1,721,078 research outputs found
Photoheterotrophy in marine prokaryotes
No full textDespite considerable advances in the understanding of the various microbial photoheterotrophic mechanisms, the role of solar radiation in the metabolism of bacterioplankton in the ocean is difficult to assess. It is already apparent that rates of CO2 fixation by prokaryotic cells may be only a part of the picture. Photophosphorylation is difficult to differentiate from respiratory phosphorylation and other types of ATP synthesis. Solar energy could by-pass ATP synthesis, instead being used to generate a proton-motive force, which in turn could be directly used for cell motility or even for importing molecules into cells. In addition, photoheterotrophic prokaryotes could actively regulate intake and use of solar energy for different metabolic functions depending on the energetic demands of the cell. The factors listed above hence require consideration when solar energy input into metabolism of oceanic photoheterotrophic prokaryotes is experimentally quantified and numerically modelled
Faster growth of the major prokaryotic versus eukaryotic CO2 fixers in the oligotrphic ocean
Full text linkBecause 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
Ultraplankton distribution in surface waters of the Mozambique Channel – flow cytometry and satellite imagery
Full text linkThe composition of ultraplankton (UP) in near-surface samples collected underway every 1 to 6 h from a ship sailing from Durban to the Seychelles was determined by flow cytometry, using both autofluorescence pigments and fluorescence DNA staining. Prochlorococcus (Pro) (17 to 160 x 103 cells ml-1) numerically dominated the ultraphytoplankton (UPP), followed by Synechococcus (Syn) (4.5 to 57 x 103 cells ml-1) and eukaryotic algae (EA) (0.6 to 4.2 x 103 cells ml-1). The abundance of heterotrophic bacterioplankton (HB) was 0.4 to 1.3 x 106 cells ml-1. A strong correlation (r = 0.8 to 0.97) was observed between sea-viewing wide field of view sensor (SeaWiFS) satellite estimates of total chlorophyll a (chl a) concentration and chl a concentration, abundance and biomass of EA as well as abundance and biomass of HB. This shows the potential for deducing spatial distributions of these 2 groups for ecosystem modelling using satellite data. Although the correlation between satellite chl a estimates and Syn chl a concentration was strong (r = 0.83 to 0.88), the correlation with its abundance and biomass was poor (r < 0.6) due to high variability (factor of 12) in cellular chl a content and to a lesser extent to diurnal cycles. The relationships were similar when either only daytime or all UP measurements were compared with the satellite data. No relationship was found between satellite data and Pro chl a concentration, abundance or biomass, even after correction for a pronounced diel cycle, suggesting that the SeaWiFS instrument might not detect Pro chl a
Differential responses of Prochlorococcus and SAR11-dominated bacterioplankton groups to atmospheric dust inputs in the tropical Northeast Atlantic Ocean
No full textMetabolic responses of indigenous dominant bacterioplankton populations to additions of dust were examined in the tropical northeast Atlantic. Subsurface seawater samples were treated with dust, added directly or indirectly as a “leachate” after its rapid dissolution in deionised water. Samples were incubated in ambient temperature and light for up to 24 h and microbial metabolic responses assessed by 35S-methionine uptake. Prochlorococcus and Low Nucleic Acid (LNA) cells were sorted by flow cytometry to determine their group-specific responses. Sorted cells were also phylogenetically affiliated using fluorescence in situ hybridisation. The High-Light adapted ecotype II dominated the Prochlorococcus group and 73 ± 14% of LNA prokaryotes belonged to the SAR11 clade of Alphaproteobacteria. Both Prochlorococcus and LNA cells were metabolically impaired by the addition of dust (40 ± 28% and 37 ± 22% decrease in 35S-methionine uptake compared to controls, respectively). However, LNA bacterioplankton showed minor positive responses to dust leachate additions (7 ± 4% increase in 35S-methionine uptake) while metabolic activity of Prochlorococcus cells decreased in the presence of dust leachate by 16 ± 11%. Thus dust dissolution in situ appears to be more deleterious to Prochlorococcus than SAR11-dominated LNA bacterioplankton and hence could initiate a compositional shift in the indigenous bacterioplankton
High bacterivory by the smallest phytoplankton in the North Atlantic Ocean
No full textPlanktonic algae <5 m in size are major fixers of inorganic carbon in the ocean1. They dominate phytoplankton biomass in post-bloom, stratified oceanic temperate waters2. Traditionally, large and small algae are viewed as having a critical growth dependence on inorganic nutrients, which the latter can better acquire at lower ambient concentrations owing to their higher surface area to volume ratios3, 4. Nonetheless, recent phosphate tracer experiments in the oligotrophic ocean5 have suggested that small algae obtain inorganic phosphate indirectly, possibly through feeding on bacterioplankton. There have been numerous microscopy-based studies of algae feeding mixotrophically6, 7 in the laboratory8, 9, 10 and field11, 12, 13, 14, as well as mathematical modelling of the ecological importance of mixotrophy15. However, because of methodological limitations16 there has not been a direct comparison of obligate heterotrophic and mixotrophic bacterivory. Here we present direct evidence that small algae carry out 40–95% of the bacterivory in the euphotic layer of the temperate North Atlantic Ocean in summer. A similar range of 37–70% was determined in the surface waters of the tropical North-East Atlantic Ocean, suggesting the global significance of mixotrophy. This finding reveals that even the smallest algae have less dependence on dissolved inorganic nutrients than previously thought, obtaining a quarter of their biomass from bacterivory. This has important implications for how we perceive nutrient acquisition and limitation of carbon-fixing protists as well as control of bacterioplankton in the ocean
Evaluation of the efficiency of metabolism of dinoflagellate phosphorus and carbon by a planktonic ciliate
No full textThe trophic transfer of nutrients through the microbial food web is a key top-down control in aquatic ecosystems which is notoriously difficult to evaluate, particularly for planktonic protists. In this study, a sensitive dual-radioactive tracer technique was developed to simultaneously assess the ingestion rate, and carbon- and phosphorus-specific assimilation efficiencies, of the marine planktonic ciliate Strobilidium neptuni feeding on the autotrophic dinoflagellate Heterocapsa triquetra. Dinoflagellate prey were simultaneously 16 h pulse labelled with NaH14CO3 and H333PO4 before being fed to the ciliate, and radioactive labels were traced into ciliate biomass and the experimental medium, as well as being monitored in the prey cells. Rates measured in short-term (10 min) incubations, as commonly used to estimate protist uptake of fluorescently labelled prey, were approximately 6 times higher and 3–6 times more variable than rates measured in longer 3–5 h incubations. The efficiency of accumulation of prey carbon (54±9%) by ciliates was lower than that of prey phosphorus (68±3%) suggesting that the phosphorus to carbon ratio in the ciliates was 1.3 times higher than in the labelled dinoflagellate biomass. Rates of phosphorus accumulation and release were combined to reveal that ciliates consumed 3.2±0.6 dinoflagellates cell?1 h?1. The assessment of carbon tracer release by ciliates was less reliable due to 14CO2 exchange between the experimental media and air. The study concludes that the dual phosphorus–carbon radioactive tracer labelling of algal prey allowed the quantification of protist herbivory and nutrient remineralisation in laboratory experiments, thereby providing a potential technique for studying planktonic microbial trophic interactions in situ
Assaying picoplankton distribution by flow cytometry of underway samples collected along a meridional transect across the Atlantic Ocean
Full text linkThe composition of picoplankton in near-surface samples collected underway at least every 6 h from a ship sailing from the Falkland Islands to the British Isles was determined by flow cytometry, using both autofluorescence markers and staining with the DNA dye SYBR Green I. Prochlorococcus (0.1 to 0.3 X 106 cells ml-l) dominated the phytoplankton in the oligotrophic northern (24" to 38" N) and southern (8" to 34" S) gyre regions. In the equatorial region (8" S to 24" N) the more productive waters supported 10 to 100 times more picoeukaryote algae and Synechococcus than in the gyres. The collection of samples underway proved to be a very economical method of obtaining a good indication of the concentration of heterotroph~c bactena and of these 3 classes of picophytoplankton in the surface mixed layer (coefficient of correlation, r2 = 0.88 to 0.98) throughout a very long transect, but correlated less well with picoplankton standing stocks in the top 200 m of the water column (r2 = 0.39 to 0.88). Daily variations in the flow cytometry profiles of stained Prochlorococcus observed over a distance of some 8000 km of the tropical and subtropical Atlantic were interpreted as an indication of their synchronised cell division in surface waters. Their estimated average minimum growth rate (0.15 d-l) was similar in both southern and northern oligotrophic gyres and in equatorial waters
P-affinity measurements of specific osmotroph populations using cell-sorting flow cytometry
No full textTo elucidate the role that the marine microbes play in global nutrient cycling, it is necessary to recognize
how various phyto- and bacterioplankton groups compete for limiting nutrients. Specific phosphate affinity
describes an organism’s ability to harvest phosphate at low concentrations from the surrounding water. For the
first time, we have taken advantage of cell-sorting flow cytometry in combination with radio-labeled phosphorus
to measure this feature of specific osmotrophic groups in natural communities. Specific phosphate affinities
for Synechococcus spp. and picoeukaryotes were measured using live, unstained cells. The results were always
lower than theoretical calculated maximum values, corresponding well with observations of P-deficiency, or
sub-optimal P supply for the osmotroph community, at the time of investigation. Fixing and staining cells
before flow sorting offers the advantage of better separation of phytoplankton and showed high sorting reproducibility
when applied to nonaxenic Synechococcuscultures. A subsequent investigation of P-leakage from isotopically
labeled, fixed, stained cells in nonaxenic cultures of Synechococcusshowed that it was only slightly larger
than the loss of 17% found when uptake of new label was stopped with adding “cold” phosphate. Possible
applications of the currently developed methodology for population specific P affinity measurements by flow
sorting are discussed
Low microbial respiration of leucine at ambient oceanic concentration in the mixed layer of the central Atlantic Ocean
No full textBacterioplankton are the primary consumers of dissolved organic matter in the ocean, thus the quantification of bacterioplankton production (BP) is essential to our understanding of carbon cycling in the largest ecosystems on Earth. We compared BP, measured as the rate of 14C-leucine or 3H-leucine uptake at close to saturating concentration (20 nmol L−1), with ambient uptake measured from dilution bioassays. We hypothesized that saturation with leucine would lead to its respiration as a carbon source, thereby not truly representing ambient BP. Seawater was collected from the photic zone throughout the Atlantic Ocean. Respiration as a proportion of total consumption (uptake + respiration) of close to ambient (0.4 nmol L−1) and close to saturating (20 nmol L−1) 14C-leucine concentrations were compared. Saturating 3H-leucine additions overestimated ambient leucine uptake at low rates (200% ± 100% ambient) and underestimated uptake at high rates (90% + 20% ambient). The proportion of total leucine uptake that was respired was threefold higher for 20 nmol L−1 14C-leucine additions than 0.4 nmol L−1 14C-leucine additions (15% ± 8% and 5% ± 4%, respectively). Consequently, microbial efficiency of leucine assimilation—an indicator of bacterioplankton growth efficiency—was significantly higher and more stable at close to ambient 14C-leucine additions than at saturating concentrations (95% ± 4% and 85% ± 8%, respectively). Thus, saturation of oligotrophic open Atlantic Ocean bacterioplankton with leucine, or other molecules indicative of microbial metabolism, leads to the measurement of a response to a nutrient addition, rather than an ambient measurement
Photoheterotrophy of bacterioplankton is ubiquitous in the surface oligotrophic ocean
Full text linkAccurate measurements in the Southern Hemisphere were obtained to test a hypothesis of the ubiquity of photoheterotrophy in the oligotrophic ocean. We present experimental results of light-enhanced uptake of methionine, leucine and ATP by bacterioplankton during two large-scale transects of the South Atlantic. Light increased the uptake of substrates by both dominant bacterioplankton groups, Prochlorococcus and SAR11, as well as for the bulk microbial community. Our consistent experimental evidence strongly indicates that photoheterotrophy is characteristic of dominant bacterioplankton populations in the global oligotrophic ocean
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