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The genus M icromonas comprises distinct genetic clades that commonly dominate eukaryotic phytoplankton community from polar to tropical waters. This phytoplankter is also recurrently infected by abundant and genetically diverse prasinoviruses. Here we report on the interplay between prasinoviruses and M icromonas with regard to the genetic diversity of this host. For 1 year, we monitored the abundance of three clades of M icromonas and their viruses in the Western English Channel, both in the environment using clade-specific probes and flow cytometry, and in the laboratory using clonal strains of M icromonas clades to assay for their viruses by plaque-forming units. We showed that the seasonal fluctuations of M icromonas clades were closely mirrored by the abundance of their corresponding viruses, indicating that the members of M icromonas genus are susceptible to viral infection, regardless of their genetic affiliation. The characterization of 45 viral isolates revealed that M icromonas clades are attacked by specific virus populations, which exhibit distinctive clade specificity, life strategies and genetic diversity. However, some viruses can also cross-infect different host clades, suggesting a mechanism of horizontal gene transfer within the M icromonas genus. This study provides novel insights into the impact of viral infection for the ecology and evolution of the prominent phytoplankter M icromonas. [ABSTRACT FROM AUTHOR]

In late 2004, the BIOSOPE cruise sailed between the equatorial influenced waters off Marquesas islands and the nutrient enriched waters of the Chilean upwelling. Along the way, it explored the Southeast Pacific gyre centred around Easter Island, which is probably the most oligotrophic oceanic region on earth. During this cruise, we under-took a vigorous effort to isolate novel photosynthetic picoplanktonic eukaryotes. Two strategies were attempted on board: enrichment of samples with culture medium and sorting of specific populations by flow cytometry based on chlorophyll fluorescence. Over 1900 pre-cultures were started and then further purified by flow cytometry, serial dilution or pipette isolation to yield a total of 212 strains. These strains were characterized morphologically and for more than 50% of them, genetically, through partial sequencing of the 18 S rRNA gene. Among the characterized strains, the largest number are stramenopiles (Heterokontophyta) with a record of 38 strains belonging to the species Pelagomonas calceolata (Pelagophyceae). Strains from the recently described genera Bolidomonas and Florenciella have been re-isolated for the first time since their description. Two other abundant groups are the Chlorophyta, especially Prasinophyceae, and the Haptophyta, especially the genera Phaeocystis and Emiliania. A limited number of heterotrophic flagellates have also been isolated, all of them closely related to known species. Finally over a dozen of unicellular cyanobacteria strains have been obtained, some forming unusual short chains. Overall our strategy was quite successful since it allowed us to isolate a large number of picoplankton strains but failed in two respects. First, apparently very few novel taxa have been obtained. One set of strains is related to Prasinoderma coloniale (Prasinococcales, Prasinophyceae) but their sequences are sufficiently different from the latter to probably belong to a new genus or species. The sequences of two other strains are phylogenetically affiliated to stramenopile environmental sequences, probably corresponding a new algal class. Second, very few strains have been obtained from the very oligotrophic central gyre itself. Future work should probably combine flow cytometry sorting with culture media and cultivation approaches specifically developed for oligotrophic water species. [ABSTRACT FROM AUTHOR]

Coastal 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., (© 2023 The Authors. Environmental Microbiology published by Applied Microbiology International and John Wiley & Sons Ltd.)

Major seasonal community reorganizations and associated biomass variations are landmarks of plankton ecology. However, the processes of plankton community turnover rates have not been fully elucidated so far. Here, we analyse patterns of planktonic protist community succession in temperate latitudes, based on quantitative taxonomic data from both microscopy counts (cells >10 μm) and ribosomal DNA metabarcoding (size fraction >3 μm, 18S rRNA gene) from plankton samples collected bimonthly over 8 years (2009-2016) at the SOMLIT-Astan station (Roscoff, Western English Channel). Based on morphology, diatoms were clearly the dominating group all year round and over the study period. Metabarcoding uncovered a wider diversity spectrum and revealed the prevalence of Dinophyceae and diatoms but also of Cryptophyta, Chlorophyta, Cercozoa, Syndiniales and Ciliophora in terms of read counts and or richness. The use of morphological and molecular analyses in combination allowed improving the taxonomic resolution and to identify the sequence of the dominant species and OTUs (18S V4 rDNA-derived taxa) that drive annual plankton successions. We detected that some of these dominant OTUs were benthic as a result of the intense tidal mixing typical of the French coasts in the English Channel. Our analysis of the temporal structure of community changes point to a strong seasonality and resilience. The temporal structure of environmental variables (especially Photosynthetic Active Radiation, temperature and macronutrients) and temporal structures generated by species life cycles and or species interactions, are key drivers of the observed cyclic annual plankton turnover., (© 2022 The Authors. Molecular Ecology published by John Wiley & Sons Ltd.)

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., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Ferrieux, Dufour, Doré, Ratin, Guéneuguès, Chasselin, Marie, Rigaut-Jalabert, Le Gall, Sciandra, Monier, Hoebeke, Corre, Xia, Liu, Scanlan, Partensky and Garczarek.)

In 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.

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 nanoplankton and picoplankton are part of a more stable ecological background on which larger protists and metazoans might develop., (© 2019 Society for Applied Microbiology and John Wiley & Sons Ltd.)

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 × 10 4 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.

Photosynthetic picoeukaryotesx 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 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 run 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.

The genus Micromonas comprises phytoplankton that show among the widest latitudinal distributions on Earth, and members of this genus are recurrently infected by prasinoviruses in contrasted thermal ecosystems. In this study, we assessed how temperature influences the interplay between the main genetic clades of this prominent microalga and their viruses. The growth of three Micromonas strains (Mic-A, Mic-B, Mic-C) and the stability of their respective lytic viruses (MicV-A, MicV-B, MicV-C) were measured over a thermal range of 4-32.5 °C. Similar growth temperature optima (T opt ) were predicted for all three hosts but Mic-B exhibited a broader thermal tolerance than Mic-A and Mic-C, suggesting distinct thermoacclimation strategies. Similarly, the MicV-C virus displayed a remarkable thermal stability compared with MicV-A and MicV-B. Despite these divergences, infection dynamics showed that temperatures below T opt lengthened lytic cycle kinetics and reduced viral yield and, notably, that infection at temperatures above T opt did not usually result in cell lysis. Two mechanisms operated depending on the temperature and the biological system. Hosts either prevented the production of viral progeny or maintained their ability to produce virions with no apparent cell lysis, pointing to a possible switch in the viral life strategy. Hence, temperature changes critically affect the outcome of Micromonas infection and have implications for ocean biogeochemistry and evolution.

Preservation of cells, choice of fixative, storage, and thawing conditions are recurrent issues for the analysis of phytoplankton by flow cytometry. We examined the effects of addition of the surfactant Pluronic F68 to glutaraldehyde-fixed photosynthetic organisms in cultures and natural samples. In particular, we examined cell losses and modifications of side scatter (a proxy of cell size) and fluorescence of natural pigments. We found that different marine phytoplankton species react differently to the action of Pluronic F68. In particular, photosynthetic prokaryotes are less sensitive than eukaryotes. Observed cell losses may result from cell lysis or from cell adhesion to the walls of plastic tubes that are commonly used for flow cytometry analysis. The addition of the surfactant, Pluronic F68, has a positive effect on cells for long-term storage. We recommend to modify current protocols for preservation of natural marine planktonic samples, by fixing them with glutaraldehyde 0.25% (final concentration) and adding Pluronic F68 at a final concentration of 0.01% in the samples before preservation. Pluronic F68 also appears effective for preserving samples without fixation for subsequent sorting, e.g. for molecular biology analyses. © 2014 International Society for Advancement of Cytometry., (© 2014 International Society for Advancement of Cytometry.)

Small photosynthetic eukaryotes are key primary producers in marine waters. In recent years, their diversity has been studied by the analysis of 18S rRNA gene sequences directly amplified and cloned from filtered natural samples. However, these clone libraries are often dominated by nonphotosynthetic organisms and few sequences from autotrophs are recovered. In the present paper, we developed a new approach based on flow cytometry. Photosynthetic pico-, nano- and phycoerythrin-containing (PE-) eukaryotes from the coastal English Channel were sorted based on their size and pigment fluorescence. 18S rRNA gene libraries were constructed from the DNA of sorted cells. We addressed methodological issues linked to the relatively low concentration of these cells. This novel approach confirmed that, in the English Channel, pico-eukaryotes are dominated by three genera Micromonas, Ostreococcus and Bathycoccus, while PE-eukaryotes are mainly cryptophytes from clade 4. It also revealed that nano-eukaryotes are dominated by haptophytes with important contributions from small diatoms and Prasinophyceae. It should be emphasized that haptophytes were nearly absent from clone libraries constructed from filtered samples, which explains why they have been overlooked in previous studies. The new strategy should be very useful to conduct similar studies on other specific populations that can be discriminated by flow cytometry (e.g. red tide organisms or uncultivated protists).