COMPARATIVE KINETIC STUDIES OF NITRATE-LIMITED GROWTH AND NITRATE UPTAKE IN PHYTOPLANKTON IN CONTINUOUS CULTURE1

A comparative study of nitrate-limited growth and nitrate uptake was carried out in chemostat cultures of Ankistrodesmus falcatus (Corda) Ralfs., Asterionella formosa Hass., and Fragilaria crotonensis Kit. In each species growth rate (μ) was related to total cell nitrogen or cell quota (q) by the empirical Droop growth function. Nitrate uptake was a function of both external N concentration and q. The apparent maximum uptake rate (V_m') at a given μ was inversely related to q – q₀, where q₀ is the minimum quota. The apparent half-saturation constant for uptake, (K_m') appears to show a slight inverse trend with μ, although statistical analysis shows that this trend is inconclusive. When q approaches q₀, V_m' is several orders of magnitude greater than μq, the calculated steady-state uptake rate. As q increases, however, the difference between these two variables decreases sharply until q approaches q_m, the cell quota for nitrogen-rich cells. At this point the difference between μq and V_m' disappears. This behavior is explained by the feedback regulation of N uptake. The inverse relationship between V_m' and q – q₀ can be described by an empirical three-parameter equation.     

APPLICATION OF THE PDMPO TECHNIQUE IN STUDYING SILICA DEPOSITION IN NATURAL POPULATIONS OF FRAGILARIA CROTONENSIS (BACILLARIOPHYCEAE) AT DIFFERENT DEPTHS IN A EUTROPHIC RESERVOIR1

At weekly intervals from July to October 2006, we measured silica deposition in the summer diatom assemblage at various depths in the eutrophic ?ímov Reservoir (Czech Republic) using PDMPO, the 2‐(4‐pyridyl)‐5{[4‐(2‐dimethylaminoethyl‐aminocarbamoyl)‐methoxy]phenyl}oxazole labeling technique. Fluorescence microscopy coupled with image analysis allows quantifying silicon (Si) deposition over time and a simple distinction between cells that are actively depositing Si and those that are not. Diatom assemblage was exclusively dominated by Fragilaria crotonensis Kitton, which formed pronounced subsurface maxima (2–6.5 m). Concentrations of the main nutrients (Si and phosphorus, P) were low over the whole season; however, at depth, the nutrient availability was higher than at the surface. Fragilaria silica deposition rates were eight times higher at the surface than at depth. Half the population was involved in silica deposition at the surface, while only 20% active cells were doing so at depth. At the surface, silica deposition was limited by P deficiency; the effect of dissolved Si (DSi) was not statistically significant. Silica deposition at depth was significantly constrained by low light availability despite the 1% average light attenuation at depth, which is supposed sufficient for photosynthesis. This study represents the first attempt to employ the PDMPO technique coupled with quantitative image analysis of PDMPO fluorescence in freshwater ecology. On the basis of our results, PDMPO probe appears to be an appropriate proxy for the study of resource limitation in natural diatom populations.     

锌对2种淡水浮游藻类增殖的影响

选用铜绿微囊藻(Microcystis aeruginosa)和脆杆藻(Fragilariasp.)为试验材料,在不同锌离子浓度下对2类微藻的增殖影响进行研究分析.藻类增长潜力(AGP)实验结果显示,在Zn2 浓度为0.02~1.00μg/L的液体培养基中,铜绿微囊藻的生长增殖快,Zn2 浓度达到100.00μg/L时,受到明显抑制.Zn2 浓度在0.02μg/L时,脆杆藻的生长繁殖快,Zn2 浓度>0.10μg/L后,则受到不同程度的抑制.     

SPATIAL AND TEMPORAL VARIATION OF EPIPSAMMIC DIATOMS IN A SPRING-FED BROOK1

Epipsammic diatom communities were sampled and quantified bimonthly from June 1983 to April 1985 at four sites in a small brook in northwest Ohio. The primary objective was to determine the extent of temporal and spatial variability in the epipsammon. The four sites, approximately 250 m apart, differed in current velocity, illumination and the amount of sand-associated detritus. Diatoms were significantly more abundant in the portion of the brook flowing through an unshaded marsh than at three heavily shaded sites located upstream. Fragilaria leptostauron (Ehr.) Hust. and Achnanthes lanceolata var. dubia Grun. dominated the epipsammic assemblage at all sites throughout the entire study period. Only Meridion circulare (Grev.) Ehr. displayed a marked seasonal distribution. Although the same species were generally found at all sites, there were sufficient differences in relative abundance that communities could be discriminated according to site. Using two canonical variate axes, all 48 samples were correctly assigned to their proper sampling site based on community composition. Data from this epipsammic assemblage support the idea developed from the river continuum concept that species comprising riverine benthic assemblages continually persist and rarely become completely absent.     

Diatoms on herbarium macrophytes as indicators for water quality

To describe the reference situation for water quality in the eutrophicated broad area De Nieuwkoopse Plassen (Holland), attached diatoms from aquatic macrophytes, collected between 1934 and 1958 have been used. Recent samples for comparison were taken in 1989.The diatoms do not indicate significant changes in salinity. Eutrophication has reduced the number of rare species and increased the abundance of meso- to hyper-eutraphentic species, particularly Fragilaria berolinensis. Reduction of total phosphorus to concentrations below 0.02 mg 1^–1 might be sufficient to restore the diatom assemblages of the 1930s.     

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.Contribution No. 234, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin.Contribution No. 234, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin.     

Production of Urea by Bacterial Decomposition of Organic Matter Including Phytoplankton

A concentrated colony of Fragilaria crotonensis collected from the surface water of Lake Suwa, which is one of the typical eutrophic lakes in Japan, and organic matter contained in untreated surface water from the same lake were subjected to aerobic decomposition by bacteria in a dark room at a temperature of 20 ± 3 °C. An exponential increase of urea with time was recorded in both of the experiments. The apparent rate constants of urea production were calculated to be 0.083 day⁻¹ for decomposition of F. crotonensis and 0.051 day⁻¹ for decomposition of the organic matter contained in the untreated surface water. This study suggests that urea production by bacterial decomposition of organic matter, including phytoplankton, may be an important source of urea in natural waters under certain conditions.