First Viruses Infecting the Marine Diatom Guinardia delicatula

The marine diatom Guinardia delicatula is a cosmopolitan species that dominates seasonal blooms in the English Channel and the North Sea. Several eukaryotic parasites are known to induce the mortality of this species. Here, we report the isolation and characterization of the first viruses that infect G. delicatula. Viruses were isolated from the Western English Channel (SOMLIT-Astan station) during the late summer bloom decline of G. delicatula. A combination of laboratory approaches revealed that these lytic viruses (GdelRNAV) are small tailless particles of 35–38 nm in diameter that replicate in the host cytoplasm where both unordered particles and crystalline arrays are formed. GdelRNAV display a linear single-stranded RNA genome of ~9 kb, including two open reading frames encoding for replication and structural polyproteins. Phylogenetic relationships based on the RNA-dependent-RNA-polymerase gene marker showed that GdelRNAV are new members of the Bacillarnavirus, a monophyletic genus belonging to the order Picornavirales. GdelRNAV are specific to several strains of G. delicatula. They were rapidly and largely produced (<12 h, 9.34 × 104 virions per host cell). Our analysis points out the host's variable viral susceptibilities during the early exponential growth phase. Interestingly, we consistently failed to isolate viruses during spring and early summer while G. delicatula developed important blooms. While our study suggests that viruses do contribute to the decline of G. delicatula's late summer bloom, they may not be the primary mortality agents during the remaining blooms at SOMLIT-Astan. Future studies should focus on the relative contribution of the viral and eukaryotic pathogens to the control of Guinardia's blooms to understand the fate of these prominent organisms in marine systems

Comparative thermophysiology of marine synechococcus CRD1 strains isolated from different thermal niches in iron-depleted areas

Marine Synechococcus cyanobacteria are ubiquitous in the ocean, a feature likely related to their extensive genetic diversity. Amongst the major lineages, clades I and IV preferentially thrive in temperate and cold, nutrient-rich waters, whilst clades II and III prefer warm, nitrogen or phosphorus-depleted waters. The existence of such cold (I/IV) and warm (II/III) thermotypes is corroborated by physiological characterization of representative strains. A fifth clade, CRD1, was recently shown to dominate the Synechococcus community in iron-depleted areas of the world ocean and to encompass three distinct ecologically significant taxonomic units (ESTUs CRD1A-C) occupying different thermal niches, suggesting that distinct thermotypes could also occur within this clade. Here, using comparative thermophysiology of strains representative of these three CRD1 ESTUs we show that the CRD1A strain MITS9220 is a warm thermotype, the CRD1B strain BIOS-U3-1 a cold temperate thermotype, and the CRD1C strain BIOS-E4-1 a warm temperate stenotherm. Curiously, the CRD1B thermotype lacks traits and/or genomic features typical of cold thermotypes. In contrast, we found specific physiological traits of the CRD1 strains compared to their clade I, II, III, and IV counterparts, including a lower growth rate and photosystem II maximal quantum yield at most temperatures and a higher turnover rate of the D1 protein. Together, our data suggests that the CRD1 clade prioritizes adaptation to low-iron conditions over temperature adaptation, even though the occurrence of several CRD1 thermotypes likely explains why the CRD1 clade as a whole occupies most iron-limited waters

Seasonal dynamics of a glycan‐degrading flavobacterial genus in a tidally mixed coastal temperate habitat

International audienceCoastal marine habitats constitute hotspots of primary productivity. In temperate regions, this is due both to massive phytoplankton blooms and dense colonisation by macroalgae that mostly store carbon as glycans, contributing substantially to local and global carbon sequestration. Because they control carbon and energy fluxes, algae‐degrading microorganisms are crucial for coastal ecosystem functions. Environmental surveys revealed consistent seasonal dynamics of alga‐associated bacterial assemblages, yet resolving what factors regulate the in situ abundance, growth rate and ecological functions of individual taxa remains a challenge. Here, we specifically investigated the seasonal dynamics of abundance and activity for a well‐known alga‐degrading marine flavobacterial genus in a tidally mixed coastal habitat of the Western English Channel. We show that members of the genus Zobellia are a stable, low‐abundance component of healthy macroalgal microbiota and can also colonise particles in the water column. This genus undergoes recurring seasonal variations with higher abundances in winter, significantly associated to biotic and abiotic variables. Zobellia can become a dominant part of bacterial communities on decaying macroalgae, showing a strong activity and high estimated in situ growth rates. These results provide insights into the seasonal dynamics and environmental constraints driving natural populations of alga‐degrading bacteria that influence coastal carbon cycling

An annotated checklist of Marine Phytoplankton taxa at the SOMLIT-Astan time series off Roscoff (Western English Channel, France): data collected from 2000 to 2010

International audienceA checklist of micro-phytoplankton. taxa based on net tow and Niskin bottle samples taken twice a month during the periods 2000-2003 and 2006-2010 at station SOMLIT-Astan (north of Roscoff, Western English Channel, France) is presented. SOMLIT-Astan is a coastal long-term monitoring station. It was established off Roscoff, where the water column seldom becomes stratified, and where continental influence is limited. Taxonomic identification was done based on light microscopy observations. The checklist includes 178 taxa (genus or species) among which 70 genera and 131 species of diatoms are recorded. Diatoms with benthic affinities make up 51% of the list of diatoms genera identified. Guinardia (especially G. delicatula) and Paralia sulcata appear as key taxa, becoming dominant in spring/summer and winter, respectively. Dinoflagellates are less diversified and never dominate. This work although not exhaustive, provides a reference list for micro-phytoplankton off Roscoff, and more generally for the permanently mixed waters of the Western English Channel, as well as information on the most common and/or abundant taxa in this habitat

Diversity and dynamics of relevant nanoplanktonic diatoms in the Western English Channel

International audienceIn the ocean, Bacillariophyta are one of the most successful protistan groups. Due to their considerable biogeochemical implications, diatom diversity, development, and seasonality have been at the center of research, specifically large-sized species. In comparison, nanoplanktonic diatoms are mostly disregarded from routine monitoring and are often underrepresented in genetic reference databases. Here, we identified and investigated the temporal dynamics of relevant nanodiatoms occurring in the Western English Channel (SOMLIT-Astan station). Coupling in situ and laboratory approaches, we revealed that nano-species from the genera Minidiscus and Thalassiosira are key components of the phytoplankton community that thrive in these coastal waters, but they display different seasonal patterns. Some species formed recurrent blooms whilst others were persistent year round. These results raise questions about their regulation in the natural environment. Over a full seasonal cycle at the monitoring station, we succeeded in isolating viruses which infect these minute diatoms, suggesting that these mortality agents may contribute to their control. Overall, our study points out the importance of considering nanodiatom communities within time-series surveys to further understand their role and fate in marine systems

Coupling between taxonomic and functional diversity in protistan coastal communities

The study of protistan functional diversity is crucial to understand the dynamics of oceanic ecological processes. We combined the metabarcoding data of various coastal ecosystems and a newly developed trait‐based approach to study the link between taxonomic and functional diversity across marine protistan communities of different size‐classes. Environmental DNA was extracted and the V4 18S rDNA genomic region was amplified and sequenced. In parallel, we tried to annotate the Operational Taxonomic Units (OTUs) from our metabarcoding dataset to 30 biological traits using published and accessible information on protists. We then developed a method to study trait correlations across protists (i.e. trade‐offs) in order to build the best functional groups. Based on the annotated OTUs and our functional groups, we demonstrated that the functional diversity of marine protist communities varied in parallel with their taxonomic diversity. The coupling between functional and taxonomic diversity was conserved across different protist size classes. However, the smallest size‐fraction was characterized by wider taxonomic and functional groups diversity, corroborating the idea that nano‐ and pico‐plankton are part of a more stable ecological background on which larger protists and metazoans might develop

Picoeukaryotes of the Micromonas genus: sentinels of a warming ocean

International audiencePhotosynthetic picoeukaryotes in the genus Micromonas show among the widest latitudinal distributions on Earth, experiencing large thermal gradients from poles to tropics. Micromonas comprises at least four different species often found in sympatry. While such ubiquity might suggest a wide thermal niche, the temperature response of the different strains is still unexplored, leaving many questions as for their ecological success over such diverse ecosystems. Using combined experiments and theory, we characterize the thermal response of eleven Micromonas strains belonging to four 1 species. We demonstrate that the variety of specific responses to temperature in the Micromonas genus makes this environmental factor an ideal marker to describe its global distribution and diversity. We then propose a diversity model for the genus Micromonas, which proves to be representative of the whole phytoplankton diversity. This prominent primary producer is therefore a sentinel organism of phytoplankton diversity at the global scale. We use the diversity within Micromonas to anticipate the potential impact of global warming on oceanic phytoplankton. We develop a dynamic, adaptive model and ran forecast simulations, exploring a range of adaptation time scales, to probe the likely responses to climate change. Results stress how biodiversity erosion depends on the ability of organisms to adapt rapidly to temperature increase

Impact de la température sur les interactions entre la microalgue marine Micromonas et les virus qui l’infectent

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Seasonal temporal dynamics of marine protists communities in tidally mixed coastal waters

Major seasonal community reorganizations and associated biomass variations are landmarksof plankton ecology. However, the processes determining marine species and communityturnover rates have not been fully elucidated so far. Here, we analyse patterns of planktonicprotist community succession in temperate latitudes, based on quantitative taxonomic datafrom both microscopy counts and ribosomal DNA metabarcoding from plankton samplescollected biweekly over 8 years (2009-2016) at the SOMLIT-Astan station (Roscoff, WesternEnglish Channel). Considering the temporal structure of community dynamics (creatingtemporal correlation), we elucidated the recurrent seasonal pattern of the dominant speciesand OTUs (rDNA-derived taxa) that drive annual plankton successions. The use ofmorphological and molecular analyses in combination allowed us to assess absolute speciesabundance while improving taxonomic resolution, and revealed a greater diversity. Overall,our results underpinned a protist community characterised by a seasonal structure, which issupported by the dominant OTUs. We detected that some were partly benthic as a result ofthe intense tidal mixing typical of the French coasts in the English Channel. While theoccurrence of these microorganisms is driven by the physical and biogeochemical conditionsof the environment, internal community processes, such as the complex network of bioticinteractions, also play a key role in shaping protist communities