3,555 research outputs found
The role of heterotrophic microflagellates in plankton communities
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1984The distribution and feeding behavior of bacterivorous micro flagellates
(2-20 μm protozoa) and their ingestion by copepods were examined in an attempt
to assess the importance of these protozoa as a trophic link between
planktonic bacteria and zooplankton. The abundance of microflagellates
relative to other picoplankton (0.2-2.0 μm) and nanoplankton (2-20 μm)
populations in water samples in the North Atlantic and in Lake Ontario and on
macroaggregates in the North Atlantic was determined using direct
microscopical and culture estimation techniques. Seasonal, vertical and
geographical changes in the density of microflagellates were generally not
greater than one order of magnitude. Microscopical counts of heterotrophic
nanoplankton (presumably microflagellates) typically ranged from a few hundred
to a few thousand m1-1 for a variety of planktonic environments. They
constituted approximately 1/3 to 1/2 of the nanoplankton in the euphotic zone
and dominated the nanoplankton in the aphotic zone. Most Probable Number
(MPN) estimation of the density of bacterivorous protozoa indicated that
microflagellates were, on average, an order of magnitude more abundant than
bacterivorous ciliates and amoebae. MPN and direct microscopical counts of
microflagellates differed by as much as 104. This discrepancy was smaller
in eutrophic environments (e.g. Continental Shelf and Lake Ontario) and on
macroscopic detrital aggregates.
All microbial populations enumerated were highly concentrated on
macroscopic detrital aggregates relative to their abundance in the water
surrounding the aggregates. Enrichment factors (the ratio of abundance of a
population on a macroaggregate to its abundance in the surrounding water)
increased along a eutrophic-to-oligotrophic gradient because of the combined
effects of an increased abundance of microorganisms on macroaggregates in
oligotrophic environments and a decreased abundance in the surrounding water
in these same environments. Average enrichment factors for direct
microscopical counts of heterotrophic nanoplankton (range = 17-114) were not
as large as enrichment factors observed for MPN estimates of the number of
bacterivorous microflagellates (range = 273-18400). Microflagellates
numerically dominated the bacterivorous protozoa cultured from macroaggregates
by one to two orders of magnitude, but ciliates and amoebae were also highly
enriched on macroaggregates. Microenvironments are therefore a potentially
important aspect for the ecology of planktonic microorganisms. Observations
on the microbial colonization of mucus sloughed by ctenophores and discarded
appendicularian houses suggest that these materials may be important sources
of macroaggregates.
Batch and continuous culture experiments were conducted with clonal
cultures of microflagellates to test their ability to grow on various types
and densities of bacteria. The doubling time of Monas sp. 1 ranged from 43 hr
(when fed the cyanobacterium Synechococcus Strain WH 8101) to 6.9 hr (when fed
the heterotrophic bacterium Serratia marinorubra). Cell yields (i.e. the
conversion of bacterial biomass into protozoan biomass) of Monas sp. 1 fed two
species of heterotrophic bacteria were greater than yields for the
microflagellate fed two species chroococcoid cyanobacteria (range = 7-68%).
Cell yields of two other species of microflagellates (Monas sp. 2 and
Cryptobia maris) were 48% and 61%, respectively, on the bacterium Pseudomonas
halodurans. Microflagellates grew in continuous culture at concentrations of
bacteria which were lower than bacterial densities required for the growth of
ciliates as shown by other investigations. Therefore, microflagellates appear
to be well-adapted for grazing bacterioplankton.
Microflagellates were also investigated for their ability to graze
bacteria attached to particles. Bodo nanorensis and Rhynchomonas nasuta both
showed a marked ability to graze attached bacteria and a limited ability to
graze unattached cells. These results suggest that microflagellates may also
be important consumers of bacteria attached to particles in the plankton and
may explain the highly elevated densities of microflagellates on
macroaggregates.
Grazing experiments performed with the copepod Acartia tonsa indicated
that heterotrophic microflagellates were ingested by the copepods at rates
comparable to the ingestion of phytoplankton of similar size. The presence of
heterotrophic microflagellates did not depress filtration rates of the
copepods, and one species (Cryptobia maris) appeared to be selectively
grazed. Survival of A. tonsa on a diet of heterotrophic microflagellates was
similar to survival on a diet of phytoplankton and was significantly longer
than survival of starved Controls or copepods fed only bacteria.
Due to their ability to grow at in-situ densities of planktonic bacteria,
their relatively high cell yields, and their acceptability as food for
zooplankton, it is concluded that bacterivorous microflagellates may
constitute an important trophic link between bacteria and zooplankton. This
link may provide a mechanism whereby organic material and energy from the
detrital food chain can be returned to the classical phytoplankton-copepod-fish food chain.This research was supported by National Science Foundation grants
OCE80-2444l and OCE82-l4928 and Ocean Industry Program grant 4473 awarded to
Dr. Laurence P. Madin, NSF Doctoral Dissertation grant OCE8l-l299l, the Woods
Hole Oceanographic Institution Education Program and the Wood Hole
Oceanographic Institution Biology Department
Two-sample Bayesian Nonparametric Hypothesis Testing
In this article we describe Bayesian nonparametric procedures for two-sample
hypothesis testing. Namely, given two sets of samples
\stackrel{\scriptscriptstyle{iid}}{\s
im} and \stackrel{\scriptscriptstyle{iid}}{\sim},
with unknown, we wish to
evaluate the evidence for the null hypothesis
versus the
alternative . Our
method is based upon a nonparametric P\'{o}lya tree prior centered either
subjectively or using an empirical procedure. We show that the P\'{o}lya tree
prior leads to an analytic expression for the marginal likelihood under the two
hypotheses and hence an explicit measure of the probability of the null
.Comment: Published at http://dx.doi.org/10.1214/14-BA914 in the Bayesian
Analysis (http://projecteuclid.org/euclid.ba) by the International Society of
Bayesian Analysis (http://bayesian.org/
Nitrogen uptake and NH4+ regeneration by pelagic microplankton and marine snow from the North Atlantic
Comparative rates of nitrogen uptake and NH4+ regeneration by plankton of \u3c153 and \u3c5 μm in size were determined in the Sargasso Sea and Gulf Stream, and by plankton associated with marine snow in the Gulf Stream during May 1982. Rates of total nitrogen uptake of Sargasso Sea phytoplankton exceeded those of the Gulf Stream phytoplankton by factors ranging from 1.8 to 5.6. Rates of microplankton NH4+ regeneration equaled or exceeded rates of NH4+ uptake in the Sargasso Sea, but in the Gulf Stream were negligible in all but one case. Significant rates of NH4+ regeneration were measured for Gulf Stream marine snow, and, in all but one case, exceeded those of NH4+ uptake. Rates of NO3– and urea uptake by the snow were less than half those of NH4+. Protozoan densities were enumerated on aliquots of the same snow particles and compared with previously reported bacterial estimates; enrichment factors of the cultivable ciliates and flagellates were 6500–9000 relative to ambient seawater. These organisms were also grazing and reproducing rapidly. Bacterial densities were also moderately enriched, but their productivity was lower than surrounding seawater bacteria. Thus, the large bacterivorous population associated with marine snow may have accounted for a substantial fraction of the observed NH4+ regeneration
Composition and degradation of salp fecal pellets: Implications for vertical flux in oceanic environments
Changes in the sinking rates, ash-free dry weights, particulate carbon and nitrogen content, and carbon:nitrogen ratios from the fecal pellets of several species of oceanic salps were examined in ten-day decomposition studies. Although bacteria and protozoa became abundant in the incubation vessels, most of the fecal pellets remained physically intact throughout the study. Bacterial activity in the pellets (measured by the rate of uptake of 3H-thymidine) increased, but microbial degradation had little effect on the sinking speeds of most of the fecal pellets. The average losses of ash-free dry weight and carbon and nitrogen content, along with changes in carbon:nitrogen ratio, were small compared to their initial values. We conclude that microbial degradation of large salp fecal pellets would not prevent the vertical flux to the deep ocean of a significant fraction of the particulate organic material contained in the pellets. The fecal pellets of oceanic salps provide a rapid, and potentially important, mechanism for the consolidation and vertical transport of organic and lithogenic material associated with minute particles in the open ocean
In Situ Diazotroph Population Dynamics Under Different Resource Ratios in the North Pacific Subtropical Gyre.
Major advances in understanding the diversity, distribution, and activity of marine N2-fixing microorganisms (diazotrophs) have been made in the past decades, however, large gaps in knowledge remain about the environmental controls on growth and mortality rates. In order to measure diazotroph net growth rates and microzooplankton grazing rates on diazotrophs, nutrient perturbation experiments and dilution grazing experiments were conducted using free-floating in situ incubation arrays in the vicinity of Station ALOHA in March 2016. Net growth rates for targeted diazotroph taxa as well as Prochlorococcus, Synechococcus and photosynthetic picoeukaryotes were determined under high (H) and low (L) nitrate:phosphate (NP) ratio conditions at four depths in the photic zone (25, 45, 75, and 100 m) using quantitative PCR and flow cytometry. Changes in the prokaryote community composition in response to HNP and LNP treatments were characterized using 16S rRNA variable region tag sequencing. Microzooplankton grazing rates on diazotrophs were measured using a modified dilution technique at two depths in the photic zone (15 and 125 m). Net growth rates for most of the targeted diazotrophs after 48 h were not stimulated as expected by LNP conditions, rather enhanced growth rates were often measured in HNP treatments. Interestingly, net growth rates of the uncultivated prymnesiophyte symbiont UCYN-A1 were stimulated in HNP treatments at 75 and 100 m, suggesting that N used for growth was acquired through continuing to fix N2 in the presence of nitrate. Net growth rates for UCYN-A1, UCYN-C, Crocosphaera sp. (UCYN-B) and the diatom symbiont Richelia (associated with Rhizosolenia) were uniformly high at 45 m (up to 1.6 ± 0.5 d-1), implying that all were growing optimally at the onset of the experiment at that depth. Differences in microzooplankton grazing rates on UCYN-A1 and UCYN-C in 15 m waters indicate that the grazer assemblage preyed preferentially on UCYN-A1. Deeper in the water column (125 m), both diazotrophs were grazed at substantial rates, suggesting grazing pressure may increase with depth in the photic zone. Constraining in situ diazotroph growth and mortality rates are important steps for improving parameterization for diazotrophs in global ecosystem models
Paleoproterozoic sterol biosynthesis and the rise of oxygen
Natural products preserved in the geological record can function as ‘molecular fossils’, providing insight into organisms and physiologies that existed in the deep past. One important group of molecular fossils is the steroidal hydrocarbons (steranes), which are the diagenetic remains of sterol lipids. Complex sterols with modified side chains are unique to eukaryotes, although simpler sterols can also be synthesized by a few bacteria. Sterol biosynthesis is an oxygen-intensive process; thus, the presence of complex steranes in ancient rocks not only signals the presence of eukaryotes, but also aerobic metabolic processes. In 1999, steranes were reported in 2.7 billion year (Gyr)-old rocks from the Pilbara Craton in Australia, suggesting a long delay between photosynthetic oxygen production and its accumulation in the atmosphere (also known as the Great Oxidation Event) 2.45–2.32 Gyr ago. However, the recent reappraisal and rejection of these steranes as contaminants pushes the oldest reported steranes forward to around 1.64 Gyr ago (ref. 6). Here we use a molecular clock approach to improve constraints on the evolution of sterol biosynthesis. We infer that stem eukaryotes shared functionally modern sterol biosynthesis genes with bacteria via horizontal gene transfer. Comparing multiple molecular clock analyses, we find that the maximum marginal probability for the divergence time of bacterial and eukaryal sterol biosynthesis genes is around 2.31 Gyr ago, concurrent with the most recent geochemical evidence for the Great Oxidation Event. Our results therefore indicate that simple sterol biosynthesis existed well before the diversification of living eukaryotes, substantially predating the oldest detected sterane biomarkers (approximately 1.64 Gyr ago), and furthermore, that the evolutionary history of sterol biosynthesis is tied to the first widespread availability of molecular oxygen in the ocean–atmosphere system
Enrichment of microbial populations in macroaggregates (marine snow) from surface waters of the North Atlantic
Marine snow particles (macroscopic detrital aggregates) were collected from surface waters throughout the western North Atlantic. Counts of phototrophic and heterotrophic picoplankton, phototrophic and heterotrophic nanoplankton, and phototrophic microplankton were made by epifluorescence microscopy. A Most Probable Number culture technique also was used to estimate the density of bacterivorous protozoa. All microbial populations enumerated were highly enriched on macroaggregates relative to their densities in the surrounding water. The degree of enrichment was greater in open ocean environments because microorganisms in the surrounding water were less abundant in the open ocean than in nearshore waters, and also because microbial density on marine snow was greater in the open ocean than in nearshore environments. Material released by ctenophores and appendicularia is a likely source of marine snow since it supported microbial populations of the same order of magnitude as were observed on SCUBA-collected particles. Heterotrophic nanoflagellates dominated the bacterivorous protozoa cultured from macroaggregates and the surrounding water, but dense populations of ciliates and amoebae also were present on particles. Protozoan populations on marine snow were so dense relative to the surrounding water as to suggest that detrital aggregates are responsible for the planktonic existence of some bacterivorous species
Seasonal analysis of protistan community structure and diversity at the USC Microbial Observatory (San Pedro Channel, North Pacific Ocean)
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109997/1/lno20105562381.pd
Squeezing in the audio gravitational wave detection band
We demonstrate the generation of broad-band continuous-wave optical squeezing
down to 200Hz using a below threshold optical parametric oscillator (OPO). The
squeezed state phase was controlled using a noise locking technique. We show
that low frequency noise sources, such as seed noise, pump noise and detuning
fluctuations, present in optical parametric amplifiers have negligible effect
on squeezing produced by a below threshold OPO. This low frequency squeezing is
ideal for improving the sensitivity of audio frequency measuring devices such
as gravitational wave detectors.Comment: 5 pages, 6 figure
Ultraviolet asymptotics of scalar and pseudoscalar correlators in hot Yang-Mills theory
Inspired by recent lattice measurements, we determine the short-distance (a
> omega >> pi T) asymptotics
of scalar (trace anomaly) and pseudoscalar (topological charge density)
correlators at 2-loop order in hot Yang-Mills theory. The results are expressed
in the form of an Operator Product Expansion. We confirm and refine the
determination of a number of Wilson coefficients; however some discrepancies
with recent literature are detected as well, and employing the correct values
might help, on the qualitative level, to understand some of the features
observed in the lattice measurements. On the other hand, the Wilson
coefficients show slow convergence and it appears uncertain whether this
approach can lead to quantitative comparisons with lattice data. Nevertheless,
as we outline, our general results might serve as theoretical starting points
for a number of perhaps phenomenologically more successful lines of
investigation.Comment: 27 pages. v2: minor improvements, published versio
- …