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1.  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.
doi:10.1038/ismej.2012.126
PMCID: PMC3580278  PMID: 23096403
SAR11; Prochlorococcus; light stimulation; flow cytometric sorting; radioisotope tracing; ATP and amino-acid uptake
2.  Extreme spatial variability in marine picoplankton and its consequences for interpreting Eulerian time-series 
Biology Letters  2005;1(3):366-369.
A high-resolution mesoscale spatial survey of picoplankton in the Celtic Sea, using flow cytometry, reveals cell concentrations of Synechococcus spp. cyanobacteria and heterotrophic bacteria that vary up to 50-fold over distances as short as 12 km. Furthermore, the range of abundances is comparable to that typically found on seasonal scales at a single location. Advection of such spatial variability through a time-series site would therefore constitute a major source of ‘error’. Consequently, attempts to model and to investigate the ecology of these globally important organisms in situ must take into account and quantify the hitherto ignored local spatial variability as a matter of necessity.
doi:10.1098/rsbl.2005.0316
PMCID: PMC1617145  PMID: 17148208
bacterioplankton; cyanobacteria; flow cytometry; patchiness
3.  High Rate of Uptake of Organic Nitrogen Compounds by Prochlorococcus Cyanobacteria as a Key to Their Dominance in Oligotrophic Oceanic Waters 
Direct evidence that marine cyanobacteria take up organic nitrogen compounds in situ at high rates is reported. About 33% of the total bacterioplankton turnover of amino acids, determined with a representative [35S]methionine precursor and flow sorting, can be assigned to Prochlorococcus spp. and 3% can be assigned to Synechococcus spp. in the oligotrophic and mesotrophic parts of the Arabian Sea, respectively. This finding may provide a mechanism for Prochlorococcus' competitive dominance over both strictly autotrophic algae and other bacteria in oligotrophic regions sustained by nutrient remineralization via a microbial loop.
doi:10.1128/AEM.69.2.1299-1304.2003
PMCID: PMC143617  PMID: 12571062
4.  Comparison of Cellular and Biomass Specific Activities of Dominant Bacterioplankton Groups in Stratified Waters of the Celtic Sea 
Applied and Environmental Microbiology  2001;67(11):5210-5218.
A flow-sorting technique was developed to determine unperturbed metabolic activities of phylogenetically characterized bacterioplankton groups with incorporation rates of [35S]methionine tracer. According to fluorescence in situ hybridization with rRNA targeted oligonucleotide probes, a clade of α-proteobacteria, related to Roseobacter spp., and a Cytophaga-Flavobacterium cluster dominated the different groups. Cytometric characterization revealed both these groups to have high DNA (HNA) content, while the α-proteobacteria exhibited high light scatter (hs) and the Cytophaga-Flavobacterium cluster exhibited low light scatter (ls). A third abundant group with low DNA (LNA) content contained cells from a SAR86 cluster of γ-proteobacteria. Cellular specific activities of the HNA-hs group were 4- and 1.7-fold higher than the activities in the HNA-ls and LNA groups, respectively. However, the higher cellular protein synthesis by the HNA-hs could simply be explained by their maintenance of a larger cellular protein biomass. Similar biomass specific activities of the different groups strongly support the main assumption that underlies the determination of bacterial production: different bacteria in a complex community incorporate amino acids at a rate proportional to their protein synthesis. The fact that the highest growth-specific rates were determined for the smallest cells of the LNA group can explain the dominance of this group in nutrient-limited waters. The metabolic activities of the three groups accounted for almost the total bacterioplankton activity, indicating their key biogeochemical role in the planktonic ecosystem of the Celtic Sea.
doi:10.1128/AEM.67.11.5210-5218.2001
PMCID: PMC93292  PMID: 11679347
5.  Determination of Total Protein Content of Bacterial Cells by SYPRO Staining and Flow Cytometry 
An assay has been developed for measuring protein biomass of marine planktonic bacteria by flow cytometry. The method was calibrated by using five species of Bacteria (an Arcobacter sp., a Cytophaga sp., an Oceanospirillum sp., a Pseudoalteromonas sp., and a Vibrio sp.) recently isolated from seawater samples and grown in culture at different temperatures. The intensity of SYPRO-protein fluorescence of these bacteria strongly correlated with their total protein content, measured by the bicinchoninic acid method to be in the range of 60 to 330 fg of protein cell−1 (r2 = 0.93, n = 34). According to the calibration, the mean biomass of planktonic bacteria from the North Sea in August 1998 was 24 fg of protein cell−1.
PMCID: PMC91485  PMID: 10388732
6.  Faster growth of the major prokaryotic versus eukaryotic CO2 fixers in the oligotrophic ocean 
Nature Communications  2014;5:3776.
Because maintenance of non-scalable cellular components—membranes and chromosomes—requires an increasing fraction of energy as cell size decreases, miniaturization comes at a considerable energetic cost for a phytoplanktonic cell. Consequently, if eukaryotes can use their superior energetic resources to acquire nutrients with more or even similar efficiency compared with prokaryotes, larger unicellular eukaryotes should be able to achieve higher growth rates than smaller cyanobacteria. Here, to test this hypothesis, we directly compare the intrinsic growth rates of phototrophic prokaryotes and eukaryotes from the equatorial to temperate South Atlantic using an original flow cytometric 14CO2-tracer approach. At the ocean basin scale, cyanobacteria double their biomass twice as frequently as the picoeukaryotes indicating that the prokaryotes are faster growing CO2 fixers, better adapted to phototrophic living in the oligotrophic open ocean—the most extensive biome on Earth.
After the energetically superior eukaryotes had evolved, prokaryotes appeared to lose control over biological CO2 fixation in all major biomes on Earth. Here the author shows that in the oligotrophic ocean, the most extensive biome on Earth, the prokaryotes fix CO2 twice as fast as eukaryotes.
doi:10.1038/ncomms4776
PMCID: PMC4015317  PMID: 24777140

Results 1-6 (6)