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Quantitative aspects of the ecology of marine planktonic ciliated protozoans, with special reference to Uronema marinum Dujardin
No full textThe 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
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
Microzooplankton grazing activity in the temperate and sub-tropical NE Atlantic: summer 1996
No full textMarine bacterioplankton can increase evaporation and gas transfer by metabolizing insoluble surfactants from the air-seawater interface
No full textHydrophobic 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, >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
No full textSamples 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
Assessing amino acid uptake by phototrophic nanoflagellates in nonaxenic cultures using flow cytometric sorting
No full textBiologically available concentrations of individual dissolved amino acids in the open ocean are generally <1 nM. Despite this, the microbial turnover of amino acids is usually measured in hours indicating high demand. It is thought that the majority of uptake is due to bacterioplankton, although protists, particularly phototrophic protists, are also expected to take up amino acids. In order to assess the ability of protists to compete with prokaryotes for amino acids at subnanomolar concentrations, we examined the direct uptake of 3H-leucine by phototrophic nanoflagellates (prasinophytes, pelagophytes and trebouxiophytes) and by associated bacteria using flow cytometric cell sorting. In contrast to 3H-leucine-assimilating bacterial copopulations, none of the six studied nanoflagellates showed measurable direct uptake of 3H-leucine, suggesting that the studied phototrophic protists were unable to utilize dissolved 3H-leucine at natural oceanic concentrations. More practically, the flow-sorting technique allowed rapid and unequivocal differentiation of organic nitrogen uptake between prokaryotic cells and eukaryotic cells in mixed microbial populations, reducing the need to establish and maintain axenic algal cultures.<br/
Comparison of cellular and biomass specific activities of dominant bacterioplankton groups in stratified waters of the Celtic Sea
Full text linkA 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 -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 -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 -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
Picoplankton community structure on the Atlantic Meridional Transect: a comparison between seasons
No full textSamples 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 & 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/
Production and turnover of particulate dimethylsulphoniopropionate during a coccolithophore bloom in the northern North Sea
No full textDimethylsulphoniopropionate (DMSP) synthesised by phytoplankton is the principal precursor of the climatically active gas dimethyl sulphide (DMS). The rates of production of particulate DMSP (DMSPp) and turnover by microzooplankton were determined in surface waters of the northern North Sea, using a dilution approach. The phytoplankton communities were characterised by DMSP-rich taxa including Emiliania huxleyi and Prorocentrum minimum and DMSPp:chlorophyll a (chl a) ratios of 64 to 162 nM ?g-1. Microzooplankton biomass varied from 25.5 to 56.7 ?g C l-1 and was dominated by oligotrich ciliates and heterotrophic dinoflagellates. DMSPp production rates ranged from 14.8 to 45.6 nM d-1 and represent a doubling time of the ambient DMSPp pool of between 1.2 and 3.1 d. Consumption rates of DMSPp by microzooplankton varied between 11.4 and 59.9 nM d-1 and were equivalent to turnover rates of the ambient DMSPp pool of between 16 and 43% d-1. In general, production rates of DMSPp were lower than those of chl a and E. huxleyi, with respective mean doubling times of 1.9, 1.5 and 1.3 d. Loss rates due to grazing were similar for DMSPp and E. huxleyi but generally significantly lower than those of the bulk phytoplankton, with mean turnover rates of 31, 30 and 40% d-1 of the standing stock of DMSPp, E. huxleyi and chl a, respectively. E. huxleyi contributed an estimated 2 to 25% of the total DMSPp production and 6 to 23% of the DMSPp ingested by microzooplankton, indicating the importance of other phytoplankton to DMSPp dynamics in ŒE. huxleyi blooms¹. At the depths sampled, DMSPp production was closely coupled to primary production and was equivalent to approximately 11% of the carbon fixation. DMSPp may be an important component of the diets of microzooplankton. Ingested DMSPp could have provided 2 to 3% of the microzooplankton carbon demand and 26 to 44% of their sulphur demand. DMSPp production and turnover rates were closely matched and suggest that in these waters microzooplankton grazing may be the principal determinant of the fate of DMSPp. The quantity of ŒDMSP¹ excreted by microzooplankton, calculated from ingestion rates, biomass and assumed growth rates and growth efficiencies, ranged from 8.0 to 41.9 nM d-1. This is equivalent to 13 to 35% d-1 of the DMSPp standing stocks and sufficient to support a daily turnover of the DMS plus dissolved DMSP (DMS+DMSPd) pool
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