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In experiments with axenic cultures of Microcystis aeruginosa, we tested whether this cyanobacterium incorporates leucine, a compound that is often used for the measurement of heterotrophic bacterioplankton production. Microcystis showed significant leucine incorporation, and the uptake of exponentially growing cells was higher than the uptake of cells in stationary growth phase. Therefore, the leucine method may not be suitable for measuring bacterial production in highly eutrophic waters with a dominance of cyanobacteria.
Die Wüste Atacama
(2000)
Simulated long-term vegetation response to alternative stocking strategies in savanna rangelands
(2000)
Mixotrophy is a widespread phenomenon among planktonic protists. It involves the combination of autotrophy and heterotrophy in varying degrees. Many phytoflagellate species ingest bacteria as a means of obtaining nutrients for photosynthesis or for supplementing their carbon budget under light limitation. Ciliates either sequester the plastids of their algal prey or harbour endosymbiotic algae. In the saline lakes of the Vestfold Hills and in Lakes Hoare and Fryxell in the McMurdo Dry Valleys the dominant phytoflagellates ingest bacteria, and there is evidence to suggest that during the winter months they lack chlorophyll and may become entirely heterotrophic. In Lake Fryxell phagotrophic pyhtoflagellates (cryptophytes) made a significant impact on bacterial production, removing up to 13% of the bacterial biomass day-1. These cryptophytes suffered predation from Plagiocampa (a ciliate), which appears to harbour them for a significant period before digesting them. We suspect that this may be equivalent to an intermediate stage in the evolution of mixotrophy. A significant number of the planktonic ciliates in Antarctic lakes were mixotrophic. The final evolutionary end point is the situation seen in Mesodinium rubrum, which now relies entirely on its cryptophycean endosymbiont and no longer ingests food. Mesodinium is the dominant ciliate in many of the saline lakes of the Vestfold Hills, which are of marine origin. It can reach abundances in excess of 60,000l-1 in Ace Lake, This ciliate is a ubiquitous member of the marine plankton worldwide and has successfully adapted to the lacustrine environment in Antarctica. The evidence suggests that among the survival strategies seen in Antarctic lake plankton, mixotrophy plays and important role among a number of the dominant protozoan species.
The growth rates of heterotrophic nanoflagellates (HNAN), mixotrophic cryptophytes, dinoflagellates and ciliates in field assemblages from Ace Lake in the Vestfold Hills (eastern Antarctica) and Lakes Fryxell and Hoare (McMurdo Dry Valleys, western Antarctica), were determined during the austral summers of 1996/1997 and 1997/1998. The response of the nanoflagellates to temperature differed between lakes in eastern and western Antarctica. In Ace Lake the available bacterial food resources had little impact on growth rate, while temperature imposed an impact, whereas in Lake Hoare increased bacterial food resources elicited an increase in growth rate. However, the incorporation of published data from across Antarctica showed that temperature had the greater effect, but that growth is probably controlled by a suite of factors not solely related to bacterial food resources and temperature. Dinoflagellates had relatively high specific growth rates (0.0057-0.384 h(-1)), which were comparable to Antarctic lake ciliates and to dinoflagellates from warmer, lower latitude locations. Temperature did not appear to impose any significant impact on growth rates. Mixotrophic cryptophytes in Lake Hoare had lower specific growth rates than HNAN (0.0029-0.0059 h(-1) and 0.0056-0.0127 h(-1), respectively). They showed a marked seasonal variation in growth rate, which was probably related to photosynthetically active radiation under the ice at different depths in the water column. Ciliates' growth rates showed no relationship between food supply and mean cell volume, but did show a response to temperature. Specific growth rates ranged between 0.0033 and 0.150 h(-1) for heterotrophic ciliates, 0.0143 h(-1) for a mixotrophic Plagiocampa species and 0.0075 h(-1) for the entirely autotrophic ciliate, Mesodinium rubrum. The data indicated that the scope for growth among planktonic Protozoa living in oligotrophic, cold extreme lake ecosystems is limited. These organisms are likely to suffer prolonged physiological stress, which may account for the highly variable growth rates seen within and between Antarctic lakes.
Population biologists have long been interested in the oscillations in population size displayed by many organisms in the field and Laboratory. A wide range of deterministic mathematical models predict that these fluctuations can be generated internally by nonlinear interactions among species and, if correct, would provide important insights for understanding and predicting the dynamics of interacting populations. We studied the dynamical behavior of a two- species aquatic Laboratory community encompassing the interactions between a demographically structured herbivore population, a primary producer, and a mineral resource, yet still amenable to description and parameterization using a mathematical model. The qualitative dynamical behavior of our experimental system, that is, cycles, equilibria, and extinction, is highly predictable by a simple nonlinear model.
An unresolved discussion in contemporary ecology deals with the relative importance of competition along environmental gradients. In deserts, local-scale differences in environmental productivity may be caused by the presence of shrubs, which represent a favorable habitat for annual populations within a nutrient-poor matrix. In this study, we attempted to test the hypothesis that facilitation of desert annuals by shrubs increase the intensity of competition among the annual plants. Such negative indirect effects have so far been ignored in studies about plant-plant interactions. We tested our hypothesis by measuring seedling survival and fecundity of four abundant annual plant species with and without neighbors in open areas and under shrub canopies in a sandy desert area. Our findings did not indicate indirect negative effects of shrubs on their understory annuals. Sensitivity to the presence of neighbors varied between species and surprisingly, the species with the smallest seeds was the only one which was not negatively affected by the presence of neighbors. In contrast to our hypothesis, there was no difference between the habitat types shrubs and openings in absolute and relative competition intensity. Our overall results suggest that negative indirect effects of shrubs are unimportant in determining demographic response of understory annual plants.
Recently, numerous studies have pointed to the importance of positive interactions in natural communities. There is now a broad consensus that the balance between negative and positive interactions should shift along environmental gradients, with competition prevailing under environmentally benign conditions and positive interactions dominating under harsh conditions. A commonly cited example of the importance of facilitation in harsh environments is the preference of desert annual plants for the areas under the canopy of shrubs. The recognition of apparently positive effects of desert shrubs on annuals, however, has been mostly based on density measurements, while fitness parameters of the understory plants have been ignored. Also, the temporal consistency of such effects has not been previously tested. Based on conceptual ideas about the balance between interference and facilitation, we predicted that positive effects of the shrubs on the understory should dominate in dry years, while in favorable years, negative effects would be stronger. We tested our hypothesis by measuring the direction and magnitude of the shrub effect on demographic responses of four desert annual plant species during four consecutive seasons of differing rainfall. The results contradicted our initial hypothesis. Depending on the species, the effect of the shrubs shifted from either negative to neutral or from neutral to positive with increasing annual rainfall. However, this trend was stronger for the effect of shrubs on plant reproductive success than on their densities. Our data highlight the importance of measuring fitness parameters in studies of plant-plant interactions. We suggest that the negative effects of shrubs on plant fitness were due to rainfall interception, while positive effects were related to increased nutrient availability beneath shrubs. However, the mechanisms by which the shrubs and annuals interact can only be resolved using an experimental approach. Our results contradict previous hypotheses about the relative importance of positive and negative interactions along environmental gradients. A simple conceptual model summarizing the proposed role of rainfall in determining the direction of shrub effects on their understory annuals is presented.
The fairy circles of Kaokoland (North-West-Namibia) : origin, distribution, and characteristics
(2000)
The phosphoinositide signaling cascade is involved in photoreception in the leech Hirudo medicinalis
(2000)
An algal culture medium was developed which reflects the extreme chemical conditions of acidic mining lakes (pH 2.7, high concentrations of iron and sulfate) and remains stable without addition of organic carbon sources. It enables controlled experiments e.g. on the heterotrophic potential of pigmented flagellates in the laboratory. Various plankton organisms isolated from acidic lakes were successfully cultivated in this medium. The growth rates of an Chlamydomonas- isolate from acidic mining lakes were assessed by measuring cell densities under pure autotrophic and heterotrophic conditions (with glucose as organic C-source) and showed values of 0.74 and 0.40, respectively.