58 research outputs found
The effects of interspecific interactions between bloom forming cyanobacteria and Scenedesmus quadricauda (chlorophyta) on their photophysiology
Eutrophication and enhanced external nutrient loading of lakes and seas are most clearly reflected by increased cyanobacterial blooms, which are often toxic. Freshwater cyanobacteria produce a number of bioactive secondary metabolites, some of which have allelopathic properties, significantly influencing the biological processes of other algae, thereby affecting species composition and succession of the phytoplankton. The goal of this work was to investigate the influence of bloom-forming cyanobacterial exudates on the photophysiology of the green alga Scenedesmus quadricauda by chlorophyll fluorescence analysis. We were able to prove the effect of algal cell-free filtrates on the performance of S. quadricauda and demonstrate for the first time that the freshwater picocyanobacterium Cyanobium gracile has strong negative impact on the coexisting green alga. Neither the cyanotoxin (MYC, CYN and ATX) producing, nor the non-toxic strains showed any systematic effect on the production of S. quadricauda. Various strains of the cyanobacterium Cylindrospermopsis raciborskii inhibited the performance of the green alga independently of their origin. Our results urge further studies for a better understanding of the factors affecting the release of allelopathic compounds and the mechanisms of their effects on target organisms
Remote Sensing of Water Quality Parameters over Lake Balaton by Using Sentinel-3 OLCI
Source at https://doi.org/10.3390/w10101428.The Ocean and Land Color Instrument (OLCI) onboard Sentinel 3A satellite was launched in February 2016. Level 2 (L2) products have been available for the public since July 2017. OLCI provides the possibility to monitor aquatic environments on 300 m spatial resolution on 9 spectral bands, which allows to retrieve detailed information about the water quality of various type of waters. It has only been a short time since L2 data became accessible, therefore validation of these products from different aquatic environments are required. In this work we study the possibility to use S3 OLCI L2 products to monitor an optically highly complex shallow lake. We test S3 OLCI-derived Chlorophyll-a (Chl-a), Colored Dissolved Organic Matter (CDOM) and Total Suspended Matter (TSM) for complex waters against in situ measurements over Lake Balaton in 2017. In addition, we tested the machine learning Gaussian process regression model, trained locally as a potential candidate to retrieve water quality parameters. We applied the automatic model selection algorithm to select the combination and number of spectral bands for the given water quality parameter to train the Gaussian Process Regression model. Lake Balaton represents different types of aquatic environments (eutrophic, mesotrophic and oligotrophic), hence being able to establish a model to monitor water quality by using S3 OLCI products might allow the generalization of the methodology
Coherent storage and phase modulation of single hard x-ray photons using nuclear excitons
Coherent storage and phase modulation of x-ray single-photon wave packets in
resonant scattering of light off nuclei is investigated theoretically. We show
that by switching off and on again the magnetic field in the nuclear sample,
phase-sensitive storage of photons in the keV regime can be achieved.
Corresponding phase modulation of the stored photon can be accomplished
if the retrieving magnetic field is rotated by . The development
of such x-ray single-photon control techniques is a first step towards
forwarding quantum optics and quantum information to shorter wavelengths and
more compact photonic devices.Comment: 12 pages, 6 figures; v2 modified to match the published version,
condensed to 4 figures, results unchange
Overlapping resonances in nuclei coupling to the atomic shells
The resonant process of nuclear excitation by electron transition (NEET) in
highly charged ions is investigated. In NEET, a bound electronic decay
transition occurs with the simultaneous excitation of the nucleus, provided
that the energies of the atomic and nuclear transition match. By varying the
atomic charge state, the atomic transition energy can be tuned to a better
match of the nuclear transition energy. We propose a new way to create the
atomic hole in highly charged ions by dielectronic capture of a free electron.
In order to derive the cross section for the three-step process composed by
dielectronic capture, NEET, and the subsequent nuclear decay, a Feshbach
projection operator formalism is developed. With the help of this formalism,
the resonances in nuclei coupling to the atomic shell and the possible
interference between several direct and resonant electronic and nuclear
processes such as radiative recombination, dielectronic recombination and
nuclear excitation by electron capture are described.Comment: 15 pages, 5 figures, updated to the published versio
Theory of nuclear excitation by electron capture for heavy ions
We investigate the resonant process of nuclear excitation by electron
capture, in which a continuum electron is captured into a bound state of an ion
with the simultaneous excitation of the nucleus. In order to derive the cross
section a Feshbach projection operator formalism is introduced. Nuclear states
and transitions are described by a nuclear collective model and making use of
experimental data. Transition rates and total cross sections for NEEC followed
by the radiative decay of the excited nucleus are calculated for various heavy
ion collision systems
Laser-induced nonresonant nuclear excitation in muonic atoms
Coherent nuclear excitation in strongly laser-driven muonic atoms is
calculated. The nuclear transition is caused by the time-dependent Coulomb
field of the oscillating charge density of the bound muon. A closed-form
analytical expression for electric multipole transitions is derived and applied
to various isotopes; the excitation probabilities are in general very small. We
compare the process with other nuclear excitation mechanisms through coupling
with atomic shells and discuss the prospects to observe it in experiment.Comment: 7 pages, 5 figure
Coherent control of the cooperative branching ratio for nuclear x-ray pumping
Coherent control of nuclear pumping in a three level system driven by x-ray
light is investigated. In single nuclei, the pumping performance is determined
by the branching ratio of the excited state populated by the x-ray pulse. Our
results are based on the observation that in ensembles of nuclei, cooperative
excitation and decay leads to a greatly modified nuclear dynamics, which we
characterize by a time-dependent cooperative branching ratio. We discuss
prospects of steering the x-ray pumping by coherently controlling the
cooperative decay. First, we study an ideal case with purely superradiant decay
and perfect control of the cooperative emission. A numerical analysis of x-ray
pumping in nuclear forward scattering with coherent control of the cooperative
decay via externally applied magnetic fields is presented. Next, we provide an
extended survey of nuclei suitable for our scheme, and propose
proof-of-principle implementations already possible with typical M\"ossbauer
nuclear systems such as . Finally, we discuss the application
of such control techniques to the population or depletion of long-lived nuclear
states.Comment: 11 pages, 8 figures; updated to the published versio
Electric dipole-forbidden nuclear transitions driven by super-intense laser fields
Electric dipole-forbidden transitions of nuclei interacting with
super-intense laser fields are investigated considering stable isotopes with
suitable low-lying first excited states. Different classes of transitions are
identified, and all magnetic sublevels corresponding to the near-resonantly
driven nuclear transition are included in the description of the nuclear
quantum system. We find that large transition matrix elements and convenient
resonance energies qualify nuclear M1 transitions as good candidates for the
coherent driving of nuclei. We discuss the implications of resonant interaction
of intense laser fields with nuclei beyond the dipole approximation for the
controlled preparation of excited nuclear states and important aspects of
possible experiments aimed at observing these effects.Comment: 20 pages, 2 tables, 3 figures, minor modifications and update to the
published versio
Photon angular distribution and nuclear-state alignment in nuclear excitation by electron capture
The alignment of nuclear states resonantly formed in nuclear excitation by
electron capture (NEEC) is studied by means of a density matrix technique. The
vibrational excitations of the nucleus are described by a collective model and
the electrons are treated in a relativistic framework. Formulas for the angular
distribution of photons emitted in the nuclear relaxation are derived. We
present numerical results for alignment parameters and photon angular
distributions for a number of heavy elements in the case of E2 nuclear
transitions. Our results are intended to help future experimental attempts to
discern NEEC from radiative recombination, which is the dominant competing
process
Cooperative effects in nuclear excitation with coherent x-ray light
The interaction between super-intense coherent x-ray light and nuclei is
studied theoretically. One of the main difficulties with driving nuclear
transitions arises from the very narrow nuclear excited state widths which
limit the coupling between laser and nuclei. In the context of direct
laser-nucleus interaction, we consider the nuclear width broadening that occurs
when in solid targets, the excitation caused by a single photon is shared by a
large number of nuclei, forming a collective excited state. Our results show
that for certain isotopes, cooperative effects may lead to an enhancement of
the nuclear excited state population by almost two orders of magnitude.
Additionally, an update of previous estimates for nuclear excited state
population and signal photons taking into account the experimental advances of
the x-ray coherent light sources is given. The presented values are an
improvement by orders of magnitude and are encouraging for the future prospects
of nuclear quantum optics.Comment: 22 pages, 4 figures, 5 tables; updated to the published version, one
additional results tabl
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