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2.  A global perspective on marine photosynthetic picoeukaryote community structure 
The ISME Journal  2013;7(5):922-936.
A central goal in ecology is to understand the factors affecting the temporal dynamics and spatial distribution of microorganisms and the underlying processes causing differences in community structure and composition. However, little is known in this respect for photosynthetic picoeukaryotes (PPEs), algae that are now recognised as major players in marine CO2 fixation. Here, we analysed dot blot hybridisation and cloning–sequencing data, using the plastid-encoded 16S rRNA gene, from seven research cruises that encompassed all four ocean biomes. We provide insights into global abundance, α- and β-diversity distribution and the environmental factors shaping PPE community structure and composition. At the class level, the most commonly encountered PPEs were Prymnesiophyceae and Chrysophyceae. These taxa displayed complementary distribution patterns, with peak abundances of Prymnesiophyceae and Chrysophyceae in waters of high (25:1) or low (12:1) nitrogen:phosphorus (N:P) ratio, respectively. Significant differences in phylogenetic composition of PPEs were demonstrated for higher taxonomic levels between ocean basins, using Unifrac analyses of clone library sequence data. Differences in composition were generally greater between basins (interbasins) than within a basin (intrabasin). These differences were primarily linked to taxonomic variation in the composition of Prymnesiophyceae and Prasinophyceae whereas Chrysophyceae were phylogenetically similar in all libraries. These data provide better knowledge of PPE community structure across the world ocean and are crucial in assessing their evolution and contribution to CO2 fixation, especially in the context of global climate change.
PMCID: PMC3635238  PMID: 23364354
photosynthetic picoeukaryotes; community structure; prymnesiophytes; chrysophytes; global scale
3.  Comparable light stimulation of organic nutrient uptake by SAR11 and Prochlorococcus in the North Atlantic subtropical gyre 
The ISME Journal  2012;7(3):603-614.
Subtropical oceanic gyres are the most extensive biomes on Earth where SAR11 and Prochlorococcus bacterioplankton numerically dominate the surface waters depleted in inorganic macronutrients as well as in dissolved organic matter. In such nutrient poor conditions bacterioplankton could become photoheterotrophic, that is, potentially enhance uptake of scarce organic molecules using the available solar radiation to energise appropriate transport systems. Here, we assessed the photoheterotrophy of the key microbial taxa in the North Atlantic oligotrophic gyre and adjacent regions using 33P-ATP, 3H-ATP and 35S-methionine tracers. Light-stimulated uptake of these substrates was assessed in two dominant bacterioplankton groups discriminated by flow cytometric sorting of tracer-labelled cells and identified using catalysed reporter deposition fluorescence in situ hybridisation. One group of cells, encompassing 48% of all bacterioplankton, were identified as members of the SAR11 clade, whereas the other group (24% of all bacterioplankton) was Prochlorococcus. When exposed to light, SAR11 cells took 31% more ATP and 32% more methionine, whereas the Prochlorococcus cells took 33% more ATP and 34% more methionine. Other bacterioplankton did not demonstrate light stimulation. Thus, the SAR11 and Prochlorococcus groups, with distinctly different light-harvesting mechanisms, used light equally to enhance, by approximately one-third, the uptake of different types of organic molecules. Our findings indicate the significance of light-driven uptake of essential organic nutrients by the dominant bacterioplankton groups in the surface waters of one of the less productive, vast regions of the world's oceans—the oligotrophic North Atlantic subtropical gyre.
PMCID: PMC3580278  PMID: 23096403
SAR11; Prochlorococcus; light stimulation; flow cytometric sorting; radioisotope tracing; ATP and amino-acid uptake
4.  Mining Genomes of Marine Cyanobacteria for Elements of Zinc Homeostasis 
Zinc is a recognized essential element for the majority of organisms, and is indispensable for the correct function of hundreds of enzymes and thousands of regulatory proteins. In aquatic photoautotrophs including cyanobacteria, zinc is thought to be required for carbonic anhydrase and alkaline phosphatase, although there is evidence that at least some carbonic anhydrases can be cambialistic, i.e., are able to acquire in vivo and function with different metal cofactors such as Co2+ and Cd2+. Given the global importance of marine phytoplankton, zinc availability in the oceans is likely to have an impact on both carbon and phosphorus cycles. Zinc concentrations in seawater vary over several orders of magnitude, and in the open oceans adopt a nutrient-like profile. Most studies on zinc handling by cyanobacteria have focused on freshwater strains and zinc toxicity; much less information is available on marine strains and zinc limitation. Several systems for zinc homeostasis have been characterized in the freshwater species Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803, but little is known about zinc requirements or zinc handling by marine species. Comparative metallo-genomics has begun to explore not only the putative zinc proteome, but also specific protein families predicted to have an involvement in zinc homeostasis, including sensors for excess and limitation (SmtB and its homologs as well as Zur), uptake systems (ZnuABC), putative intracellular zinc chaperones (COG0523) and metallothioneins (BmtA), and efflux pumps (ZiaA and its homologs).
PMCID: PMC3323870  PMID: 22514551
zinc limitation; Zur; COG0523; SmtA; BmtA
5.  Genome Sequence of Ostreococcus tauri Virus OtV-2 Throws Light on the Role of Picoeukaryote Niche Separation in the Ocean▿ 
Journal of Virology  2011;85(9):4520-4529.
Ostreococcus tauri, a unicellular marine green alga, is the smallest known free-living eukaryote and is ubiquitous in the surface oceans. The ecological success of this organism has been attributed to distinct low- and high-light-adapted ecotypes existing in different niches at a range of depths in the ocean. Viruses have already been characterized that infect the high-light-adapted strains. Ostreococcus tauri virus (OtV) isolate OtV-2 is a large double-stranded DNA algal virus that infects a low-light-adapted strain of O. tauri and was assigned to the algal virus family Phycodnaviridae, genus Prasinovirus. Our working hypothesis for this study was that different viruses infecting high- versus low-light-adapted O. tauri strains would provide clues to propagation strategies that would give them selective advantages within their particular light niche. Sequence analysis of the 184,409-bp linear OtV-2 genome revealed a range of core functional genes exclusive to this low-light genotype and included a variety of unexpected genes, such as those encoding an RNA polymerase sigma factor, at least four DNA methyltransferases, a cytochrome b5, and a high-affinity phosphate transporter. It is clear that OtV-2 has acquired a range of potentially functional genes from its host, other eukaryotes, and even bacteria over evolutionary time. Such piecemeal accretion of genes is a trademark of large double-stranded DNA viruses that has allowed them to adapt their propagation strategies to keep up with host niche separation in the sunlit layers of the oceanic environment.
PMCID: PMC3126241  PMID: 21289127
6.  Plastid 16S rRNA Gene Diversity among Eukaryotic Picophytoplankton Sorted by Flow Cytometry from the South Pacific Ocean 
PLoS ONE  2011;6(4):e18979.
The genetic diversity of photosynthetic picoeukaryotes was investigated in the South East Pacific Ocean. Genetic libraries of the plastid 16S rRNA gene were constructed on picoeukaryote populations sorted by flow cytometry, using two different primer sets, OXY107F/OXY1313R commonly used to amplify oxygenic organisms, and PLA491F/OXY1313R, biased towards plastids of marine algae. Surprisingly, the two sets revealed quite different photosynthetic picoeukaryote diversity patterns, which were moreover different from what we previously reported using the 18S rRNA nuclear gene as a marker. The first 16S primer set revealed many sequences related to Pelagophyceae and Dictyochophyceae, the second 16S primer set was heavily biased toward Prymnesiophyceae, while 18S sequences were dominated by Prasinophyceae, Chrysophyceae and Haptophyta. Primer mismatches with major algal lineages is probably one reason behind this discrepancy. However, other reasons, such as DNA accessibility or gene copy numbers, may be also critical. Based on plastid 16S rRNA gene sequences, the structure of photosynthetic picoeukaryotes varied along the BIOSOPE transect vertically and horizontally. In oligotrophic regions, Pelagophyceae, Chrysophyceae, and Prymnesiophyceae dominated. Pelagophyceae were prevalent at the DCM depth and Chrysophyceae at the surface. In mesotrophic regions Pelagophyceae were still important but Chlorophyta contribution increased. Phylogenetic analysis revealed a new clade of Prasinophyceae (clade 16S-IX), which seems to be restricted to hyper-oligotrophic stations. Our data suggest that a single gene marker, even as widely used as 18S rRNA, provides a biased view of eukaryotic communities and that the use of several markers is necessary to obtain a complete image.
PMCID: PMC3084246  PMID: 21552558
7.  Functional Characterization of Synechocystis sp. Strain PCC 6803 pst1 and pst2 Gene Clusters Reveals a Novel Strategy for Phosphate Uptake in a Freshwater Cyanobacterium▿  
Journal of Bacteriology  2010;192(13):3512-3523.
Synechocystis sp. strain PCC 6803 possesses two putative ABC-type inorganic phosphate (Pi) transporters with three associated Pi-binding proteins (PBPs), SphX (encoded by sll0679), PstS1 (encoded by sll0680), and PstS2 (encoded by slr1247), organized in two spatially discrete gene clusters, pst1 and pst2. We used a combination of mutagenesis, gene expression, and radiotracer uptake analyses to functionally characterize the role of these PBPs and associated gene clusters. Quantitative PCR (qPCR) demonstrated that pstS1 was expressed at a high level in Pi-replete conditions compared to sphX or pstS2. However, a Pi stress shift increased expression of pstS2 318-fold after 48 h, compared to 43-fold for pstS1 and 37-fold for sphX. A shift to high-light conditions caused a transient increase of all PBPs, whereas N stress primarily increased expression of sphX. Interposon mutagenesis of each PBP demonstrated that disruption of pstS1 alone caused constitutive expression of pho regulon genes, implicating PstS1 as a major component of the Pi sensing machinery. The pstS1 mutant was also transformation incompetent. 32Pi radiotracer uptake experiments using pst1 and pst2 deletion mutants showed that Pst1 acts as a low-affinity, high-velocity transporter (Ks, 3.7 ± 0.7 μM; Vmax, 31.18 ± 3.96 fmol cell−1 min−1) and Pst2 acts as a high-affinity, low-velocity system (Ks, 0.07 ± 0.01 μM; Vmax, 0.88 ± 0.11 fmol cell−1 min−1). These Pi ABC transporters thus exhibit differences in both kinetic and regulatory properties, the former trait potentially dramatically increasing the dynamic range of Pi transport into the cell, which has potential implications for our understanding of the ecological success of this key microbial group.
PMCID: PMC2897655  PMID: 20435726
8.  Groups without Cultured Representatives Dominate Eukaryotic Picophytoplankton in the Oligotrophic South East Pacific Ocean 
PLoS ONE  2009;4(10):e7657.
Photosynthetic picoeukaryotes (PPE) with a cell size less than 3 µm play a critical role in oceanic primary production. In recent years, the composition of marine picoeukaryote communities has been intensively investigated by molecular approaches, but their photosynthetic fraction remains poorly characterized. This is largely because the classical approach that relies on constructing 18S rRNA gene clone libraries from filtered seawater samples using universal eukaryotic primers is heavily biased toward heterotrophs, especially alveolates and stramenopiles, despite the fact that autotrophic cells in general outnumber heterotrophic ones in the euphotic zone.
Methodology/Principal Findings
In order to better assess the composition of the eukaryotic picophytoplankton in the South East Pacific Ocean, encompassing the most oligotrophic oceanic regions on earth, we used a novel approach based on flow cytometry sorting followed by construction of 18S rRNA gene clone libraries. This strategy dramatically increased the recovery of sequences from putative autotrophic groups. The composition of the PPE community appeared highly variable both vertically down the water column and horizontally across the South East Pacific Ocean. In the central gyre, uncultivated lineages dominated: a recently discovered clade of Prasinophyceae (IX), clades of marine Chrysophyceae and Haptophyta, the latter division containing a potentially new class besides Prymnesiophyceae and Pavlophyceae. In contrast, on the edge of the gyre and in the coastal Chilean upwelling, groups with cultivated representatives (Prasinophyceae clade VII and Mamiellales) dominated.
Our data demonstrate that a very large fraction of the eukaryotic picophytoplankton still escapes cultivation. The use of flow cytometry sorting should prove very useful to better characterize specific plankton populations by molecular approaches such as gene cloning or metagenomics, and also to obtain into culture strains representative of these novel groups.
PMCID: PMC2764088  PMID: 19893617
9.  Phosphate Acquisition Components of the Myxococcus xanthus Pho Regulon Are Regulated by both Phosphate Availability and Development▿  
Journal of Bacteriology  2008;190(6):1997-2003.
In many organisms, phosphatase expression and phosphate (P) uptake are coordinately regulated by the Pho regulon. In Myxococcus xanthus P limitation initiates multicellular development, a process associated with changes in phosphatase expression. We sought here to characterize the link between P acquisition and development in this bacterium, an organism capable of preying upon other microorganisms as a sole nutrient source. M. xanthus seems to possess no significant internal P stores, as reducing the P concentration to less than 10 μM retarded growth within one doubling time. Pyrophosphate, polyphosphate, and glyceraldehyde-3-phosphate could support growth as sole P sources, although many other P-containing biomolecules could not (including nucleic acids and phospholipids). Several Pho regulon promoters were found to be highly active during vegetative growth, and P limitation specifically induced pstSCAB, AcPA1, and pho3 promoter activity and repressed pit expression. Enhanced pstSCAB and pho3 promoter activities in a phoP4 mutant (in the presence of high and low concentrations of P) suggested that PhoP4 acts as a repressor of these genes. However, in a phoP4 background, the activities of pstSCAB remained P regulated, suggesting that there is additional regulation by a P-sensitive factor. Initiation of multicellular development caused immediate down-regulation of Pho regulon genes and caused pstSCAB and pho3 promoter activities to become P insensitive. Hence, P acquisition components of the M. xanthus Pho regulon are regulated by both P availability and development, with developmental down-regulation overriding up-regulation by P limitation. These observations suggest that when development is initiated, subsequent changes in P availability become irrelevant to the population, which presumably has sufficient intrinsic P to ensure completion of the developmental program.
PMCID: PMC2258891  PMID: 18178740
10.  Unraveling the genomic mosaic of a ubiquitous genus of marine cyanobacteria 
Genome Biology  2008;9(5):R90.
Local niche occupancy of marine Synechococcus lineages is facilitated by lateral gene transfers. Genomic islands act as repositories for these transferred genes.
The picocyanobacterial genus Synechococcus occurs over wide oceanic expanses, having colonized most available niches in the photic zone. Large scale distribution patterns of the different Synechococcus clades (based on 16S rRNA gene markers) suggest the occurrence of two major lifestyles ('opportunists'/'specialists'), corresponding to two distinct broad habitats ('coastal'/'open ocean'). Yet, the genetic basis of niche partitioning is still poorly understood in this ecologically important group.
Here, we compare the genomes of 11 marine Synechococcus isolates, representing 10 distinct lineages. Phylogenies inferred from the core genome allowed us to refine the taxonomic relationships between clades by revealing a clear dichotomy within the main subcluster, reminiscent of the two aforementioned lifestyles. Genome size is strongly correlated with the cumulative lengths of hypervariable regions (or 'islands'). One of these, encompassing most genes encoding the light-harvesting phycobilisome rod complexes, is involved in adaptation to changes in light quality and has clearly been transferred between members of different Synechococcus lineages. Furthermore, we observed that two strains (RS9917 and WH5701) that have similar pigmentation and physiology have an unusually high number of genes in common, given their phylogenetic distance.
We propose that while members of a given marine Synechococcus lineage may have the same broad geographical distribution, local niche occupancy is facilitated by lateral gene transfers, a process in which genomic islands play a key role as a repository for transferred genes. Our work also highlights the need for developing picocyanobacterial systematics based on genome-derived parameters combined with ecological and physiological data.
PMCID: PMC2441476  PMID: 18507822
11.  Diversity and evolution of phycobilisomes in marine Synechococcus spp.: a comparative genomics study 
Genome Biology  2007;8(12):R259.
By comparing Synechococcus genomes, candidate genes required for the production of phycobiliproteins, which are part of the light-harvesting antenna complexes called phycobilisomes, were identified. Phylogenetic analyses suggest that the phycobilisome core evolved together with the core genome, whereas rods evolved independently.
Marine Synechococcus owe their specific vivid color (ranging from blue-green to orange) to their large extrinsic antenna complexes called phycobilisomes, comprising a central allophycocyanin core and rods of variable phycobiliprotein composition. Three major pigment types can be defined depending on the major phycobiliprotein found in the rods (phycocyanin, phycoerythrin I or phycoerythrin II). Among strains containing both phycoerythrins I and II, four subtypes can be distinguished based on the ratio of the two chromophores bound to these phycobiliproteins. Genomes of eleven marine Synechococcus strains recently became available with one to four strains per pigment type or subtype, allowing an unprecedented comparative genomics study of genes involved in phycobilisome metabolism.
By carefully comparing the Synechococcus genomes, we have retrieved candidate genes potentially required for the synthesis of phycobiliproteins in each pigment type. This includes linker polypeptides, phycobilin lyases and a number of novel genes of uncharacterized function. Interestingly, strains belonging to a given pigment type have similar phycobilisome gene complements and organization, independent of the core genome phylogeny (as assessed using concatenated ribosomal proteins). While phylogenetic trees based on concatenated allophycocyanin protein sequences are congruent with the latter, those based on phycocyanin and phycoerythrin notably differ and match the Synechococcus pigment types.
We conclude that the phycobilisome core has likely evolved together with the core genome, while rods must have evolved independently, possibly by lateral transfer of phycobilisome rod genes or gene clusters between Synechococcus strains, either via viruses or by natural transformation, allowing rapid adaptation to a variety of light niches.
PMCID: PMC2246261  PMID: 18062815
12.  Prochlorococcus Ecotype Abundances in the North Atlantic Ocean As Revealed by an Improved Quantitative PCR Method†  
The cyanobacterium Prochlorococcus numerically dominates the photosynthetic community in the tropical and subtropical regions of the world's oceans. Six evolutionary lineages of Prochlorococcus have been described, and their distinctive physiologies and genomes indicate that these lineages are “ecotypes” and should have different oceanic distributions. Two methods recently developed to quantify these ecotypes in the field, probe hybridization and quantitative PCR (QPCR), have shown that this is indeed the case. To facilitate a global investigation of these ecotypes, we modified our QPCR protocol to significantly increase its speed, sensitivity, and accessibility and validated the method in the western and eastern North Atlantic Ocean. We showed that all six ecotypes had distinct distributions that varied with depth and location, and, with the exception of the deeper waters at the western North Atlantic site, the total Prochlorococcus counts determined by QPCR matched the total counts measured by flow cytometry. Clone library analyses of the deeper western North Atlantic waters revealed ecotypes that are not represented in the culture collections with which the QPCR primers were designed, explaining this discrepancy. Finally, similar patterns of relative ecotype abundance were obtained in QPCR and probe hybridization analyses of the same field samples, which could allow comparisons between studies.
PMCID: PMC1352191  PMID: 16391112
13.  Clade-Specific 16S Ribosomal DNA Oligonucleotides Reveal the Predominance of a Single Marine Synechococcus Clade throughout a Stratified Water Column in the Red Sea 
Phylogenetic relationships among members of the marine Synechococcus genus were determined following sequencing of the 16S ribosomal DNA (rDNA) from 31 novel cultured isolates from the Red Sea and several other oceanic environments. This revealed a large genetic diversity within the marine Synechococcus cluster consistent with earlier work but also identified three novel clades not previously recognized. Phylogenetic analyses showed one clade, containing halotolerant isolates lacking phycoerythrin (PE) and including strains capable, or not, of utilizing nitrate as the sole N source, which clustered within the MC-A (Synechococcus subcluster 5.1) lineage. Two copies of the 16S rRNA gene are present in marine Synechococcus genomes, and cloning and sequencing of these copies from Synechococcus sp. strain WH 7803 and genomic information from Synechococcus sp. strain WH 8102 reveal these to be identical. Based on the 16S rDNA sequence information, clade-specific oligonucleotides for the marine Synechococcus genus were designed and their specificity was optimized. Using dot blot hybridization technology, these probes were used to determine the in situ community structure of marine Synechococcus populations in the Red Sea at the time of a Synechococcus maximum during April 1999. A predominance of genotypes representative of a single clade was found, and these genotypes were common among strains isolated into culture. Conversely, strains lacking PE, which were also relatively easily isolated into culture, represented only a minor component of the Synechococcus population. Genotypes corresponding to well-studied laboratory strains also appeared to be poorly represented in this stratified water column in the Red Sea.
PMCID: PMC154553  PMID: 12732508
14.  Niche-Partitioning of Prochlorococcus Populations in a Stratified Water Column in the Eastern North Atlantic Ocean† 
The in situ community structure of Prochlorococcus populations in the eastern North Atlantic Ocean was examined by analysis of Prochlorococcus 16S rDNA sequences with three independent approaches: cloning and sequencing, hybridization to specific oligonucleotide probes, and denaturing gradient gel electrophoresis (DGGE). The hybridization of high-light (HL) and low-light (LL) Prochlorococcus genotype-specific probes to two depth profiles of PCR-amplified 16S rDNA sequences revealed that in these two stratified water columns, an obvious niche-partitioning of Prochlorococcus genotypes occurred. In each water column a shift from the HL to the LL genotype was observed, a transition correlating with the depth of the surface mixed layer (SML). Only the HL genotype was found in the SML in each water column, whereas the LL genotype was distributed below the SML. The range of in situ irradiance to which each genotype was subjected within these distinct niches was consistent with growth irradiance studies of cultured HL- and LL-adapted Prochlorococcus strains. DGGE analysis and the sequencing of Prochlorococcus 16S rDNA clones were in full agreement with the genotype-specific oligonucleotide probe hybridization data. These observations of a partitioning of Prochlorococcus genotypes in a stratified water column provide a genetic basis for the dim and bright Prochlorococcus populations observed in flow cytometric signatures in several oceanic provinces.
PMCID: PMC91382  PMID: 10347047

Results 1-14 (14)