Dianne B. Seale

Phagotrophy by a flagellate selects for colonial prey: A possible origin of multicellularity

Predation was a powerful selective force promoting increased morphological complexity in a unicellular prey held in constant environmental conditions. The green alga, Chlorella vulgaris, is a well-studied eukaryote, which has retained its normal unicellular form in cultures in our laboratories for thousands of generations. For the experiments reported here, steady-state unicellular C. vulgaris continuous cultures were inoculated with the predator Ochromonas vallescia, a phagotrophic flagellated protist (‘flagellate’). Within less than 100 generations of the prey, a multicellular Chlorella growth form became dominant in the culture (subsequently repeated in other cultures). The prey Chlorella first formed globose clusters of tens to hundreds of cells. After about 10–20 generations in the presence of the phagotroph, eight-celled colonies predominated. These colonies retained the eight-celled form indefinitely in continuous culture and when plated onto agar. These self-replicating, stable colonies were virtually immune to predation by the flagellate, but small enough that each Chlorella cell was exposed directly to the nutrient medium.

Waterborne lead affects feeding neurotransmitter levels of juvenile fathead minnows (Pimephales promelas)

Fathead minnows (Pimephales promelas) were exposed for a total of 4 wk to 0, 0.5 or 1.0 mg/I lead as lead acetate. After 14-day acclimation to flake food, separate groups were fed either 20 1-day, 2-day or 7-day old Daphnia magna on alternate days for 14 days and tested for total time spent feeding, failed attempts and foraging distance. Time spent feeding on daphnids and number of miscues were significantly higher among lead-exposed groups than in control fish (P<0.05). Except for a high significance among fish exposed to 1.0 mg/l lead and feeding on the largest sized prey (7-day olds), reaction distance showed no dose-response. After 4 wk, body lead ranged from not detectable in the controls to 44.2 ± 2.5 mg/l in the 1.0 mg/l lead exposed groups. Examination of whole brain neurotransmitters indicated a significant increase in both serotonin and norepinephrine levels (P < 0.01) but no change in dopamine in response to lead intoxication.

Xenopus laevis larvae (Amphibia, Anura) as model suspension feeders

Larvae of the South African clawed frog, Xenopus laevis (Daudin), are efficient, obligate suspension feeders. We examine the relationship between the ambient particle concentration offered these larvae as food and their filtering, ingestion, and buccal pumping rates. We demonstrate that: (i) the larvae can sense and respond to a broad range of particle concentrations, down to 0.2 mg 1−1 (dry weight); (ii) their metabolic needs theoretically can be met by particle concentrations as low as 5 mg 1−1; and (iii) their patterns of regulation of filtering and ingestion fit predictions from certain models used to describe zooplankton feeding dynamics. Two such models are discussed: the modified Monod (Michaelis-Menten) model, with a lower threshold below which the tadpoles do not feed, and an energy optimization model. Both the models and the observed behavior of the tadpoles allow for stability of populations of food organisms. Tadpole feeding dynamics apparently are compatible with both the predictions and assumptions of these models, suggesting similar regulation of feeding by tadpoles and zooplankton. However, the size, morphology, and behavior of X. laevis larvae make their feeding regulation uniquely accessible to direct observation.

Excretion of total phosphorus, dissolved reactive phosphorus, ammonia, and urea by Lake MichiganMysis relicta

Rates of nutrient release byMysis relicta in Lake Michigan were measured on five nights at a 45-m station near Milwaukee, Wisconsin, U.S.A., in the summer of 1977. Nocturnal vertical migrations of the mysids were monitored with both echosounder tracings and vertical net tows. Estimates of the total areal dry mass of the mysids ranged from 600 to 1 820 mg m−2. Rates of release of dissolved reactive phosphorus, total phosphorus, ammonia, and urea were measured in dark incubations on shipboard. Excretion experiments were initiated immediately after mysids were collected from each of several vertical net hauls. The depths of maximum mysid densities corresponded approximately with a deep phytoplankton peak located in the vicinity of the thermocline. Semiquantitative ‘demands’ for N and P by phytoplankton within this peak were obtained from14C estimates of primary production from a previous study, assuming a constant C:N:P ratio for the algae. These algal nutrient ‘demands’ were compared to potential N and P release by the mysids to obtain a first approximation of the relative rates of nutrient supply and demand for the field phytoplankton populations. Our analysis indicates that mysids may directly supply about 1–10% of the daily N and P ‘demands’ of the phytoplankton in the deep peak. However, indirect interactions betweenMysis relicta and other organisms, such as small zooplankton and fishes, could be major factors in nutrient recycling within the metalimnion and subthermocline region of Lake Michigan.

Ecology of micro-organisms in a small closed system - Potential benefits and problems for Space Station

The inevitble presence on the space station of microorganisms associated with crew members and their environment will have the potential for both benefits and a range of problems including illness and corrosion of materials. This report reviews the literature presenting information about microorganisms pertinent to Environmental Control and Life Support (ECLS) on the space station. The perspective of the report is ecological, viewing the space station as an ecosystem in which biological relationships are affected by factors such as zero gravity and by closure of a small volume of space. Potential sites and activities of microorganisms on the space station and their environmental limits, microbial standards for the space station, monitoring and control methods, effects of space factors on microorganisms, and extraterrestrial contamination are discussed.

Influence of experimental conditions on nitrogenous excretion by Lake MichiganMysis relicta (Lovén): laboratory studies with animals acclimated inFragilaria

In laboratory experiments, rates of excretion of ammonia and urea by Lake MichiganMysis relicta were compared for animals incubated in the presence and the absence of algal food (Fragilaria crotonensis chemostat outflow). Prior to experiments, all animals were acclimated to laboratory conditions and the experimental food for 2–4 weeks. Algae used in experiments were enriched in the dark with nutrients (N and P) prior to experimental incubation.There were no significant differences in the ammonia and urea excretion rates of mysids incubated in filtered water compared to those inFragilaria cultures, or in ammonia excretion rates either as a function of individual size or sex of the mysid or of its having been held at 5 °C, 10 °C, or 15 °C. Ammonia excretion rates measured between 4 and 8 h of incubation were significantly higher than those between 1–3 h and 8–15 h, for mysids held both in filtered water and inFragilaria cultures. The results are compared to those from shipboard incubations in a previous study and are discussed with respect to physiological regulation of nitrogenous waste production.

Rotifer size distribution changes during transient phases in open cultures

In laboratory studies, rotifers (Brachionus calyciflorus) were monitored under well-defined environmental conditions at different supply rates of a unicellular algal food (Chlorella vulgaris). Rotifer size frequency distributions are described for conditions of steady-state growth, exponential increase, and starvation. Temporal fluctuations in size-age structure are described for cultures during transient conditions during the approach to a steady state and following step changes in food supply rate. The size structures of the populations displayed definite and reproducible shifts among typical patterns during transient conditions, reflecting the physiological and other dynamic processes that underlay the population dynamics. Size structure probably is a key variable that should be included in models for predicting growth dynamics during transient growth conditions.

Turbidostat Culture of Brachionus calyciflorus: An Experimental System to Assess Biological Limits on Population Growth

The turbidostat is a continuous culture device in which the population density of an organism (or its food) is held constant, and the specific growth rate (r) of the population becomes a dependent variable. Turbidostat cultures are powerful tools for assessing biological limits on population growth. In theory, and in practice, if the food level is high enough to alleviate the effects of food limitation, the r in a turbidostat should equal rmax, the maximum specific growth rate of the population. Steady states can be readily achieved, in which the growth rate is balanced by losses: dilution, metabolism, and death. Hence, populations can be sustained at rmax for extended periods of time to assess key questions concerning biological limits on population growth: What is the maximum growth rate for a species? What are the cost-benefit trade-offs for partitioning energy to achieve maximum growth? How can theory and empirical results be integrated to reveal the nature of the environmental limiting factor? What are practical environmental constraints on population growth?

Use of semicontinuous culture methods for examining competitive outcome between two freshwater rotifers (genus Brachionus) growing on a single algal resource

By definition, simple resource competition takes place when two or more populations grow at the expense of the same single limiting resource (Hsu et al. 1977). The limiting resource is extracted from the common environment, and the growth rates of the competitors decline as they deplete the resource (Fredrickson and Stephanopoulos 1981). In the simplest case, only one resource is assumed to determine the growth rates of both species. The competitive mechanism is differential consumption and growth on this limiting nutrient.

The Use of Models to Predict Potential Contamination Aboard Orbital Vehicles

A model of fungal growth on air-exposed, nonnutritive solid surfaces, developed for utilization aboard orbital vehicles is presented. A unique feature of this testable model is that the development of a fungal mycelium can facilitate its own growth by condensation of water vapor from its environment directly onto fungal hyphae. The fungal growth rate is limited by the rate of supply of volatile nutrients and fungal biomass is limited by either the supply of nonvolatile nutrients or by metabolic loss processes. The model discussed is structurally simple, but its dynamics can be quite complex. Biofilm accumulation can vary from a simple linear increase to sustained exponential growth, depending on the values of the environmental variable and model parameters. The results of the model are consistent with data from aquatic biofilm studies, insofar as the two types of systems are comparable. It is shown that the model presented is experimentally testable and provides a platform for the interpretation of observational data that may be directly relevant to the question of growth of organisms aboard the proposed Space Station.