278 research outputs found
Resting spores of the freshwater diatoms Acanthoceras and Urosolenia
Diatom resting spores are a widespread, but sometimes misconstrued component of siliceous microfossil assemblages. We illustrate and discuss resting spore morphology found in populations of Acanthoceras and Urosolenia , two widely distributed freshwater genera. Taxonomic status of these genera and the potential paleolimnologic interpretation of resting spores are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43094/1/10933_2004_Article_BF00680035.pd
Actinella species (Bacillariophyta) from an Amazon black water floodplain lake (Amazonas - Brazil)
Novel Sex Cells and Evidence for Sex Pheromones in Diatoms
BACKGROUND: Diatoms belong to the stramenopiles, one of the largest groups of eukaryotes, which are primarily characterized by a presence of an anterior flagellum with tubular mastigonemes and usually a second, smooth flagellum. Based on cell wall morphology, diatoms have historically been divided into centrics and pennates, of which only the former have flagella and only on the sperm. Molecular phylogenies show the pennates to have evolved from among the centrics. However, the timing of flagellum loss--whether before the evolution of the pennate lineage or after--is unknown, because sexual reproduction has been so little studied in the 'araphid' basal pennate lineages, to which Pseudostaurosira belongs. METHODS/PRINCIPAL FINDING: Sexual reproduction of an araphid pennate, Pseudostaurosira trainorii, was studied with light microscopy (including time lapse observations and immunofluorescence staining observed under confocal scanning laser microscopy) and SEM. We show that the species produces motile male gametes. Motility is mostly associated with the extrusion and retrieval of microtubule-based 'threads', which are structures hitherto unknown in stramenopiles, their number varying from one to three per cell. We also report experimental evidence for sex pheromones that reciprocally stimulate sexualization of compatible clones and orientate motility of the male gametes after an initial 'random walk'. CONCLUSIONS/SIGNIFICANCE: The threads superficially resemble flagella, in that both are produced by male gametes and contain microtubules. However, one striking difference is that threads cannot beat or undulate and have no motility of their own, and they do not bear mastigonemes. Threads are sticky and catch and draw objects, including eggs. The motility conferred by the threads is probably crucial for sexual reproduction of P. trainorii, because this diatom is non-motile in its vegetative stage but obligately outbreeding. Our pheromone experiments are the first studies in which gametogenesis has been induced in diatoms by cell-free exudates, opening new possibilities for molecular 'dissection' of sexualization
Conversion of self-assembled monolayers into nanocrystalline graphene: Structure and electric transport
Graphene-based materials have been suggested for applications ranging from
nanoelectronics to nanobiotechnology. However, the realization of
graphene-based technologies will require large quantities of free-standing
two-dimensional (2D) carbon materials with tuneable physical and chemical
properties. Bottom-up approaches via molecular self-assembly have great
potential to fulfil this demand. Here, we report on the fabrication and
characterization of graphene made by electron-radiation induced cross-linking
of aromatic self-assembled monolayers (SAMs) and their subsequent annealing. In
this process, the SAM is converted into a nanocrystalline graphene sheet with
well defined thickness and arbitrary dimensions. Electric transport data
demonstrate that this transformation is accompanied by an insulator to metal
transition that can be utilized to control electrical properties such as
conductivity, electron mobility and ambipolar electric field effect of the
fabricated graphene sheets. The suggested route opens broad prospects towards
the engineering of free-standing 2D carbon materials with tuneable properties
on various solid substrates and on holey substrates as suspended membranes.Comment: 30 pages, 5 figure
How UV photolysis accelerates the biodegradation and mineralization of sulfadiazine (SD)
Granulite facies Iower crustal xenoliths from the Eifel, West Germany: petrological and geochemical aspects
Evolution of the upper mantle beneath the Southern Baikal Rift Zone: A Sr-Nd isotope study of xenoliths from the Bartoy Volcanoes
- …