The effect of Cyanophyta upon zooplankton in a eutrophic tropical lake (Lake Valencia,Venezuela)

The effect which Cyanophyta have upon the zooplankton varies according to the form of the alga (mucilaginous colonies or filaments) and its abundance. Periodical blooms of Microcystis aeruginosa were not detrimental for the zooplankton, in spite of the fact that copepods, cladocerans and rotifers consume small colonies. High concentrations of Lyngbya limnetica and Oscillatoria limnetica in Lake Valencia, Venezuela, proved to be inhibitory for cladocerans. A total absence of cladocerans was detected when filaments increased.     

Simulation of cellular compaction and internalization in mammalian embryo development as driven by minimization of surface energy

A model based upon minimization of surface energy is proposed as an explanation for compaction and internalization of cells during mammalian embryo development. The model is used to simulate and graphically display these phenomena on a computer.     

In vitro modification of Candida albicans invasiveness

Candida albicans produces germ-tubes (GT) when it is incubated in animal or human serum. This dimorphism is responsible for its invasive ability.The purpose of the present paper is (1) to evaluate the ability of rat peritoneal macrophages to inhibit GT production of ingested Candida albicans, obtained from immunized rats and then activated in vitro with Candida-induced lymphokines; (2) to determinate any possible alteration of phagocytic and candidacidal activities.The phagocytes were obtained from rats immunized with viable C. albicans. Some of them were exposed to Candida-induced lymphokines in order to activate the macrophages in vitro. The monolayers of activated, immune and normal macrophages were infected with a C. albicans suspension during 4 hr.Activated macrophages presented not only the highest phagocytic and candidacidal activities but a noticeable inhibition of GT formation and incremented candidacidal activity.     

Iron-Chelating Compounds Produced by Soil Pseudomonads: Correlation with Fungal Growth Inhibition

Strains of Pseudomonas putida, Pseudomonas sp., and Pseudomonas aeruginosa were examined for their ability to grow in the presence of the iron chelator, ethylenediamine-di-(o-hydroxyphenylacetic acid). In vitro fungal inhibition assays showed that the isolates varied in their ability to inhibit the growth of representative fungal plant pathogens. Fungal inhibition in vitro was superior to that of previously reported Pseudomonas sp. Studies with Fusarium oxysporum forma sp. lycopersici and a susceptible tomato cultivar demonstrated that Pseudomonas putida PPU3.1 was able to significantly reduce wilt disease.     

Factors Determining Annual Changes in Bacterial Photosynthetic Pigments in Holomictic Lake Cisó, Spain

The pigments and biomass of anoxygenic phototrophic bacteria were measured during a year cycle in Lake Cisó (Girona, Spain). Two genera, Chromatium and Chlorobium, accounted for most of the bacterial population. The bacteria were present throughout the year despite complete mixing of the lake during fall and winter. This was possible because the sulfide production in the sediment was high enough to make the lake anaerobic to the very surface. Solar radiation, temperature, and biomass of Chromatium sp. were found to be important in determining pigment concentrations by correlation analysis. Sulfide concentration and biomass of Chlorobium spp. were found to be unimportant. A path analysis was performed to determine what percentage of the variability of pigments could be explained by the variables studied. Since a high percentage could be explained, it was possible to conclude that solar radiation, temperature, and biomass of Chromatium sp. were the main variables.     

Population growth in random environments

This paper reviews some recent advances in single population stochastic differential equation growth models. They are a natural way to model population growth in a randomly varying environment. The question of which calculus, Itô or Stratonovich, is preferable is addressed. The two calculi coincide when the noise term is linear, if we take into account the differences in the interpretation of the parameters. This clarifies, among other things, the controversy on the theory of niche limiting similarity proposed by May and MacArthur. The effects of correlations in the environmental fluctuations and statistical methods for estimating parameters and for prediction based on a single population trajectory are mentioned. Applications to fisheries, wildlife management and particularly to environmental impact assessment are now becoming possible and are proposed in this paper.     

Collagen polysomes: Site of hydroxylation of proline residues

The biosynthesis of collagen on polysomes has been studied by using a newly devised method for obtaining polysomes in high yield from stationary-phase mouse fibroblast (line 3T6; Goldberg &, Green, 1967). These polysomes were completely disaggregated to monosomes by brief exposure to ribonuclease and they lost most of their radioactivity to the top of the sucrose gradients as a result of a 30-minute chase with unlabeled proline. After a ten-minute pulse with [³H]proline, nascent collagen peptides could be identified in these polysomes on sucrose gradients. Most of the proline residues susceptible to hydroxylation by collagen proline hydroxylase were found, in most cases, to be already hydroxylated in these nascent peptides. The nascent nature of these peptides was confirmed by the observation that treatment of the polysomes with RNase transferred the radioactive collagen peptides to the monosome area and these peptides could subsequently be removed to the soluble material at the top of the gradient upon treatment with puromycin. These findings therefore, show clearly that the hydroxylation of proline residues is occurring, in vivo under normal conditions, on nascent collagen chains. In no case was the degree of hydroxylation of the released collagen chains higher than that on the nascent collagen peptides. It seems likely, therefore, that the major site of proline hydroxylation is the nascent collagen peptide.     

Inactivation of 1,6-Diphosphatase by Glucose in Yeast

Fructose-1,6-diphosphatase was derepressed in Saccharomyces cerevisiae by incubation in media containing non-sugar carbon sources. Addition of glucose to a derepressed culture led to a rapid loss of the measurable activity of the enzyme. Fructose and mannose also produced inactivation, but 2-deoxyglucose was ineffective. Experiments with cycloheximide indicated that the inactivation does not require protein synthesis. It was also shown that the process is not energy-dependent. The reappearance of the enzyme was dependent on an energy source and was prevented by cycloheximide. These results suggest that fructose diphosphatase inactivation is irreversible and that reappearance of enzyme activity implies de novo synthesis. Screening of different genera of yeasts has shown that the inactivation of fructose diphosphatase is a relatively widespread phenomenon.     

Discrimination between groups of chromosomes and individual chromosomes in the normal human karyotype

The karyotypes of 200 normal individuals are examined in a study designed to investigate the extent to which the cytologist's classification of chromosomes into the A–G autosomal groups and identification of individual chromosomes are reflected in the values of measurements made on these chromosomes. The statistical technique of discriminant function analysis is employed to obtain linear functions of the chromosomal measurements which may be used to classify and identify chromosomes. The results of the study indicate that while chromosomes can be grouped with a reasonably high probability of success using measurements alone, identification of individual chromosomes is somewhat more difficult.     

POLYMORPHISM IN THE DESMID XANTHIDIUM REGULARE AND ITS TAXONOMIC IMPLICATIONS

This paper refers to a case of polymorphism in the desmid genus Xanthidium Ehr. It is based on material from Lake Dais Irmaios, the main body of water in the Zoological and Botanical Garden in Recife, Pernambuco, northeastern Brazil, collected at 4 different times of the year during 1967 and 1968. A detailed examination of almost 1300 specimens showed an enormous variety in form of Xanthidium regulare Nordst., X. fragile Borge, and X. pseudoregulare Borge, thus allowing the authors to draw the following conclusions: (1) the name X. regulare Nordst. should be retained until further and more detailed studies on form variation within the species are available; (2) the names X. regulare Nordst. var. asteptum Nordst. in Borge, X. regulare Nordst. var. sexangulare Grönbl., X. regulare Nordst. var. sexangulare Grönbl. f. robustior Grönbl., X. fragile Borge, X. fragile Borge forma, and X. fragile Borge var. depauperatum Borge should be considered synonymous, all referring to a single variety of X. regulare Nordst., var. asteptum Nordst. in Borge emend. C. Bic. & L. M. Carv.; (3) X. pseudoregulare Borge must be treated as a variety of X. regulare Nordst. and must be called X. regulare Nordst. var. pseudoregulare (Barge) C. Bic. & L. M. Carv. Finally, a key is given to the 3 varieties of X. regulare Nordst. proposed in the present paper.