Lipid bilayer polypeptide interactions studied by molecular dynamics simulation

A model membrane with a polypeptide alpha-helix inserted has been simulated by molecular dynamics at a temperature well above the gel/liquid crystalline phase transition temperature. Order parameters of the lipids and other equilibrium and dynamic quantities have been calculated. Three systems, polyglycine constrained into an alphahelical configuration, glycophorin with similarly conformationally constrained backbone and finally glycophorin free to change its backbone conformation, have been studied. In all cases there was an ordering of the chains close to the helix. This effect was, however, much smaller for glycophorin with its rather bulky side chains than for polyglycine. The dynamics of the lipids were affected by the neighbouring helix, not drastically however. Lateral diffusion and reorientational time correlations of lipids close to the helix were slower than for the bulk ones, but not more than two or three times. Thus, we did not find any evidence of bound or frozen boundary lipids.     

The influence of earthworms (Lumbricidae) on the nitrogen dynamics in the soil litter system of a deciduous forest

Summary The influence of earthworms (Aporrectodea caliginosa (Savigny) and Lumbricus castaneus (Savigny)) on the rate of nitrogen net mineralization of the soil was studied in the laboratory and in the field. The additional mineralization of nitrogen cause by the burrowing activity of the substrat feeding earthworm A. caliginosa (N _L )was directly correlated to the biomass of the lumbricids independently of their number. A rise in temperature caused an exponential increase in N _L values. The Q ₁₀ value of this process (2.18) was found to be much higher than that of the nitrogen mineralization without earthworms (Q ₁₀=1.22). At 15°C the N _L value caused by A. caliginosa was calculated to be about 250 g N g^-1 fresh body wt d^-1. Using the experimentally determined exponential relationship between temperature and N _L values, the additional nitrogen mineralization caused by a population of A. caliginosa in a beechwood on limestone was calculated to be 4.23 kg ha^-1 a^-1.In contrast to A. caliginosa the litter dwelling species L. castaneus lost considerable amounts of biomass (56%) during the 4 week incubation period. Only 1/3 of the nitrogen equivalent to the weight loss of the animals was recovered in the mineral nitrogen pool.The addition of litter (old beech leaf litter, freshly fallen beech and ash leaf litter) had a pronounced effect on both nitrogen net mineralization and N _L values of the soil. Presence of old beech leaves caused an increase in both values, wheres the other litter types effected a decrease in nitrogen net mineralization. Fragmented ash litter was found to have the most distinct effect on N _L values (-69%) and nitrogen net mineralization (-74%).     

Population Density and Spatial Pattern of Heterodera glycines in Relation to Soybean Phenology

Population dynamics of Heterodera glycines (SCN) were influenced by initial nematode population density in soil, soybean root growth pattern, soil type, and environmental conditions in two field experiments. Low initial populations (Pi) of SCN increased more rapidly during the growing season than high Pi and resulted in greater numbers of nematodes at harvest. Egg and juvenile (J2) populations increased within 2-6 weeks after planting when early-season soil temperatures were 20 C and above and were delayed by soil temperatures of 17 C or below in May and early June. Frequencies of occurrence and number of nematodes decreased with increasing depth and distance from center of the soybean row. Spatial pattern of SCN paralleled that of soybean roots. Higher clay content in the subsoil 30-45 cm deep in one field restricted soil penetration by roots, indirectly influencing vertical distribution of SCN. Shoot dry weight was a good indicator of the effect of SCN on seed yield. Root dry weight was poorly correlated with soybean growth and yield. The relationship of yield (seed weight) to Pi was best described by a quadratic equation at one site, but did not fit any regression model tested at the second site.     

Repeated floristical observations on islets in the Aegean

Repeated observations are made on species and numbers of individuals in some islets. The changes after 14 years are reported for 11 islets. They include several extinctions and invasions. Great changes in numbers of individuals were found, especially for many of the annuals, in some of them as great as between 0–10 and over 10 000 individuals. The greatest constancy was found in a few dominant and subdominant perennials.Dedicated to Prof.K. H. Rechinger on the occasion of his 80th birthday.     

Variation in length and age at sexual maturity of Atlantic groundfish: a reply

Synopsis The assertion has been made by Halliday (1987) that trends in size and age at maturity of Atlantic groundfish published by Beacham (1983a, b, c, d, e, f) are artifacts induced by errors in determining the sex of an individual, distinguishing between immature and mature fish, sampling fish outside of the regular spawning season, and by nonrandom sampling of the population. In particular, Halliday asserts that for the Atlantic argentine,Argentina silus analysis, the conclusions of Beacham (1983a) that: (1) median length at sexual maturity declined over time; and (2) males matured at older ages than did females are invalid owing to biases in both sampling and analysis. In fact, if some of the biases indicated by Halliday were significant, then the decline in median length at sexual maturity is enhanced and the conclusions of Beacham (1983a) reinforced. Size and age at sexual maturity are dynamic characters in many vertebrate populations, and the fact that they should change for Atlantic groundfish should not be surprising given variable exploitation patterns in the fisheries since 1960.     

Comparison of population dynamics between slow- and fast-growing strains of the rotifer Brachionus calyciflorus pallas in continuous culture

Summary The population dynamics of a slow- and a fast-growing strain of the rotifer Brachionus calyciflorus are compared. Rotifers were grown in steady-states, at various specific growth rates (), in both two-stage chemostat and turbidostat cultures on the green alga Chlorella pyrenoidosa. Population variables, including specific ingestion (I), loss (L) and filtration (F) rates, yield (Y), production (P) and half-saturation coefficient of growth (K _g), were calculated using a growth model based on saturation kinetics. I, L, F and K _g were shown to be higher and Y and P lower for the fast-growing strain. Differences between the two strains with regard to these variables may represent tradeoffs associated with the faster potential growth rate. Steady-state relationships between these values and for the fast-growing strain, however, deviated from model predictions which suggest a possible shift from carbon to non-carbon growth limitation.     

How important is seed predation to recruitment in stable populations of long-lived perennials?

Summary The many evolutionary modifications to seed biology in response to seed predation do not necessarily imply that seed predators have an important impact on population recruitment. This is because competition between individual plants for rare safe sites can cause an oversupply of seeds so far as a population is concerned. The importance of seed losses to population recruitment at any point in time is related to the abundance of safe sites (Fig. 1): it is zero when safe sites are absent, negligible when safe sites are rare, and greatest when safe sites are numerous enough for recruitment to be limited by seed supply. Here I interpret the impact of severe seed losses on population recruitment in four species of long-lived perennials (Eucalyptus baxteri, Leptospermum juniperinum, Casuarina pusilla and L. myrsinoides) by considering these losses in terms of the overall seed dynamics of the populations. I focus on seed supply and seedling survival, as a measure of the current abundance of safe sites, and the maintenance of seed banks, as a measure of the ability of populations to exploit any future changes in safe site abundance. Insect seed predators destroyed about 95% of total seeds in each case. However, these losses do not necessarily have an important impact on population recruitment, because: (i) in most years recruitment appears to be limited by a rarity of safe sites, and not by seed supply (which was as high as 43 germinable seeds/m²/yr); and (ii) the losses did not prevent the establishment of large seed banks (ranging from >30 to >1000 viable seeds/m²) potentially capable of exploiting temporary conditions favourable for recruitment. In contrast to the situation with many annual plants, patterns of recruitment in stable populations of long-lived perennials are often extremely complex, and the significance of seed losses therefore difficult to determine.     

Relationships among recent models for insect population dynamics with variable rates of development

A classification scheme for those population models which allow variation in development rates is proposed, based on two ways of modifying standard age-structured models. The resulting classes of models are termed development index models and sojourn time models. General formulations for the two classes of models are developed from two basic balance equations, and numerous specific models from the literature are shown to fit into the scheme. Concepts from competing risks theory are shown to be important in understanding the interplay between mortality and maturation. Relationships among the classes are investigated both for the most general forms of the models and for the simpler forms often used. The scheme can provide guidance in developing appropriate insect population models for specific modelling situations.Contribution 3878871     

Effects of cooling water discharge on the structure and dynamics of epilithic algal communities in the northern Baltic

The Forsmark Biotest Basin is a shallow coastal ecosystem that receives brackish cooling-water discharge from a nuclear power plant. The effects of the discharge on epilithic algal communities were investigated by analysing samples taken every third week throughout one year at 11 sites differentially affected by temperature and/or flow rate enhancement. Community variation was summarized in a canonical correspondence analysis (CCA) of species abundances as a function of site and date. The temperature increase favoured blue-green algae at the expense of red and brown algae. Blue-green algae were however abundant in summer in stagnant water, whether heated or not, and some red and brown algae became abundant in winter in heated sites with flowing water. Green algae and diatoms increased in biomass in the heated sites, but not in relative cover-abundance. The absence of ice and snow cover at sites with heated and/or flowing water caused autumn species to persist into winter, because of the higher light intensity (compared with natural conditions) and the absence of the mechanical abrasion by ice. The thermal discharge lowered species diversity (Shannon-Weaver index) both in summer and winter at sites with flowing water, but not at sites with quiescent or stagnant water. CCA showed alternate periods of stability and rapid change within the seasonal cycle. Individual species were placed according to their optimum; red and brown algae in winter/spring, green algae in spring/summer, blue-green algae in summer, and diatoms at various times. Exceptions to this pattern were species endo- or epiphytic on species of a different group. Analysis of the effects of temperature, flow rate and ice cover on the seasonal pattern of particular species showed that different species respond in individualistic ways to different combinations of these environmental variables.     

Effect of Moniliformis moniliformis density on distribution within the definitive host population (Rattus norvegicus)

The population dynamics of Moniliformis moniliformis was studied in ‘free-ranging’ laboratory rats, Rattus norvegicus, presented with different relative density levels of M. moniliformis in cockroaches, Periplaneta americana. Changes in selected population parameters of the negative binomial distribution were evaluated as indicators of changes in aggregation. A significant increase in the degree of aggregation of parasites occurred as a result of the increase in relative density of infective stages available to the rats. This increase in aggregation was due to the increase in over-dispersion that occurred in female rats only. The degree of aggregation in females was found to be significantly higher than that in males at both treatment levels. The best indicators of the degree of aggregation were found to be the ratio of the variance to the relative density and the ratio of the log-variance to log-relative density. Changes in k were not correlated with changes in over-dispersion or the relative density.