Feeding preferences and grazing rates of Pfiesteria piscicida and a cryptoperidiniopsoid preying on fish blood cells and algal prey

The grazing rates and feeding preferences of the dinoflagellates Pfiesteria piscicida and a cryptoperidiniopsoid on the alga Rhodomonas sp. and fish blood cells were calculated at different ratios of the two food types and at different total food densities. Data from 6 h grazing periods within microcosms were used to calculate grazing rates. Grazing rates of both dinoflagellates increased linearly with an increased ratio of blood cells to Rhodomonas, and P. piscicida had a higher maximum grazing rate than the cryptoperidiniopsoid. The grazing rate of P. piscicida on Rhodomonas also increased with increased Rhodomonas densities relative to the blood cells, but increased densities of Rhodomonas did not increase the grazing rate of the cryptoperidiniopsoid, suggesting a lower feeding threshold for this species. Both dinoflagellates demonstrated a preference for fish blood cells over Rhodomonas cells, with no significant difference in the index of preference between the two species. Total food abundance affected the degree of preference differently for each dinoflagellate species. A higher index of feeding preference was attained by P. piscicida when resource levels were high, while the cryptoperidiniopsoid did not show this response. A preference for fish blood cells occurred at all food ratios for both dinoflagellates, including when blood cells were scarce relative to the alternate food type (15% of total available food). These results suggest that these strains of P. piscicida and the cryptoperidiniopsoid share similar feeding preferences for the prey types tested, although cryptoperidiniopsoids have not been associated with fish kills.     

Observations on limnological conditions associated with a fish kill of Oreochromis niloticus in Lake Chivero following collapse of an algal bloom

Possible causes of deaths of Oreochromis niloticus in Lake Chivero were examined in relation to changes in limnological conditions monitored over a 25‐month period. The fish deaths coincided with the collapse of an algal bloom that had developed and builtup in the lake for 8 months. Chlorophyll a and dissolved oxygen increased to average concentrations of 42.4 μg l^?1 and 10.9 mg l^?1 respectively prior to the collapse of the bloom. Dissolved oxygen decreased when the bloom started to die off and coincided with the fish deaths when the average surface dissolved oxygen concentration in the lake was 3.9 mg l^?1 and was at a depth of 5 m <2 mg l^?1. Mortality probably resulted from depressed oxygen levels caused by the high oxygen demand from the massive algal die‐off and released algal toxins. This is the first time that die‐off of algae has been linked to fish‐kills in Lake Chivero as occurs in other hypereutrophic systems.     

Seasonal variations of diatoms and dinoflagellates in a shallow,temperate estuary,with emphasis on neritic assemblages

Seasonal changes in the diatom and dinoflagellate assemblages were examined in the neritic zone of the Urdaibai estuary (north Spain) with regard to some major physical and chemical variables during an annual cycle. A total of 81 diatoms and 38 dinoflagellates were identified and quantified during the study period. Both groups displayed a distinctive pattern of seasonal succession. The seasonal distribution of the Shannon index showed a trend of increasing values from the upper estuary to the lower neritic segment. The diatom diversity maxima were observed in February, April and September, and dinoflagellate maxima in April–May, July and October. Diatoms dominated the assemblages, reaching 1×10⁶ cells l^–1 from April to September. A shift from large diatoms and dinoflagellates to small bloom-forming taxa was observed during winter–early spring. A spring diatom bloom composed of Rhizosolenia spp. was observed in April, while small chain-forming taxa (chiefly Chaetoceros spp.) dominated from June to September. Cell maxima for both groups in late summer were produced by the diatoms Chaetoceros salsugineum and Skeletonema costatum, and by the dinoflagellates Heterocapsa pygmaea and Peridinium quinquecorne. Silicate availability by river supply and strong tidal-mixing of the water column seem to determine the year-round dominance of diatoms over dinoflagellates.     

The Influence of Selected Environmental Variables on Fish Assemblage Structure in a Range of Southeast African Estuaries

This study compared the ichthyofaunal assemblages in 10 Eastern Cape Province, South Africa, estuaries to identify whether there were any structural variations between the assemblages and to determine the environmental and physical factors linked to these variations. Species belonging to both the marine migrant and estuarine resident groups, by consistent representation within each estuary grouping, were responsible for the separation of fish assemblage structures between the different types of systems. The data further revealed that fish assemblages in the three main types of estuaries investigated were distinct, linked primarily to estuary mouth status and estuary size.     

Trophic cascading effects of predatory fish on leaf litter processing in a Japanese stream

A manipulative field experiment to test for trophic cascading effects of predatory fish on detritus processing by benthic invertebrates was performed in stream channels running through a wetland forest in northern Japan. To control for fish effects on benthic invertebrates, two simple treatments (fish-present and fish-absent) were established for 4 weeks, with two common predatory fish, rainbow trout (Oncorhynchus mykiss) and freshwater sculpin (Cottus nozawae), being introduced into and excluded from stream cages. At the end of experiment, the biomass of the dominant detritivore, an amphipod (Jesogammarus jezoensis), was significantly less in the fish-present treatment (0.56 g m^–2 in dry mass on average) than that in the fish-absent treatment (1.32 g m^–2), there being no significant treatment effect evident for the second-dominant detritivore, coleopteran larvae (Optioservus kubotai). The loss of oak leaves (Quercus crispla) from litter bags in the fish-present treatment (0.31 g week^–1 in dry mass on average) was significantly less than in the fish-absent treatment (0.54 g week^–1). Predator-induced lower biomass and likely lowered foraging activities of the J. jezoensis were responsible for the suppression of litter processing efficiency. In contrast, the standing crop of fine particulate organic matter did not differ significantly between the treatments. The experimental results revealed that the predatory fish had an indirect but significant effect on leaf litter processing in the stream.     

Impact of a Flood Event on the Planktonic Food Web of the Schelde Estuary (Belgium) in Spring 1998

To evaluate the effect of short-term changes in discharge on plankton dynamics, a station in the upper reaches of the Schelde estuary was monitored during 1 month in spring 1998 with a sampling frequency of 1–2 days. During this monitoring period, a flood event occurred during which discharge increased about 5-fold. This flood event had a strong effect on the composition of the planktonic community. Rotifers were strongly negatively affected by the flood event while densities of heterotrophic nanoflagellates and scuticociliates increased. Chlorophyll a concentration and abundance of bacteria, oligotrich ciliates and crustacean zooplankton did not respond clearly to the flood event. Although the flood event lasted only a few days it took more than 2 weeks for the planktonic community to return to its original composition.     

Controls on microbial CO2 production: a comparison of surface and subsurface soil horizons

Although a significant amount of the organic C stored in soil resides in subsurface horizons, the dynamics of subsurface C stores are not well understood. The objective of this study was to determine if changes in soil moisture, temperature, and nutrient levels have similar effects on the mineralization of surface (0–25 cm) and subsurface (below 25 cm) C stores. Samples were collected from a 2 m deep unsaturated mollisol profile located near Santa Barbara, CA, USA. In a series of experiments, we measured the influence of nutrient additions (N and P), soil temperature (10–35°C), and soil water potential (?0.5 to ?10 MPa) on the microbial mineralization of native soil organic C. Surface and subsurface soils were slightly different with respect to the effects of water potential on microbial CO₂ production; C mineralization rates in surface soils were more affected by conditions of moderate drought than rates in subsurface soils. With respect to the effects of soil temperature and nutrient levels on C mineralization rates, subsurface horizons were significantly more sensitive to increases in temperature or nutrient availability than surface horizons. The mean Q₁₀ value for C mineralization rates was 3.0 in surface horizons and 3.9 in subsurface horizons. The addition of either N or P had negligible effects on microbial CO₂ production in surface soil layers; in the subsurface horizons, the addition of either N or P increased CO₂ production by up to 450% relative to the control. The results of these experiments suggest that alterations of the soil environment may have different effects on CO₂ production through the profile and that the mineralization of subsurface C stores may be particularly susceptible to increases in temperature or nutrient inputs to soil.     

The application of a molecular clock based on molecular sequences and the fossil record to explain biogeographic distributions within the Alexandrium tamarense "species complex" (Dinophyceae)

The cosmopolitan dinoflagellate genus Alexandrium, and especially the A. tamarense species complex, contain both toxic and nontoxic strains. An understanding of their evolution and paleogeography is a necessary precursor to unraveling the development and spread of toxic forms. The inclusion of more strains into the existing phylogenetic trees of the Alexandrium tamarense species complex from large subunit rDNA sequences has confirmed that geographic distribution is consistent with the molecular clades but not with the three morphologically defined species that constitute the complex. In addition, a new clade has been discovered, representing Mediterranean nontoxic strains. The dinoflagellates fossil record was used to calibrate a molecular clock: key dates used in this calibration are the origins of the Peridiniales (estimated at 190 MYA), Gonyaulacaceae (180 MYA), and Ceratiaceae (145 MYA). Based on the data set analyzed, the origin of the genus Alexandrium was estimated to be around late Cretaceous (77 MYA), with its earliest possible origination in the mid Cretaceous (119 MYA). The A. tamarense species complex potentially diverged around the early Neogene (23 MYA), with a possible first appearance in the late Paleogene (45 MYA). A paleobiogeographic scenario for Alexandrium is based on (1) the calculated possible ages of origination for the genus and its constituent groups; (2) paleogeographic events determined by plate movements, changing ocean configurations and currents, as well as climatic fluctuations; and (3) the present geographic distribution of the various clades of the Alexandrium tamarense species complex.     

Detection of adeno- and lentiviral (HIV1) contaminations on laboratory surfaces as a tool for the surveillance of biosafety standards

Aims: As a biosafety laboratory, we survey the handling of adenovirus type 5 (Ad5) and HIV1‐derived lentivirus in contained‐use facilities in Switzerland to identify insufficiencies of the safety precautions taken by the laboratories. Methods and Results: In the past 9 years, we took 430 swab samples from various types of surfaces in research laboratories. Samples were examined for Ad5 contaminations by real‐time PCR and infectivity assay or for the presence of lentivirus (HIV1) nucleic acids by real‐time (RT) PCR. Samples collected from centrifuges did not only contain Ad5 DNA more frequently but also exhibited higher numbers of Ad5 and lentiviral (HIV1) DNA copies than swabs from any other area of sampling. Five of ten samples containing infectious Ad5 particles or lentivirus (HIV1) RNA were found in samples taken from centrifuges. Ad5 contamination rates were higher in the tube holder and lower on the inner wall of the rotor chamber in centrifuges that were fitted with aerosol tight covers compared to centrifuges without covers. Conclusions: Our results allowed the comparison of hygiene standards of different laboratories and lead to the identification of centrifuges as hotspots for contaminations. Significance and Impact of the Study: Based on our results, we propose to use the collected data as a tool for rating future swab results. Furthermore, the amount of Ad5 and HIV1‐derived lentivirus DNA could serve as an indicator of the level of good laboratory practice in contained‐use laboratories handling these viral vectors.     

Effects of L-Glutamate Transport Inhibition by a Conformationally Restricted Glutamate Analogue (2S,1'S,2'R)-2-(Carboxycyclopropyl)Glycine (L-CCG III) on Metabolism in Brain Tissue In Vitro Analysed by NMR Spectroscopy

(2S,1'S,2'R)-2-(Carboxycyclopropyl)glycine (L-CCG III) was a substrate of Na⁺-dependent glutamate transporters (GluT) in Xenopus laevis oocytes (IC₅₀ 13 and 2 M for, respectively, EAAT 1 and EAAT 2) and caused an apparent inhibition of [³H]L-glutamate uptake in mini-slices of guinea pig cerebral cortex (IC₅₀ 12 M). In slices (350 M) of guinea pig cerebral cortex, 5 M L-CCG III increased both the flux of label through pyruvate carboxylase and the fractional enrichment of glutamate, GABA, glutamine and lactate, but had no effect on total metabolite pool sizes. At 50 M L-CCG III decreased incorporation of ¹³C from [3-¹³C]-pyruvate into glutamate C4, glutamine C4, lactate C3 and alanine C3. The total metabolite pool sizes were also decreased with no change in the fractional enrichment. Furthermore, L-CCG III was accumulated in the tissue, probably via GluT. At lower concentration, L-CCG III would compete with L-glutamate for GluT and the changes probably reflect a compensation for the missing L-glutamate. At 50 M, intracellular L-CCG III could reach > 10 mM and metabolism might be affected directly.