Diel rhythmicity in amino acid uptake by Prochlorococcus

The marine cyanobacterium Prochlorococcus, the most abundant phototrophic organism on Earth, numerically dominates the phytoplankton in nitrogen (N)-depleted oceanic gyres. Alongside inorganic N sources such as nitrite and ammonium, natural populations of this genus also acquire organic N, specifically amino acids. Here, we investigated using isotopic tracer and flow cytometric cell sorting techniques whether amino acid uptake by Prochlorococcus is subject to a diel rhythmicity, and if so, whether this was linked to a specific cell cycle stage. We observed, in contrast to diurnally similar methionine uptake rates by Synechococcus cells, obvious diurnal rhythms in methionine uptake by Prochlorococcus cells in the tropical Atlantic. These rhythms were confirmed using reproducible cyclostat experiments with a light synchronised axenic Prochlorococcus (PCC9511 strain) culture and 35S-methionine and 3H-leucine tracers. Cells acquired the tracers at lower rates around dawn and higher rates around dusk despite >104 times higher concentration of ammonium in the medium, presumably because amino acids can be directly incorporated into protein. Leucine uptake rates by cells in the S+G2 cell cycle stage were consistently 2.2 times higher than those of cells at the G1 stage. Furthermore, S+G2 cells up-regulated amino acid uptake 3.5 times from dawn to dusk to boost protein synthesis prior to cell division. Because Prochlorococcus populations can account from 13% at midday, and up to 42% at dusk, of total microbial uptake of methionine and probably of other amino acids in N-depleted oceanic waters, this genus exerts diurnally variable, strong competitive pressure on other bacterioplankton populations

Biomarker-defined clusters by level of Type 2 inflammatory involvement in severe asthma

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Quantitative aspects of the ecology of marine planktonic ciliated protozoans, with special reference to Uronema marinum Dujardin

The seasonal abundance of ciliates and other microseston components a I t Calshot was investigated between January 1974 and March 1975. Samples were collected by pumping and were size fractionated. Ciliates were found to be present throughout the year. The standing crops of both tintinnids and naked ciliates were at a maximum in late spring; a second peak was found in the autumn for tintinnids. Thirteen species of tintinnid were found although two species, StenosWella ventricosa and Tintinnopsis beroidea, dominated the tintinnid,assemblage throughout the year. Some tintinnid species showed seasonal succession and could be divided into seasonal groups.Laboratory studies were carried out A The bacteriovorous ciliateUronema marinum. The effects of salinity, food type and temperature-on reproduction were investigated. Experiments were carried out to determine rates of consumption, growth and reproduction in relation to food concentration and temperature. I Mean cell volumes and gross growth efficiencies were also measured. Spectrophotometry, using calibration by bacterial organic carbon measurements,' was used to determine food concentration. A method was developed for separating ciliates from their food for biomass estimation using the ciliate's migratory behaviourv Models were derived for consumption, growth and reproduction based on the forcing functions of food concentration and temperature. Tentative estimates were made for the- production of naked ciliate; and 'their consumption of bacteria at Calshot by integrating the field work data with the models of consumption and growth. Estimates were also made for the production of tintinnids and their consumption of nanophytoplankton using field study data and-literature derived information on consumption and reproduction. The results obtained suggest that the grazing of bacteria by naked ciliates is negligible in proportion to bacterial standing crop and production but that tintinnids may be important' in controlling phytoplankton production.</p

Assaying picoplankton distribution by flow cytometry of underway samples collected along a meridional transect across the Atlantic Ocean

The 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

Marine bacterioplankton can increase evaporation and gas transfer by metabolizing insoluble surfactants from the air-seawater interface

Hydrophobic surfactants at the air–sea interface can retard evaporative and gaseous exchange between the atmosphere and the ocean. While numerous studies have examined the metabolic role of bacterioneuston at the air–sea interface, the interactions between hydrophobic surfactants and bacterioplankton are not well constrained. A novel experimental design was developed, using Vibrio natriegens and 3H-labelled hexadecanoic acid tracer, to determine how the bacterial metabolism of fatty acids affects evaporative fluxes. In abiotic systems, &gt;92% of the added hexadecanoic acid remained at the air–water interface. In contrast, the presence of V. natriegens cells draws down insoluble hexadecanoic acid from the air–water interface as an exponential function of time. The exponents characterizing the removal of hexadecanoic acid from the interface co-vary with the concentration of V. natriegens cells in the underlying water, with the largest exponent corresponding to the highest cell abundance. Radiochemical budgets show that evaporative fluxes from the system are linearly proportional to the quantity of hexadecanoic acid at the interface. Thus, bacterioplankton could influence the rate of evaporation and gas transfer in the ocean through the metabolism of otherwise insoluble surfactants.<br/

Bacterial growth and grazing loss in contrasting areas of North and South Atlantic

Samples were collected from the top 200 m of the water column at 50 stations during two cruises in different, near equinoctial seasons on an Atlantic transect near the 20°W meridian between 50°N and 50°S. These samples were analysed to determine characteristics of the heterotrophic bacterial populations. Flow cytometry was used to enumerate these bacteria and determine their average size so as to calculate their biomass. Heterotrophic bacterial production, and the rate of grazing of these bacteria by heterotrophic nanoplankton in the main depth layers, were determined using 3H thymidine and 14C leucine techniques. The biomass of heterotrophic nanoplankton in these layers was determined using a glucosaminidase assay. Five provinces were distinguished along the transect and characterized by average values of all measured parameters. The relative composition and activity of the microbial community in the water columns within each province changed little between the two cruises. Lowest heterotrophic bacterial biomass of 1–2 mg C m–3 and production of 0.1–0.2 mg C m–3 day–1 were found in the northern and southern Atlantic gyres, and were relatively similar in both seasons. Biomass and production were 2–4 times higher in the northern and southern temperate waters, and in equatorial waters, than in the gyres and tended to show more seasonal variation. Production and biomass in the layer below the pycnocline were lower by 10–30% and about 50%, respectively, than values determined in the surface mixed layer, and varied less with latitude. Depth-integrated values of these two parameters were generally of similar size in the mixed water layer and the layer of the chlorophyll maximum and pycnocline, and tended to vary with season. The specific growth rate of heterotrophic bacteria was in the range 0.05 to 0.12 day–1 in the top mixed layer at all latitudes. In spite of the elevated temperatures, bacterial growth appears to be restricted by a shortage of nutrients so that the microbial community cycles very slowly, with a turnover time of the order of 1 week or more. The depth-integrated biomass of heterotrophic nanoplankton was generally about 100% of the heterotrophic bacterial biomass in the same water. Grazing by these nanoplankton at the rate measured could consume all of the new production of heterotrophic bacteria in all waters, and they probably control the populations of both heterotrophic and phototrophic bacteria

Picoplankton community structure on the Atlantic Meridional Transect: a comparison between seasons

Samples collected from 10 depths at 25 stations in September–October 1996 and 12 depths at 28 stations in April–May 1997 on an Atlantic Meridional Transect between the British Isles and the Falkland Islands were analysed by flow cytometry to determine the numbers and biomass of four categories of picoplankton: Prochlorococcus spp, Synechococcus spp, picoeukaryotic phytoplankton and heterotrophic bacteria. The composition of the picoplankton communities confirmed earlier findings (Zubkov, Sleigh, Tarran, Burkill &amp; Leakey, 1998) about distinctive regions along the transect and indicated that the stations should be grouped into five provinces: northern temperate, northern Atlantic gyre, equatorial, southern Atlantic gyre and southern temperate, with an intrusion of upwelling water off the coast of Mauritania between the northern Atlantic gyre and equatorial waters. Prochlorococcus was the most numerous phototrophic organism in waters of both northern and southern gyres and in the equatorial region, at concentrations in excess of 0.1×106ml?1; it also dominated plant biomass in the gyres, but the biomass of the larger picoeukaryotic algae equalled that of Prochlorococcus in the equatorial region; higher standing stocks of both Prochlorococcus and picoeukaryotes were present in spring than in autumn in waters of both gyres. In temperate waters at both ends of the transect the numbers and biomass of picoeukaryotes and, more locally, of Synechococcus increased, and the Synechococcus, particularly, were more numerous in spring than in autumn. There was a pronounced southward shift of the main populations of both Synechococcus and Prochlorococcus in April–May in comparison to those of September–October, associated with seasonal changes in solar radiation, the abundance of Prochlorococcus dropping sharply near the 17°C contour, while Synechococcus was still present at temperatures below 10°C. Picoeukaryotes were more tolerant of low temperatures and lower light levels, often being more abundant in samples from greater depths, where they contributed to the deep chlorophyll maximum. Heterotrophic bacterial numbers and biomass tended to be highest in those samples where phototrophic biomass was greatest, with peaks in temperate and equatorial waters, which were shifted southwards in April–May compared with September–October.<br/