Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

Organic Acid Exudation by Wild Herbs in Response to Elevated Al Concentrations

In acidic soils, monomeric aluminium (Al³⁺) can reach levelsthat are toxic to plants, thus preventing many species fromgrowing there. Organic acids chelate Al and render it non-toxic.It has been shown that exudation of organic acids by Al-tolerantcrops increases their tolerance to Al. We have extended thisobservation to wild plants by comparing the ability of ten herbsto exude organic acids in response to elevated Al levels. Wehypothesized that exudation of organic acids was related tothe ability of plants to grow on Al-rich soils. Two grasseswere grown in rhizotrons in soils with 41 and 63 µM reactiveAl. Organic acids were sampled from root tips connected to anintact plant-root system.Deschampsia flexuosa (L.) Trin. exudedmore malic acid when grown in the soil with the highest Al content.Five forbs and five grasses were also exposed to three Al levels(0, 25 and 75 µM) in a hydroponic system.Rumex acetosellaL. and Viscaria vulgaris Bernh. increased exudation of oxalicacid and Galium saxatile auct. non L. and Veronica officinalisL. increased exudation of citric acid in response to elevatedAl. The distribution of the forbs in the field as describedby soil pH was negatively related to the amount of organic acidsexuded in response to Al. In contrast, none of the grasses exudedhigher amounts of organic acids with increasing Al concentrationin the hydroponic experiment. Copyright 2001 Annals of BotanyCompany Citrate, malate, aluminium detoxification, rhizotron, hydroponics, rhizosphere, Carex pilulifera,Deschampsia flexuosa , Festuca gigantea, Galium saxatile, Geum urbanum, Holcus mollis,Milium effusum , Rumex acetosella, Veronica officinalis, Viscaria vulgaris     

Packing properties of 1-alkanols and alkanes in a phospholipid membrane

We have used vibrating tube densitometry to investigate the packing properties of four alkanes and a homologous series of ten alcohols in fluid-phase membranes of dimyristoyl phosphatidylcholine (DMPC). It was found that the volume change of transferring these compounds from their pure states into the membrane, DeltaV(m)(pure-->mem), was positive for small (C4-C6) 1-alkanols while it was negative for larger alcohols and all alkanes. The magnitude of DeltaV(m)(pure-->mem) ranged from about +4 cm3/mol for alcohols with an alkyl chain about half the length of the fatty acids of DMPC, to -10 to -15 cm3/mol for the alkanes and long chain alcohols. On the basis of these observations, previously published information on the structure of the membrane-solute complexes and the free volume properties of (pure) phospholipid membranes, we suggest that two effects dominate the packing properties of hydrophobic solutes in DMPC. First, perturbation of the tightly packed interfacial zone around the ester bonds and first few methylene groups of DMPC brings about a positive contribution to DeltaV(m)(pure-->mem). This effect dominates the volume behavior for alcohols like 1-butanol, 1-pentanol and 1-hexanol. More hydrophobic solutes penetrate into the membrane core, which is loosely packed. In this region, they partially occupy interstitial (or free-) volume, which bring about a denser molecular packing and generate a negative contribution to DeltaV(m)(pure-->mem).     

The Mitochondrial 2-Oxoglutarate Carrier Is Part of a Metabolic Pathway That Mediates Glucose- and Glutamine-stimulated Insulin Secretion

Glucose-stimulated insulin secretion from pancreatic islet β-cells is dependent in part on pyruvate cycling through the pyruvate/isocitrate pathway, which generates cytosolic α-ketoglutarate, also known as 2-oxoglutarate (2OG). Here, we have investigated if mitochondrial transport of 2OG through the 2-oxoglutarate carrier (OGC) participates in control of nutrient-stimulated insulin secretion. Suppression of OGC in clonal pancreatic β-cells (832/13 cells) and isolated rat islets by adenovirus-mediated delivery of small interfering RNA significantly decreased glucose-stimulated insulin secretion. OGC suppression also reduced insulin secretion in response to glutamine plus the glutamate dehydrogenase activator 2-amino-2-norbornane carboxylic acid. Nutrient-stimulated increases in glucose usage, glucose oxidation, glutamine oxidation, or ATP:ADP ratio were not affected by OGC knockdown, whereas suppression of OGC resulted in a significant decrease in the NADPH:NADP⁺ ratio during stimulation with glucose but not glutamine + 2-amino-2-norbornane carboxylic acid. Finally, OGC suppression reduced insulin secretion in response to a membrane-permeant 2OG analog, dimethyl-2OG. These data reveal that the OGC is part of a mechanism of fuel-stimulated insulin secretion that is common to glucose, amino acid, and organic acid secretagogues, involving flux through the pyruvate/isocitrate cycling pathway. Although the components of this pathway must remain intact for appropriate stimulus-secretion coupling, production of NADPH does not appear to be the universal second messenger signal generated by these reactions.     

Intraspecific hybrids of Arabidopsis thaliana revealed no gross alterations in endopolyploidy, DNA methylation, histone modifications and transcript levels

Arabidopsis accessions Col-0 and C24 and their reciprocal hybrids were employed as a model system to investigate the potential relationship between changes in DNA methylation, chromatin structure, endopolyploidization and gene expression in heterotic genotypes. Nucleolus size, endopolyploidization level and distribution of DNA and histone H3 methylation at the microscopic level does not differ between parents and their hybrids. Methylation sensitive amplified polymorphism revealed a largely constant pattern of DNA methylation (97% of signals analyzed) after intraspecific crosses. The parental expression profile of selected genes was maintained in hybrid offspring. No correlation was found between expression pattern and DNA methylation levels at restriction sites within 5′ regulatory regions. Thus, the results revealed only minor changes of chromatin properties and other nuclear features in response to intraspecific hybridization in Arabidopsis thaliana.     

A model of the protein–pigment baseplate complex in chlorosomes of photosynthetic green bacteria

In contrast to photosynthetic reaction centers, which share the same structural architecture, more variety is found in the light-harvesting antenna systems of phototrophic organisms. The largest antenna system described, so far, is the chlorosome found in anoxygenic green bacteria, as well as in a recently discovered aerobic phototroph. Chlorosomes are the only antenna system, in which the major light-harvesting pigments are organized in self-assembled supramolecular aggregates rather than on protein scaffolds. This unique feature is believed to explain why some green bacteria are able to carry out photosynthesis at very low light intensities. Encasing the chlorosome pigments is a protein-lipid monolayer including an additional antenna complex: the baseplate, a two-dimensional paracrystalline structure containing the chlorosome protein CsmA and bacteriochlorophyll a (BChl a). In this article, we review current knowledge of the baseplate antenna complex, which physically and functionally connects the chlorosome pigments to the reaction centers via the Fenna–Matthews–Olson protein, with special emphasis on the well-studied green sulfur bacterium Chlorobaculum tepidum (previously Chlorobium tepidum). A possible role for the baseplate in the biogenesis of chlorosomes is discussed. In the final part, we present a structural model of the baseplate through combination of a recent NMR structure of CsmA and simulation of circular dichroism and optical spectra for the CsmA–BChl a complex.     

Understanding of basic mechanisms of β-cell function and survival

Type 1 and type 2 diabetes are both diseases of insulin insufficiency, although they develop by distinct pathways. The recent surge in the incidence of type 2 diabetes and the chronic ailments confronted by patients with either form of the disease highlight the need for better understanding of β-cell biology. In this review, we present recent work focused on this goal. Our hope is that basic research being conducted in this and other laboratories will ultimately contribute to the development of methods for enhancing β-cell function and survival in the context of both major forms of diabetes. Our strategy for understanding the β-cell involves a multidisciplinary approach in which tools from the traditional fields of biochemistry, enzymology, and physiology are teamed with newer technologies from the fields of molecular biology, gene discovery, cell and developmental biology, and biophysical chemistry. We have focused on two important aspects of β-cell biology in our studies: β-cell function, specifically the metabolic regulatory mechanisms involved in glucose-stimulated insulin secretion, and β-cell resistance to immune attack, with emphasis on resistance to inflammatory cytokines and reactive oxygen species.     

Independent introduction of two lactase-persistence alleles into human populations reflects different history of adaptation to milk culture

The T₋₁₃₉₁₀ variant located in the enhancer element of the lactase (LCT) gene correlates perfectly with lactase persistence (LP) in Eurasian populations whereas the variant is almost nonexistent among Sub-Saharan African populations, showing high prevalence of LP. Here, we report identification of two new mutations among Saudis, also known for the high prevalence of LP. We confirmed the absence of the European T₋₁₃₉₁₀ and established two new mutations found as a compound allele: T/G₋₁₃₉₁₅ within the −13910 enhancer region and a synonymous SNP in the exon 17 of the MCM6 gene T/C₋₃₇₁₂, −3712 bp from the LCT gene. The compound allele is driven to a high prevalence among Middle East population(s). Our functional analyses in vitro showed that both SNPs of the compound allele, located 10 kb apart, are required for the enhancer effect, most probably mediated through the binding of the hepatic nuclear factor 1 α (HNF1α). High selection coefficient (s) ~0.04 for LP phenotype was found for both T₋₁₃₉₁₀ and the compound allele. The European T₋₁₃₉₁₀ and the earlier identified East African G₋₁₃₉₀₇ LP allele share the same ancestral background and most likely the same history, probably related to the same cattle domestication event. In contrast, the compound Arab allele shows a different, highly divergent ancestral haplotype, suggesting that these two major global LP alleles have arisen independently, the latter perhaps in response to camel milk consumption. These results support the convergent evolution of the LP in diverse populations, most probably reflecting different histories of adaptation to milk culture.     

SLC9A6 mutations cause X-linked mental retardation, microcephaly, epilepsy, and ataxia, a phenotype mimicking Angelman syndrome

Linkage analysis and DNA sequencing in a family exhibiting an X-linked mental retardation (XLMR) syndrome, characterized by microcephaly, epilepsy, ataxia, and absent speech and resembling Angelman syndrome, identified a deletion in the SLC9A6 gene encoding the Na(+)/H(+) exchanger NHE6. Subsequently, other mutations were found in a male with mental retardation (MR) who had been investigated for Angelman syndrome and in two XLMR families with epilepsy and ataxia, including the family designated as having Christianson syndrome. Therefore, mutations in SLC9A6 cause X-linked mental retardation. Additionally, males with findings suggestive of unexplained Angelman syndrome should be considered as potential candidates for SLC9A6 mutations.