary significantly as a function of the type of AHL, their concentration, and response time. AHL-based quorum sensing may be thought of as a global language among marine bacteria, but it is highly complex.

Coccolithovirus facilitation of carbon export in the North Atlantic

Marine phytoplankton account for approximately half of global primary productivity1, making their fate an important driver of the marine carbon cycle. Viruses are thought to recycle more than one-quarter of oceanic photosynthetically fixed organic carbon2, which can stimulate nutrient regeneration, primary production and upper ocean respiration2 via lytic infection and the ‘virus shunt’. Ultimately, this limits the trophic transfer of carbon and energy to both higher food webs and the deep ocean2. Using imagery taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite, along with a suite of diagnostic lipid- and gene-based molecular biomarkers, in situ optical sensors and sediment traps, we show that Coccolithovirus infections of mesoscale (~100 km) Emiliania huxleyi blooms in the North Atlantic are coupled with particle aggregation, high zooplankton grazing and greater downward vertical fluxes of both particulate organic and particulate inorganic carbon from the upper mixed layer. Our analyses captured blooms in different phases of infection (early, late and post) and revealed the highest export flux in ‘early-infected blooms’ with sinking particles being disproportionately enriched with infected cells and subsequently remineralized at depth in the mesopelagic. Our findings reveal viral infection as a previously unrecognized ecosystem process enhancing biological pump efficiency.Using a combination of remote-sensing technologies, lipidomics and gene-based biomarkers, the authors demonstrate a coupling between viral infection of an Emiliania huxleyi bloom and the export of organic and inorganic carbon from the photic zone.

Spatial and Temporal Robustness of Sr/Ca‐SST Calibrations in Red Sea Corals: Evidence for Influence of Mean Annual Temperature on Calibration Slopes

We thank the crew of the M/V Dream Island for help in the collection of the coral cores. This study was funded by the King Abdullah University of Science and Technology (KAUST Award Nos. USA 00002 and KSA 00011) and the U.S. National Science Foundation (NSF Award No. OCE-1031288) grants to K. Hughen. This research was also supported by the National Research Foundation Singapore and the Singapore Ministry of Education under the Research Centres of Excellence initiative and by the National Research Foundation Singapore under its Singapore NRF Fellowship scheme (National Research Fellowship Award to N. Goodkin, NRFF-2012-03). The coral Sr/Ca data generated in this study can be found in Table S5 and are archived at the NOAA NCDC World Data Center for Paleoclimatology at http://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets.

Diverse diazotrophs are present on sinking particles in the North Pacific Subtropical Gyre

Sinking particles transport carbon and nutrients from the surface ocean into the deep sea and are considered hot spots for bacterial diversity and activity. In the oligotrophic oceans, nitrogen (N2)-fixing organisms (diazotrophs) are an important source of new N but the extent to which these organisms are present and exported on sinking particles is not well known. Sinking particles were collected every 6 h over a 2-day period using net traps deployed at 150 m in the North Pacific Subtropical Gyre. The bacterial community and composition of diazotrophs associated with individual and bulk sinking particles was assessed using 16S rRNA and nifH gene amplicon sequencing. The bacterial community composition in bulk particles remained remarkably consistent throughout time and space while large variations of individually picked particles were observed. This difference suggests that unique biogeochemical conditions within individual particles may offer distinct ecological niches for specialized bacterial taxa. Compared to surrounding seawater, particle samples were enriched in different size classes of globally significant N2-fixing cyanobacteria including Trichodesmium, symbionts of diatoms, and the unicellular cyanobacteria Crocosphaera and UCYN-A. The particles also contained nifH gene sequences of diverse non-cyanobacterial diazotrophs suggesting that particles could be loci for N2 fixation by heterotrophic bacteria. The results demonstrate that diverse diazotrophs were present on particles and that new N may thereby be directly exported from surface waters on sinking particles.