Phytoplankton Community Structure, Biomass, and Off-flavor: Pond Size Relationships in Louisiana Catfish Ponds

Abstract.— Many aquaculture studies are conducted in relatively small research ponds and the results are then extrapolated to larger commercial ponds. Implicit in this research is the assumption that there is no relationship between pond size and phytoplankton composition. Study objectives were to assess phytoplankton composition and biomass by several methods in 17 channel catfish Ictalurus punctatus ponds at the Aqua‐culture Research Station, Louisiana Agricultural Experiment Station in Baton Rouge, Louisiana, USA. Pond size ranged from 0.04–0.60 ha. Sampling occurred weekly from 10 September – 1 October 1997. Water temperatures coincided with a transition from summer to fall‐winter conditions. Biomass was assessed by cell counts and quantification of photopigments. Concentrations of dissolved off‐flavor compounds (2‐methylisoborneol and geosmin) were assessed by gas chromatography/mass spectroscopy of water column samples. Cell count data showed differences in dominant species, biovolume, and diagnostic pigment signatures among ponds. The smaller ponds had more diverse phytoplankton composition compared to the larger ponds, whereas chlorophyll levels were nearly an order of magnitude lower in the smaller ponds. Ultraplanktonic (2–20 μm) unicellular cyanobacteria dominated the numerical counts on most sampling dates; however, biovolume transformations of cell count data reduced the dominance of this component relative to cryptophytes, diatoms, and filamentous cyanobacteria. Pigment and microscopic analyses were well correlated. Unialgal isolates of dominant taxa from these samples indicated the presence of at least five genera of off‐flavor producers in these ponds; these taxa included Anabaena, Aphanizomenon, Pseudanabaena, as well as two species of Oscillatoria. Care in extrapolating results from smaller‐sized research ponds to larger commercial ponds is warranted, as is the potential for taxa other than Oscillatoria and Anabaena in forming off‐flavor compounds.     

The excretion of Cryptococcus neoformans (Sanfelice) VUILLEMIN 1894 by pet birds

A survey on taxonomy, diagnosis, distribution and pathogenicity of Cryptococcus neoformans is given. The investigation presented here concerns 1046 fecal samples of avian species, which can be kept in the living area of humans. All together only 15 strains have been isolated, 9 of which originated from the same breeder.     

Untersuchungen zum Nachweis latenter Kontaminationen vonErwinia carotovora var.atroseptica (van Hall) Dye an in vitro-Pflanzen von Kartoffeln

Zusammenfassung Es wurde geprüft, mit welcher Nachweissicherheit in vitro-Klone auf latenten Befall vonErwinia carotovora var.atroseptica (Eca) untersucht werden k?nnen. Als Testmethoden wurden ELISA-daten von in vitro-Pflanzen, Topfpflanzen und Knollen sowie der Bakteriennachweis an in vitro-Proben auf N?hrb?den verglichen. W?hrend die richtige Erfassung Eca-freier Klone mit allen Nachweismethoden fast fehlerfrei gelang, war die Ausgrenzung der positiven Proben mit ELISA teilweise nur mit unzureichender Testsicherheit m?glich. Die N?hrbodentests erwiesen sich als ungeeignet. Die besten Ergebnisse lieferte der ELISA-Knollentest, der aber wegen der Gefahr der Eca-Rekontamination ausscheidet. Aus Ermangelung rationeller Alternativen wird vorerst, trotz schwankender Nachweissicherheit, dem ELISA an in vitro-Pflanzen der Vorzug gegeben. Der Gefahr m?glicher Fehlaussagen wird durch eine mehrfach gestaffelte Kombination von Wiederholungen und Verwendung eines Auswertungsverfahrens mit rechnerischer Grenzwertermittlung entgegengewirkt.

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Summary Erwinia carotovora var.atroseptica (Eca) can survive for many years in tissue culture without signs of bacterial infection on the plants or of it being visible on the medium. Because the infection can be transmitted from one subcultivation to the next, there is a need for appropriate tests to detect latent contamination. Latent infection of in vitro plants, glasshouse plants and tubers by Eca was studied using ELISA and two nutrient media tests in trials to determine the levels of detection which could be achieved. Attempts were made to improve the interpretation of the ELISA results by comparing it with other methods of analysis. The plant material originated from in vitro material of known disease status. Sap samples for ELISA were pressed from whole in vitro plants, pieces from the stem bases of glasshouse plants and the heel-end of tubers. ELISA readings were taken at 405 nm using two photometers. The level used for discriminating between negative and positive results was based on calculating (mean and spread of values respectively). YEB-medium in liquid form and the Luria-Bertani Medium in liquid and solidified (agar) form were the test media (Table 1). Finely cut nodal segments of in vitro plants were placed in liquid media and incubated for five weeks. In vitro leaves which had been pierced were laid on the agar medium. ELISA proved to be inadequate when the results from repeated subcultures were compared (Table 2). The reproducibility of the results varied in sign and value. Readings obtained with negative samples were in better agreement with the disease status of the plants than those from positive ones. Using calculated separation levels gave sharper differentiation in the results than using fixed values. The ELISA results, interpreted on the basis of , varied considerably when applied to in vitro plants in the glasshouse (Fig. 1). Complete detection of the bacteria was only successful in the first test with in vitro plants and in tuber tests. Detection of latent Eca contamination was unsuccessful using the test nutrient media. Only 1 in 116 replicates gave the clouding expected with the nutrient media. There was strong growth of bacteria after two days in all tests with dilutions of pure cultures. False positives did not occur i.e. tests withErwinia-free clones remained clear. On the basis of these results a clear indication of latent Eca contamination on in vitro plants is not possible using ELISA, since the same clones gave positive or negative results depending on the time of testing. The poor reliability of the test must be due to the levels of the pathogen on in vitro plants (10²/g), lower than the detection level of ELISA (10³ cells ml⁻¹). The failure of latent bacteria to grow in the test nutrient media may be due to the attachment of the Eca pathogen to the cell wall structure of the in vitro plants. For reliable detection an enzymatic pre-treatment is presumably necessary, which would allow the pathogen to grow actively in the test media. No improvement of detection levels can be expected using ELISA. Glasshouse grown plants give inconsistent results similar to in vitro cultures. The degree of detection achievable with tuber tests renders it impractical, since the danger of recontamination by Eca during tuber multiplication in the glasshouse is too great. However other approaches to detection described in the literature are either uncertain or too expensive, and it appears that there is no alternative to ELISA for the detection of latent Eca contamination. The possible danger of false results can be counteracted to some extent by the combined use of replication and evaluations based on calculated separation values.

     

Subsystems,flowpaths, and the spatial variability of nitrogen in a fluvial ecosystem

Nutrient dynamics in rivers affect biogeochemical fluxes from land to oceans and the atmosphere. Fluvial ecosystems are thus important environments for understanding spatial variability in nutrient concentrations. At the San Pedro River in semi-arid Arizona, USA, we investigated how variability in dissolved inorganic nitrogen (DIN) was regulated by subsystem type and hydrological flowpaths. The three subsystems we compared were the riparian zone, parafluvial (gravel bar) zone, and surface stream. DIN concentration was greater in the riparian zone than in the surface stream, suggesting that the riparian zone retains DIN and is a source of N for the surface stream. Parafluvial zones were too variable to generalize how they regulate DIN. Our hypothesis that subsystem type regulates DIN oxidation was too simple. The riparian and parafluvial zones host a mosaic of oxidizing and reducing conditions, as they exhibited highly variable ammonium to nitrate (NH₄⁺:NO₃⁻) ratios. Surface stream DIN was dominated by NO₃⁻. Along a subsurface flowpath in the riparian zone, we did not observe spatial covariation among the N forms and transformations involved in mineralization. We also compared spatial variability in solute concentrations between flowpaths and non-flowpath reference areas. Our mixed results suggest that spatial variability is regulated in part by flowpaths, but also by solute-specific processing length along a flowpath. To understand the distribution of N in fluvial ecosystems, subsystem type and flowpaths are readily discernable guides, but they should be coupled with other mechanistic factors such as biota and soil type.     

Bose-Einstein condensation of cesium

Bose-Einstein condensation of cesium atoms is achieved by evaporative cooling using optical trapping techniques. The ability to tune the interactions between the ultracold atoms by an external magnetic field is crucial to obtain the condensate and offers intriguing features for potential applications. We explore various regimes of condensate self-interaction (attractive, repulsive, and null interaction strength) and demonstrate properties of imploding, exploding, and non-interacting quantum matter.