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The marine picoeukaryote Bathycoccus prasinos has been considered a cosmopolitan alga, although recent studies indicate two ecotypes exist, Clade BI (B. prasinos) and Clade BII. Viruses that infect Bathycoccus Clade BI are known (BpVs), but not that infect BII. We isolated three dsDNA prasinoviruses from the Sargasso Sea against Clade BII isolate RCC716. The BII-Vs do not infect BI, and two (BII-V2 and BII-V3) have larger genomes (~210 kb) than BI-Viruses and BII-V1. BII-Vs share ~90% of their proteins, and between 65% to 83% of their proteins with sequenced BpVs. Phylogenomic reconstructions and PolB analyses establish close-relatedness of BII-V2 and BII-V3, yet BII-V2 has 10-fold higher infectivity and induces greater mortality on host isolate RCC716. BII-V1 is more distant, has a shorter latent period, and infects both available BII isolates, RCC716 and RCC715, while BII-V2 and BII-V3 do not exhibit productive infection of the latter in our experiments. Global metagenome analyses show Clade BI and BII algal relative abundances correlate positively with their respective viruses. The distributions delineate BI/BpVs as occupying lower temperature mesotrophic and coastal systems, whereas BII/BII-Vs occupy warmer temperature, higher salinity ecosystems. Accordingly, with molecular diagnostic support, we name Clade BII Bathycoccus calidus sp. nov. and propose that molecular diversity within this new species likely connects to the differentiated host-virus dynamics observed in our time course experiments. Overall, the tightly linked biogeography of Bathycoccus host and virus clades observed herein supports species-level host specificity, with strain-level variations in infection parameters.

Estimates of the fitness effects of spontaneous mutations are important for understanding the adaptive potential of species. Here, we present the results of mutation accumulation experiments over 265–512 sequential generations in four species of marine unicellular green algae, Ostreococcus tauri RCC4221, Ostreococcus mediterraneus RCC2590, Micromonas pusilla RCC299, and Bathycoccus prasinos RCC1105. Cell division rates, taken as a proxy for fitness, systematically decline over the course of the experiment in O. tauri, but not in the three other species where the MA experiments were carried out over a smaller number of generations. However, evidence of mutation accumulation in 24 MA lines arises when they are exposed to stressful conditions, such as changes in osmolarity or exposure to herbicides. The selection coefficients, estimated from the number of cell divisions/day, varies significantly between the different environmental conditions tested in MA lines, providing evidence for advantageous and deleterious effects of spontaneous mutations. This suggests a common environmental dependence of the fitness effects of mutations and allows the minimum mutation/genome/generation rates to be inferred at 0.0037 in these species.

Bathycoccus and Ostreococcus are broadly distributed marine picoprasinophyte algae. We enumerated small phytoplankton using flow cytometry and qPCR assays for phylogenetically distinct Bathycoccus clades BI and BII and Ostreococcus clades OI and OII. Among 259 photic-zone samples from transects and time-series, Ostreococcus maxima occurred in the North Pacific coastal upwelling for OI (36 713 ± 1485 copies ml−1) and the Kuroshio Front for OII (50 189 ± 561 copies ml−1) and the two overlapped only in frontal regions. The Bathycoccus overlapped more often with maxima along Line-P for BI (10 667 ± 1299 copies ml−1) and the tropical Atlantic for BII (4125 ± 339 copies ml−1). Only BII and OII were detected at warm oligotrophic sites, accounting for 34 ± 13 of 1589 ± 448 eukaryotic phytoplankton cells ml−1 (annual average) at Station ALOHA's deep chlorophyll maximum. Significant distributional and molecular differences lead us to propose that Bathycoccus clade BII represents a separate species which tolerates higher temperature oceanic conditions than Bathycoccus prasinos (BI). Morphological differences were not evident, but quick-freeze deep-etch electron microscopy provided insight into Bathycoccus scale formation. Our results highlight the importance of quantitative seasonal abundance data for inferring ecological distributions and demonstrate significant, differential picoprasinophyte contributions in mesotrophic and open-ocean waters. © 2017 The Authors. Environmental Microbiology published by Society for Applied Microbiology and John Wiley & Sons Ltd.

We characterized molecularly the first freshwater member ever reported for the family Bathycoccaceae in Lake Musters (Argentinean Patagonia). Members of this family are extremely numerous and play a key ecological role in marine systems as primary producers. We cloned a fragment comprising the SSU rRNA gene + ITS region from environmental DNA using specific mamiellophyte primers. The unique SSU rRNA gene sequence obtained clustered robustly with Bathycoccus prasinos. Analysis of the two-dimensional structure of the ITS region showed the presence of a typical supplementary helix in the ITS-2 region, a synapomorphy of Bathycoccaceae, which confirmed further its phylogenetic placement. We finally discuss the possible causes for the presence of this organism in Lake Musters. Fil: Lara, Enrique. Universite de Neuchatel. Faculte de Sciences; Suiza. Consejo Superior de Investigaciones Científicas. Real Jardín Botánico; España Fil: Fernández, Leonardo D.. Universidad Bernardo O’Higgin; Chile Fil: Schiaffino, María Romina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia del Noroeste de la Provincia de Buenos Aires. Universidad Nacional del Noroeste de la Provincia de Buenos Aires. Centro de Investigaciones y Transferencia del Noroeste de la Provincia de Buenos Aires; Argentina Fil: Izaguirre, Irina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; Argentina

Prasinoviruses infecting unicellular green algae in the order Mamiellales (class Mamiellophyceae) are commonly found in coastal marine waters where their host species frequently abound. We tested 40 Ostreococcus tauri viruses on 13 independently isolated wild-type O. tauri strains, 4 wild-type O. lucimarinus strains, 1 Ostreococcus sp. (“ Ostreococcus mediterraneus ”) clade D strain, and 1 representative species of each of two other related species of Mamiellales, Bathycoccus prasinos and Micromonas pusilla . Thirty-four out of 40 viruses infected only O. tauri , 5 could infect one other species of the Ostreococcus genus, and 1 infected two other Ostreococcus spp., but none of them infected the other genera. We observed that the overall susceptibility pattern of Ostreococcus strains to viruses was related to the size of two host chromosomes known to show intraspecific size variations, that genetically related viruses tended to infect the same host strains, and that viruses carrying inteins were strictly strain specific. Comparison of two complete O. tauri virus proteomes revealed at least three predicted proteins to be candidate viral specificity determinants.

Eukaryotic algae within the picoplankton size class (≤2 μm in diameter) are important marine primary producers, but their spatial and ecological distributions are not well characterized. Here, we studied three picoeukaryotic prasinophyte genera and their cyanobacterial counterparts, Prochlorococcus and Synechococcus , during two cruises along a North Pacific transect characterized by different ecological regimes. Picoeukaryotes and Synechococcus reached maximum abundances of 1.44 × 10 5 and 3.37 × 10 5 cells · ml −1 , respectively, in mesotrophic waters, while Prochlorococcus reached 1.95 × 10 5 cells · ml −1 in the oligotrophic ocean. Of the picoeukaryotes, Bathycoccus was present at all stations in both cruises, reaching 21,368 ± 327 18S rRNA gene copies · ml −1 . Micromonas and Ostreococcus clade OI were detected only in mesotrophic and coastal waters and Ostreococcus clade OII only in the oligotrophic ocean. To resolve proposed Bathycoccus ecotypes, we established genetic distances for 1,104 marker genes using targeted metagenomes and the Bathycoccus prasinos genome. The analysis was anchored in comparative genome analysis of three Ostreococcus species for which physiological and environmental data are available to facilitate data interpretation. We established that two Bathycoccus ecotypes exist, named here BI (represented by coastal isolate Bathycoccus prasinos ) and BII. These share 82% ± 6% nucleotide identity across homologs, while the Ostreococcus spp. share 75% ± 8%. We developed and applied an analysis of ecomarkers to metatranscriptomes sequenced here and published -omics data from the same region. The results indicated that the Bathycoccus ecotypes cooccur more often than Ostreococcus clades OI and OII do. Exploratory analyses of relative transcript abundances suggest that Bathycoccus NRT2.1 and AMT2.2 are high-affinity NO 3 − and low-affinity NH 4 + transporters, respectively, with close homologs in multiple picoprasinophytes. Additionally, in the open ocean, where dissolved iron concentrations were low (0.08 nM), there appeared to be a shift to the use of nickel superoxide dismutases (SODs) from Mn/Fe/Cu SODs closer inshore. Our study documents the distribution of picophytoplankton along a North Pacific ecological gradient and offers new concepts and techniques for investigating their biogeography.

The characterization by chromatographic (TLC, HPLC) and spectroscopic (visible and mass spectrometric and, in part, 1 H NMR, circular dichroism methods) and structural elucidation of 10 minor carotenoids isolated from prasinophytes ( Bathycoccus prasinos, Micromonas pusilla, Mantoniella squamata and a clone IIA2) are reported. These C 40 -carotenoids comprise the aldehydes micromonal and anhydromicromonal and the corresponding allylic prim carotenols micromonol and anhydromicromonol, and three natural butenolides derived from uriolide, namely anhydrouriolide, deepoxyuriolide and 3′-dehydrouriolide. In addition there are two prasinoxanthin related carotenoids, anhydroprasinoxanthin with a γ-end group and preprasinoxanthin, as well as a dihydrolutein. Several of the new carotenoids have short chromophores with saturated C-7,8 bond. A C-2,3 double bond is another frequent structural feature.

Rapid shifts in picoeukaryote community structure were observed during a CO2 perturbation experiment in which we followed the development of phytoplankton blooms in nutrient-amended mesocosms under present day or predicted future atmospheric pCO2 (750 μatm, seawater pH 7.8). Analysis of rbcL clone libraries (encoding the large subunit of RubisCO) and specific QPCR assays showed that two prasinophytes closely related to Micromonas pusilla and Bathycoccus prasinos were present but responded very differently to high CO2/acidification. We found that the abundance of Micromonas-like phylotypes was significantly higher (>20-fold) under elevated CO2/low pH, whereas the Bathycoccus-like phylotypes were more evenly distributed between treatments and dominated the prasinophyte community under ambient conditions. Rapid shifts in picoeukaryote community structure were observed during a CO2 perturbation experiment in which we followed the development of phytoplankton blooms in nutrient-amended mesocosms under present day or predicted future atmospheric pCO2 (750 μatm, seawater pH 7.8). Analysis of rbcL clone libraries (encoding the large subunit of RubisCO) and specific QPCR assays showed that two prasinophytes closely related to Micromonas pusilla and Bathycoccus prasinos were present but responded very differently to high CO2/acidification. We found that the abundance of Micromonas-like phylotypes was significantly higher (>20-fold) under elevated CO2/low pH, whereas the Bathycoccus-like phylotypes were more evenly distributed between treatments and dominated the prasinophyte community under ambient conditions.

Mantoniella squamata (Prasinophyceae, Chlorophyta) is presently considered to be one of the most primitive green plants. The structure of the spider-web-like scales that cover the flagellar and body surfaces of this organism is described. By contrast to current theory, the scales may be divided into several different morphological types, one form being resricted to the flagella and two to the cell body. The cells contain trichocyst-like organelles similar to those found in the related species Mamiella gilva. In the order Mamiellales, which contains four genera and seven described species, the simplest and thus perhaps the most primitive species is the recently described Bathycoccus prasinos Eikrem et Throndsen, the cells of which lack flagella and are covered with only one type of plate scale.

A non-motile, picoplanktonic species, Bathycoccus prasinos gen. et sp. nov. (Mamiellaceae, Mamiellales, Prasinophyceae) is described from the Gulf of Naples, Italy. Resembling a comma, the cells ar...

BackgroundBathycoccus prasinos is an extremely small cosmopolitan marine green alga whose cells are covered with intricate spider's web patterned scales that develop within the Golgi cisternae before their transport to the cell surface. The objective of this work is to sequence and analyze its genome, and to present a comparative analysis with other known genomes of the green lineage.ResearchIts small genome of 15 Mb consists of 19 chromosomes and lacks transposons. Although 70% of all B. prasinos genes share similarities with other Viridiplantae genes, up to 428 genes were probably acquired by horizontal gene transfer, mainly from other eukaryotes. Two chromosomes, one big and one small, are atypical, an unusual synapomorphic feature within the Mamiellales. Genes on these atypical outlier chromosomes show lower GC content and a significant fraction of putative horizontal gene transfer genes. Whereas the small outlier chromosome lacks colinearity with other Mamiellales and contains many unknown genes without homologs in other species, the big outlier shows a higher intron content, increased expression levels and a unique clustering pattern of housekeeping functionalities. Four gene families are highly expanded in B. prasinos, including sialyltransferases, sialidases, ankyrin repeats and zinc ion-binding genes, and we hypothesize that these genes are associated with the process of scale biogenesis.ConclusionThe minimal genomes of the Mamiellophyceae provide a baseline for evolutionary and functional analyses of metabolic processes in green plants.