433 research outputs found
Plasticity of Total and Intracellular Phosphorus Quotas in Microcystis aeruginosa Cultures and Lake Erie Algal Assemblages
Blooms of the potentially toxic cyanobacterium Microcystis are common events globally, and as a result significant resources continue to be dedicated to monitoring and controlling these events. Recent studies have shown that a significant proportion of total cell-associated phosphorus (P) in marine phytoplankton can be surface adsorbed; as a result studies completed to date do not accurately report the P demands of these organisms. In this study we measure the total cell-associated and intracellular P as well as growth rates of two toxic strains of Microcystis aeruginosa KĂŒtz grown under a range of P concentrations. The results show that the intracellular P pool in Microcystis represents a percentage of total cell-associated P (50â90%) similar to what has been reported for actively growing algae in marine systems. Intracellular P concentrations (39â147âfgâcellâ1) generally increased with increasing P concentrations in the growth medium, but growth rate and the ratio of total cell-associated to intracellular P remained generally stable. Intracellular P quotas and growth rates in cells grown under the different P treatments illustrate the ability of this organism to successfully respond to changes in ambient P loads, and thus have implications for ecosystem scale productivity models employing P concentrations to predict algal bloom events
Seasonal Expression of the Picocyanobacterial Phosphonate Transporter Gene phnD in the Sargasso Sea
In phosphorus-limited marine environments, picocyanobacteria (Synechococcus and Prochlorococcus spp.) can hydrolyze naturally occurring phosphonates as a P source. Utilization of 2-aminoethylphosphonate (2-AEP) is dependent on expression of the phn genes, encoding functions required for uptake, and CâP bond cleavage. Prior work has indicated that expression of picocyanobacterial phnD, encoding the phosphonate binding protein of the phosphonate ABC transporter, is a proxy for the assimilation of phosphonates in natural assemblages of Synechococcus spp. and Prochlorococcus spp (Ilikchyan et al., 2009). In this study, we expand this work to assess seasonal phnD expression in the Sargasso Sea. By RT-PCR, our data confirm that phnD expression is constitutive for the Prochlorococcus spp. detected, but in Synechococcus spp. phnD transcription follows patterns of phosphorus availability in the mixed layer. Specifically, our data suggest that phnD is repressed in the spring when P is bioavailable following deep winter mixing. In the fall, phnD expression follows a depth-dependent pattern reflecting depleted P at the surface following summertime drawdown, and elevated P at depth
Ammonium recycling supports toxic Planktothrix blooms in Sandusky Bay, Lake Erie: Evidence from stable isotope and metatranscriptome data
Sandusky Bay, Lake Erie, receives high nutrient loadings (nitrogen and phosphorus) from the Sandusky River, which drains an agricultural watershed. Eutrophication and cyanobacterial harmful algal blooms (cyanoHABs) persist throughout summer. Planktothrix agardhii is the dominant bloom-forming species and the main producer of microcystins in Sandusky Bay. Non-N2 fixing cyanobacteria, such as Planktothrix and Microcystis, thrive on chemically reduced forms of nitrogen, such as ammonium (NH4+) and urea. Ammonium regeneration and potential uptake rates and total microbial community demand for NH4+ were quantified in Sandusky Bay. Potential NH4+ uptake rates in the light increased from June to August at all stations. Dark uptake rates also increased seasonally and, by the end of August, were on par with light uptake rates. Regeneration rates followed a similar pattern and were significantly higher in August than June. Ammonium uptake kinetics during a Planktothrix-dominated bloom in Sandusky Bay and a Microcystis-dominated bloom in Maumee Bay were also compared. The highest half saturation constant (Km) in Sandusky Bay was measured in June and decreased throughout the season. In contrast, Km values in Maumee Bay were lowest at the beginning of summer and increased in October. A significant increase in Vmax in Sandusky Bay was observed between July and the end of August, reflective of intense competition for depleted NH4+. Metatranscriptome results from Sandusky Bay show a shift from cyanophycin synthetase (luxury NH4+ uptake; cphA1) expression in early summer to cyanophycinase (intracellular N mobilization; cphB/cphA2) expression in August, supporting the interpretation that the microbial community is nitrogen-starved in late summer. Combined, our results show that, in late summer, when nitrogen concentrations are low, cyanoHABs in Sandusky Bay rely on regenerated NH4+ to support growth and toxin production. Increased dark NH4+ uptake late in summer suggests an important heterotrophic contribution to NH4+ depletion in the phycosphere. Kinetic experiments in the two bays suggest a competitive advantage for Planktothrix over Microcystis in Sandusky Bay due to its higher affinity for NH4+ at low concentrations
Community dynamics and function of algae and bacteria during winter in central European great lakes
Abundant phytoplankton and bacteria were identified by microscopy and high-throughput 16S rRNA tag Illumina sequencing of samples from water- and ice phases collected during winter at two central European Great Lakes, Balaton and FertĆ (Neusiedlersee). Bacterial reads at all sites were dominated (\u3e85%) by Bacteroidetes and Proteobacteria. Amongst phototrophs, microscopy and 16S sequencing revealed that both phytoplankton and cyanobacteria were represented, with a median of 1500 cyanobacterial sequence reads amongst 13 samples analyzed. The sequence analysis compared replicate Balaton and FertĆ ice and water samples with an outgroup from three Hungarian soda lakes. In particular, both water and ice from FertĆ contained high contributions from cyanobacteria. Two percent of total reads identified to the level of family in water at FertĆ were dominated by a single operational taxonomic unit (OTU) of a cyanobacterium within the Rivulariaceae, which was largely absent from ice. Conversely, ice samples from both lakes yielded an abundant OTU assigned to a Flavobacterium sp. known to be associated with freshwater ice. Principal Coordinates Analysis (PCoA) revealed that the ice communities from all sites were similar to one another, and that the water communities did not cluster together. Fluorescence emission spectra obtained at 77 K confirmed the presence of intact cyanobacteria in FertĆ water and ice. Photosynthetic characterization of phototrophs resident in water and ice analyzed by assay of acid-stable photosynthetic H14CO3â incorporation showed that communities from both phases were photosynthetically active, thus adding to growing recognition of ice-covered lakes as viable habitat for phototrophs
Effect of Phosphorus Amendments on Present Day Plankton Communities in Pelagic Lake Erie
To address questions regarding the potential impact of elevated total phosphorus (TP) inputs (due to relaxed regulations of TP loading), a series of TP enrichment experiments were conducted at pelagic stations in the 3 hydrologically distinct basins of Lake Erie. Results of nutrient assimilation measurements and assays for nutrient bioavailability suggest that the chemical speciation, and not concentration, of nitrogenous compounds may influence phytoplankton community structure; this in turn may lead to the selective proliferation of cyanobacteria in the eastern basin of the lake. Assays with cyanobacterial bioluminescent reporter systems for P and N availability as well as N-tot:P-tot assimilation ratios from on-deck incubation experiments support this work. Considered in the context of a microbial food web relative to a grazing food web, the results imply that alterations in current TP loading controls may lead to alterations in the phytoplankton community structure in the different basins of the Lake Erie system
Transitions in microbial communities along a 1600âŻkm freshwater trophic gradient
This study examined vertically-resolved patterns in microbial community structure across a freshwater trophic gradient extending 1600 km from the oligotrophic waters of Lake Superior to the eutrophic waters of Lake Erie, the most anthropogenically influenced of the Laurentian Great Lakes system. Planktonic bacterial communities clustered by Principal Coordinates Analysis (PCoA) on UniFrac distance matrices into four groups representing the epilimnion and hypolimnion of the upper Great Lakes (Lakes Superior and Huron), Lake Superior\u27s northern bays (Nipigon and Black bays), and Lake Erie. The microbes within the upper Great Lakes hypolimnion were the most divergent of these groups with elevated abundance of Planctomycetes and Chloroflexi compared to the surface mixed layer. Statistical tests of the correlation between distance matrices identified temperature and sample depth as the most influential community structuring parameters, reflecting the strong UniFrac clustering separating mixed-layer and hypolimnetic samples. Analyzing mixed-layer samples alone showed clustering patterns were correlated with nutrient concentrations. Operational taxonomic units (OTU) which were differentially distributed among these conditions often accounted for a large portion of the reads returned. While limited in coverage of temporal variability, this study contributes a detailed description of community variability that can be related to other large freshwater systems characterized by changing trophic state
Evidence for Reionization at z ~ 6: Detection of a Gunn-Peterson Trough in a z=6.28 Quasar
We present moderate resolution Keck spectroscopy of quasars at z=5.82, 5.99
and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the
Ly Alpha absorption in the spectra of these quasars evolves strongly with
redshift. To z~5.7, the Ly Alpha absorption evolves as expected from an
extrapolation from lower redshifts. However, in the highest redshift object,
SDSSp J103027.10+052455.0 (z=6.28), the average transmitted flux is
0.0038+-0.0026 times that of the continuum level over 8450 A < lambda < 8710 A
(5.95<z(abs)<6.16), consistent with zero flux. Thus the flux level drops by a
factor of >150, and is consistent with zero flux in the Ly Alpha forest region
immediately blueward of the Ly Alpha emission line, compared with a drop by a
factor of ~10 at z(abs)~5.3. A similar break is seen at Ly Beta; because of the
decreased oscillator strength of this transition, this allows us to put a
considerably stronger limit, tau(eff) > 20, on the optical depth to Ly Alpha
absorption at z=6.
This is a clear detection of a complete Gunn-Peterson trough, caused by
neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen
fraction in the intergalactic medium would result in an undetectable flux in
the Ly Alpha forest region. Therefore, the existence of the Gunn-Peterson
trough by itself does not indicate that the quasar is observed prior to the
reionization epoch. However, the fast evolution of the mean absorption in these
high-redshift quasars suggests that the mean ionizing background along the line
of sight to this quasar has declined significantly from z~5 to 6, and the
universe is approaching the reionization epoch at z~6.Comment: Revised version (2001 Sep 4) accepted by the Astronomical Journal
(minor changes
High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data IV: Luminosity Function from the Fall Equatorial Stripe Sampl
This is the fourth paper in a series aimed at finding high-redshift quasars
from five-color imaging data taken along the Celestial Equator by the SDSS.
during its commissioning phase. In this paper, we use the color-selected sample
of 39 luminous high-redshift quasars presented in Paper III to derive the
evolution of the quasar luminosity function over the range of 3.6<z<5.0, and
-27.5<M_1450<-25.5 (Omega=1, H_0=50 km s^-1 Mpc^-1). We use the selection
function derived in Paper III to correct for sample incompleteness. The
luminosity function is estimated using three different methods: (1) the 1/V_a
estimator; (2) a maximum likelihood solution, assuming that the density of
quasars depends exponentially on redshift and as a power law in luminosity and
(3) Lynden-Bell's non-parametric C^- estimator. All three methods give
consistent results. The luminous quasar density decreases by a factor of ~ 6
from z=3.5 to z=5.0, consistent with the decline seen from several previous
optical surveys at z<4.5. The luminosity function follows psi(L) ~ L^{-2.5} for
z~4 at the bright end, significantly flatter than the bright end luminosity
function psi(L) \propto L^{-3.5} found in previous studies for z<3, suggesting
that the shape of the quasar luminosity function evolves with redshift as well,
and that the quasar evolution from z=2 to 5 cannot be described as pure
luminosity evolution. Possible selection biases and the effect of dust
extinction on the redshift evolution of the quasar density are also discussed.Comment: AJ accepted, with minor change
High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data II: The Spring Equatorial Stripe
This is the second paper in a series aimed at finding high-redshift quasars
from five-color (u'g'r'i'z') imaging data taken along the Celestial Equator by
the Sloan Digital Sky Survey (SDSS) during its commissioning phase. In this
paper, we present 22 high-redshift quasars (z>3.6) discovered from ~250 deg^2
of data in the spring Equatorial Stripe, plus photometry for two previously
known high-redshift quasars in the same region of sky. Our success rate of
identifying high-redshift quasars is 68%. Five of the newly discovered quasars
have redshifts higher than 4.6 (z=4.62, 4.69, 4.70, 4.92 and 5.03). All the
quasars have i* < 20.2 with absolute magnitude -28.8 < M_B < -26.1 (h=0.5,
q_0=0.5). Several of the quasars show unusual emission and absorption features
in their spectra, including an object at z=4.62 without detectable emission
lines, and a Broad Absorption Line (BAL) quasar at z=4.92.Comment: 28 pages, AJ in press (Jan 2000), final version with minor changes;
high resolution finding charts available at
http://www.astro.princeton.edu/~fan/paper/qso2.htm
Weak Lensing with SDSS Commissioning Data: The Galaxy-Mass Correlation Function To 1/h Mpc
(abridged) We present measurements of galaxy-galaxy lensing from early
commissioning imaging data from the Sloan Digital Sky Survey (SDSS). We measure
a mean tangential shear around a stacked sample of foreground galaxies in three
bandpasses out to angular radii of 600'', detecting the shear signal at very
high statistical significance. The shear profile is well described by a
power-law. A variety of rigorous tests demonstrate the reality of the
gravitational lensing signal and confirm the uncertainty estimates. We
interpret our results by modeling the mass distributions of the foreground
galaxies as approximately isothermal spheres characterized by a velocity
dispersion and a truncation radius. The velocity dispersion is constrained to
be 150-190 km/s at 95% confidence (145-195 km/s including systematic
uncertainties), consistent with previous determinations but with smaller error
bars. Our detection of shear at large angular radii sets a 95% confidence lower
limit , corresponding to a physical radius of
kpc, implying that galaxy halos extend to very large radii. However, it is
likely that this is being biased high by diffuse matter in the halos of groups
and clusters. We also present a preliminary determination of the galaxy-mass
correlation function finding a correlation length similar to the galaxy
autocorrelation function and consistency with a low matter density universe
with modest bias. The full SDSS will cover an area 44 times larger and provide
spectroscopic redshifts for the foreground galaxies, making it possible to
greatly improve the precision of these constraints, measure additional
parameters such as halo shape, and measure the properties of dark matter halos
separately for many different classes of galaxies.Comment: 28 pages, 11 figures, submitted to A
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