Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus are major contributors to oceanic primary production. The genera are genetically diverse, comprising several known ecotypes or lineages. However, little is known of the distribution of these lineages over large geographic areas. Here, we analysed the relative abundance of Prochlorococcus ecotypes and Synechococcus lineages at the ocean basin scale along an Atlantic Meridional Transect (AMT) using dot blot hybridization and fluorescence in situ hybridization (FISH) techniques. The transect covered several contrasting oceanic provinces (gyres, upwelling, temperate regions) as well as environmentally 'equivalent' regions in the northern and southern hemisphere (northern and southern gyres and temperate regions). Flow cytometric data revealed a discrete separation in abundance of major picocyanobacterial genera. Prochlorococcus reached highest abundance in oligotrophic regions, while more mesotrophic waters were dominated by Synechococcus. Individual genetic lineages of both Prochlorococcus and Synechococcus showed highly similar distributions in corresponding regions in the northern and southern hemisphere. In addition, Prochlorococcus showed a distinctive depth distribution, with HLI and HLII ecotypes near the surface and co-occurring LL ecotypes further down in the water column. Conversely, Synechococcus generally revealed no obvious depth preference, but did show highly specific distribution at the horizontal scale, with clades I and IV particularly dominating temperate, mesotrophic waters in both the northern and southern hemispheres. The data clearly reveal that specific picocyanobacterial lineages proliferate in similar oceanic provinces separated by large spatial scales. Furthermore, comparison with an earlier AMT dataset suggests that basin scale distribution patterns for Prochlorococcus ecotypes are remarkably reproducible from year to year.

Low nucleic acid (LNA) bacterioplankton (sorted by flow cytometry) were characterised in surface water samples along a meridional transect from 48°N to 40°S across the Atlantic Ocean. The LNA bacterioplankton abundance and metabolic activity, assessed by their 35 S-methionine uptake rate, were similar along the transect, representing 36 ± 6 and 36 ± 11% of total bacterioplankton, respectively. Fluorescence in situ hybridisation analysis of the flow-sorted cells revealed that the LNA bacterioplankton population was dominated (59 ± 4%) by and contained virtually all the identifiable SAR11 clade cells throughout the Atlantic Ocean. Therefore, the present study provides ecological characterisation of this flow-sorted group and suggests both phylogenetic and functional constancy of the LNA bacterioplankton at the basin-scale. KEY WORDS: Flow cytometry sorting · Cell metabolic activity · SAR11 clade · CARD-FISH · Radioactive tracer labelling · Atlantic Ocean Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 45: [107][108][109][110][111][112][113] 2006 into the South Subtropical Frontal Zone (SSFZ, Belkin & Gordon 1996). As it was not possible to work on every sample collected along the transect, we selected representative samples for a basin-scale study. MATERIALS AND METHODSSampling area. The material was collected and measurements were made primarily on one Atlantic Meridional Transect (AMT) cruise on board the RRS 'James Clark Ross', cruise no. JR91, transect no. AMT-13, in September and October 2003 (Fig. 1). Up to 24 seawater samples were collected approximately every 12 h from a range of depths in the top 300 m using a rosette of 24 × 20 l Niskin bottles mounted on a conductivity-temperature-density (CTD) profiler. The Bpl cells were routinely fixed with 1% paraformaldehyde (PFA) final concentration.The abundance of Bpl was determined at every station , and for the present study 9 stations were chosen along the transect to represent the basin-scale range of surface waters (Fig. 1). The samples for the study of turnover rates of methionine (Met) by total bacterioplankton and by the LNA group sorted by flow cytometry were collected from depths of 3 to 7 m. Methionine uptake was chosen because of the high specific activity of the 35 S-methionine precursor and because previous studies have shown that microbial uptake of this amino acid are strongly correlated with the uptake of other amino acids such as leucine (Zubkov et al. 2004). The analyses also included the phylogenetic affiliation of the flow-sorted LNA cells using fluorescence in situ hybridisation (FISH). To test that the results obtained from the selected stations were representative of the Atlantic surface waters, an additional 15 stations ( Fig. 1) were included for comparison of the relative LNA abundance and Met uptake rates.Additional samples for phylogenetic analyses of bacterioplankton were collected from the Northern Atlantic at 5 stations on a later cruise on the RRS 'Discovery', cruise...

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Recent applications of culture-independent, molecular methods have revealed unexpectedly high diversity in a variety of functional and phylogenetic groups of microorganisms in the ocean. However, none of the existing research tools are free from significant limitations, such as PCR and cloning biases, low phylogenetic resolution and others. Here, we employed novel, single-cell sequencing techniques to assess the composition of small (o10 lm diameter), heterotrophic protists from the Gulf of Maine. Single cells were isolated by flow cytometry, their genomes amplified, and 18S rRNA marker genes were amplified and sequenced. We compared the results to traditional environmental PCR cloning of sorted cells. The diversity of heterotrophic protists was significantly higher in the library of single amplified genomes (SAGs) than in environmental PCR clone libraries of the 18S rRNA gene, obtained from the same coastal sample. Libraries of SAGs, but not clones contained several recently discovered, uncultured groups, including picobiliphytes and novel marine stramenopiles. Clone, but not SAG, libraries contained several large clusters of identical and nearly identical sequences of Dinophyceae, Cercozoa and Stramenopiles. Similar results were obtained using two alternative primer sets, suggesting that PCR biases may not be the only explanation for the observed patterns. Instead, differences in the number of 18S rRNA gene copies among the various protist taxa probably had a significant role in determining the PCR clone composition. These results show that single-cell sequencing has the potential to more accurately assess protistan community composition than previously established methods. In addition, the creation of SAG libraries opens opportunities for the analysis of multiple genes or entire genomes of the uncultured protist groups.