The distribution of protozoa among soil aggregates

Abstract The distribution of 5 groups of protozoan cells among soil aggregates from an arable field soil was investigated, by incubating each aggregate (1–2 mm) in a ring on an agar plate with a prey bacterium and detecting emergent-active forms of each group from respective aggregates. The detection of each protozoan group was especially affected by scattered times of the excystment and the rapid re-encystment owing to food exhaustion. The cumulative number of protozoan-positive rings obtained through observation at short time intervals, therefore, provided us with more reliable results.
It was shown that the distribution of protozoan cells was not even but greatly conglomerative among soil aggregates.     

Catalysis of protein disulfide bond isomerization in a homogeneous substrate

Protein disulfide isomerase (PDI) catalyzes the rearrangement of nonnative disulfide bonds in the endoplasmic reticulum of eukaryotic cells, a process that often limits the rate at which polypeptide chains fold into a native protein conformation. The mechanism of the reaction catalyzed by PDI is unclear. In assays involving protein substrates, the reaction appears to involve the complete reduction of some or all of its nonnative disulfide bonds followed by oxidation of the resulting dithiols. The substrates in these assays are, however, heterogeneous, which complicates mechanistic analyses. Here, we report the first analysis of disulfide bond isomerization in a homogeneous substrate. Our substrate is based on tachyplesin I, a 17-mer peptide that folds into a beta hairpin stabilized by two disulfide bonds. We describe the chemical synthesis of a variant of tachyplesin I in which its two disulfide bonds are in a nonnative state and side chains near its N and C terminus contain a fluorescence donor (tryptophan) and acceptor (N(epsilon)-dansyllysine). Fluorescence resonance energy transfer from 280 to 465 nm increases by 28-fold upon isomerization of the disulfide bonds into their native state (which has a lower E(o') = -0.313 V than does PDI). We use this continuous assay to analyze catalysis by wild-type human PDI and a variant in which the C-terminal cysteine residue within each Cys-Gly-His-Cys active site is replaced with alanine. We find that wild-type PDI catalyzes the isomerization of the substrate with kcat/K(M) = 1.7 x 10(5) M(-1) s(-1), which is the largest value yet reported for catalysis of disulfide bond isomerization. The variant, which is a poor catalyst of disulfide bond reduction and dithiol oxidation, retains virtually all of the activity of wild-type PDI in catalysis of disulfide bond isomerization. Thus, the C-terminal cysteine residues play an insignificant role in the isomerization of the disulfide bonds in nonnative tachyplesin I. We conclude that catalysis of disulfide bond isomerization by PDI does not necessarily involve a cycle of substrate reduction/oxidation.     

A test of the cosmopolitan distribution of fresh-water protozoans

Summary The cosmopolitan distribution and invasion of fresh-water protozoans were examined on Abaco Island, Bahamas by comparing: (1) samples from surface waters; (2) containers of sterile water exposed to colonization; and (3) cultured soil samples. The distributional pattern of protozoans in the various surface waters was not uniform; the number of species found varied widely (from 1–32 sp.). Out of 74 species from surface waters and 119 species from the soil cultures, 20 species were common to both studies. Other species common only to soil samples may act as a reservoir group which furnishes colonizing species should a suitable aquatic habitat be present. Very few species (range of 0–6 per container) were collected from jars of exposed sterile water. The variation in numbers and kinds of species found in surface waters may be due to variations in water quality or variation of invaders, while differences in number of species found in the containers may indicate some variation in air-borne invasion rate since water quality was essentially the same.     

Biodiversity of terrestrial protozoa appears homogeneous across local and global spatial scales

Free-living microbes are by far the most abundant group of organisms in the biosphere, yet estimates of global species richness remain nebulous, and there is no consensus regarding the likely geographical distribution of species. Both uncertainties are addressed by the suggestion that the vast abundance of microbes may drive their ubiquitous random dispersal; for this would also make it likely that global species richness is relatively low. Here we test the idea of ubiquitous dispersal of testate amoebae and ciliates living in soil. We analysed their abundance and species richness in 150 soil samples collected from the one-hectare grassland site at Sourhope in Scotland, and in comparable published data from 1500 soil samples collected worldwide. Following taxonomic revision and removal of synonyms, there remained a total of 186 taxa (91 testate and 95 ciliate) recorded from both Sourhope and other places in the world. A fundamental pattern of random spatial distribution of species was revealed in species that are relatively rare. This probably arises from random dispersal, for when localised population growth occurs, the distributions become aggregated, as in virtually all metazoan species. We find no evidence for geographically-restricted protozoan morphospecies at spatial scales of 4 m², 10,000 m², or worldwide. Species that are locally rare or abundant are similarly rare or abundant on a global scale. Approximately one third of the global diversity of soil protozoa was found at the one-hectare grassland site in Scotland, but this is a minimum figure, for recorded species richness is proportional to sampling effort, as shown here.     

The heterogeneous distribution of acid hydrolases within a homogeneous population of cultured mammalian cells

1. Chinese-hamster ovary fibroblasts were cultured to provide a homogeneous cell population. Homogenates obtained from these cells were fractionated by centrifugation techniques and the resulting fractions were analysed for protein and for enzymes representative of certain subcellular particles. 2. Unlike those in rat liver homogenates, the mitochondrial and lysosomal populations proved impossible to separate by differential centrifugation owing to the similarity of their sedimentation properties. Their resolution was possible by using isopycnic centrifugation in a continuous sucrose density gradient. 3. The mitochondrial population equilibrated at a density of 1.17g.cm(-3) as in rat liver homogenates. However, the lysosomal population equilibrated at a lower rather than a higher density position than the mitochondria and the probable reasons for this are discussed. 4. The lysosomal population subdivided into two groups characterized by differences in acid hydrolase content and equilibrium densities. The fraction with a density of 1.15g.cm(-3) contained the majority of arylsulphatases A and B, of cathepsin and of beta-acetylglucosaminidase activities, whereas that with a density of 1.09g.cm(-3) contained the majority of the acid phosphatase and acid ribonuclease activities. The probable division of the lysosomal population of a single cell into a number of distinguishable subgroups is suggested.     

The interstitial ciliated protozoa of a Mediterranean microcommunity

The main features of an interstitial ciliate community, living in the coastal sand of the Mediterranean sea, were analyzed during a one-year survey, carried out on 113 samples. The community was composed of 56 species, 31 of which belong to 8 resident genera. Total density varied from 0 to 410 individuals cm^–3 and followed a Spring-Summer and an Autumn-Winter trend, not related to temperature or to any single abiotic variable (interstitial dissolved O₂, grain sand size, salinity), although the density of some taxa was related to one abiotic factor. During Spring, diversity increased by a synchronous bloom involving the whole community. Some taxa, such as the predator Lacrymaria and its prey Frontonia, were significantly associated. The finding of the simultaneous bloom of congeneric species, like that of the genus Remanella suggests that they respond to the same environmental factors, and avoid interspecific competition.     

The structure of the ventricle of Elliptio complanatus, a fresh-water lamellibranch

The morphology of the ventricle of the fresh-water lamellibranch, Elliptio complanatus, was investigated. Contrary to the condition reported previously in Tritogonia verrucosa, the two atria in Elliptio communicate with the ventricular lumen through separate openings, each guarded by an atrio-ventricular valve. Fixation of ventricle for electron microscopy with 2.5% buffered glutaraldehyde did not appear to shrink the tissue, in spite of the low blood osmolarity to which the muscle is adapted. Ventricle tissue is composed of smooth muscle fibers, containing a central nucleus, glycogen, mitochondria, paramyosin, dense bodies and “attachment plaques,” much like the ventricle of the salt-water clam, Venus (Mercenaria) mercenaria.     

The rise of model protozoa

It is timely to evaluate the role of protozoa as model organisms given their diversity, abundance and versatility as well as the economic and ethical pressures placed on animal-based experimentation. We first define the term model organism and then examine through examples why protozoa make good models. Our examples reflect major issues including evolution, ecology, population and community biology, disease, the role of organelles, ageing, space travel, toxicity and teaching. We conclude by recognising that although protozoa may in some cases not completely mimic tissue- or whole-animal-level processes, they are extremely flexible and their use should be embraced. Finally, we offer advice on obtaining emergent model protozoa.     

The plasmalemma redox system of a fresh-water alga and membrane electrical parameters

In order to learn more details about the plasmalemma redox system in the alga Hydrodictyon reticulatum the electrophysiological characteristics of the plasma membrane as influenced by artificial electron acceptors were estimated. Ferricyanide anion as well as TTF+ depolarized the membrane potential, the effect being more marked in the dark than in the light. In the presence of ferricyanide the membrane resistance in the light was decreased by about 29% on the average, in the dark it remained unchanged. On the other hand, TTF+ brought about a large increase in membrane resistance, notwithstanding the light conditions. The results are discussed in view of the impairing influence of both electron acceptors on the active inflow of chloride ions (Rybová, R. et al. (1990) Bot. Acta 103, 404-407). The electrogenic outflow of electrons appears to make the largest contribution to the membrane depolarization. The inhibition of a pumping mechanism coupling the active uptake of Cl- in some way to the transmembrane electron flow is envisaged as a plausible explanation for membrane resistance increase by TTF+.     

The differentiation of sclerocytes in fresh-water sponges grown in a silica-poor medium

Abstract. Fresh-water sponges ( Ephydatia fluviatilis ) were cultivated in a mineral medium containing as little silica as technically possible (less than 15 μg/1). Some of their cells secreted an internal, slender, flexible rod. If, and only if, silica was added later to the medium, these rods were completed into spicules. It is suggested that these flexible rods correspond to the organic axes of normal spicules. This would mean that the differentiation of sclerocytes does not depend on the presence of usable concentrations of silica.     

Characterisation of the substrate specificity of homogeneous vaccinia virus uracil-DNA glycosylase

The decision to stop smallpox vaccination and the loss of specific immunity in a large proportion of the population could jeopardise world health due to the possibility of a natural or provoked re-emergence of smallpox. Therefore, it is mandatory to improve the current capability to prevent or treat such infections. The DNA repair protein uracil-DNA glycosylase (UNG) is one of the viral enzymes important for poxvirus pathogenesis. Consequently, the inhibition of UNG could be a rational strategy for the treatment of infections with poxviruses. In order to develop inhibitor assays for UNG, as a first step, we have characterised the recombinant vaccinia virus UNG (vUNG) and compared it with the human nuclear form (hUNG2) and catalytic fragment (hUNG) UNG. In contrast to hUNG2, vUNG is strongly inhibited in the presence of 7.5 mM MgCl₂. We have shown that highly purified vUNG is not inhibited by a specific uracil-DNA glycosylase inhibitor. Interestingly, both viral and human enzymes preferentially excise uracil when it is opposite to cytosine. The present study provides the basis for the design of specific inhibitors for vUNG.     

The dynamics of an open,isothermal homogeneous,single enzyme,two substrate system

The dynamics of an isothermal, open, well mixed reaction system, wherein an enzyme reacts with either of two substrates but not both simultaneously, in a Michaelis-Menten fashion, are investigated. An open well mixed environment has been previously postulated as being a useful model for the living cell. The existence of multiple steady operating conditions, and limit cycles are shown to be impossible. Phase plane plots depicting the system's transient approach to the steady state are presented, and the effect of system parameters on the shape of the integral curves are discussed.     

The scope of mission-orientation in Dutch fresh-water ecology

In response to growing concern about environmental problems ecologists have engaged in a variety of mission-oriented efforts in which they claim to have taken into account the objective of helping to solve environmental problems in their research strategies or research programmes. The significance of these efforts is evaluated here in terms of both the theoretical development of the field of ecology and its orientation towards social objectives.Three examples of mission-orientation are analyzed on the basis of a case-study of Dutch fresh-water ecology; (1) ecosystems research within the framework of the International Biological Programme; (2) landscape ecology, and (3) ecological research on the management of fresh-water resources. These examples demonstrate that in principle the scope of missionorientation in ecology can be broad. In Dutch fresh-water ecology, however, two specific approaches have become particularly institutionalized. The ecologists tended to opt either for theory-centered approaches close to the type of research carried out by ecologists developing the field regardless of any societal mission or for problem-centered approaches without much emphasis on theory development. Types of mission-orientation which can be placed between these extremes have been established only to a limited extent in Dutch fresh-water ecology.