Experimental Diatom Dissolution and the Quantification of Microfossil Preservation in Sediments

Four laboratory experiments on fresh, modern diatoms collected from lakes in the Northern Great Plains of North America were carried out to assess the effects of dissolution on diatom abundance and composition. Marked differences in mean dissolution susceptibility exist between species, despite sometimes significant intra- specific variation between heterovalves. Twenty-four taxa were ranked according to susceptibility to dissolution using an exponential decay model of valve abundance. This dissolution ranking was used to derive two weighted indices of sample preservation. A third index (F) was based on a simple binary classification of valve morphology into dissolved and pristine categories, as distinguished by light microscopy (LM). When compared against rank indices and a measure of species diversity, this diatom dissolution index was found to be the best predictor of the progress of dissolution as estimated by total valve abundance or biogenic silica (BiSi) loss. Strong empirical relationships between F index values and diatom abundance (r2 = 0.84, n = 32) and BiSi (r2 = 0.89, n = 32) were developed and applied to a diatom sequence from a short core from Devils Lake, North Dakota, and compared to diatom-inferred and observed salinity at this site. The F index is a simple, effective diagnostic tool to assess important aspects of diatom preservation. The index can provide insight into Si cycling and record changes in conditions pertinent to diatom dissolution, and has a role in validation of transfer functions or other inferences derived from compositional data

The historical dependency of organic carbon burial efficiency

Many studies have viewed lakes as quasi-static systems with regard to the rate of organic carbon (OC) burial, assuming that the dominant control on BE is sediment mineralization. However, in systems undergoing eutrophication or oligotrophication (i.e., altered nutrient loading), or climatic forcing, the changes in primary production will vary on both longer (> 10 yr) and shorter (seasonal) timescales, influencing the rate of OC accumulation and subsequent permanent burial. Here, we consider the extent to which permanent OC burial reflects changing production in a deep monomictic lake (Rostherne Mere, UK) that has been culturally eutrophied (present TP>200 μg L-1), but has undergone recent reductions in nutrient loading. We compare multi-year dynamics of OC fluxes using sediment traps to longer-term burial rates estimated from two 210Pb-dated sediment cores. The recent sediment record demonstrates that most of the autochthonous OC is preserved (∼95% of OC captured in the deep trap and 86% of the NEP in the contemporary system), contrary to widely held assumptions that this more labile, algal-dominated OC component is not well preserved in lake sediments. A revised method for calculating BE for lakes which have undergone changes in primary productivity in recent decades is developed, which reduces some of problems inherent in existing approaches using historical sediment records averaged over the last 25-150 yr. We suggest that an appreciation of lakes in all biomes as ecosystems responding dynamically to recent human impact and climate change (for example) can improve up-scaled regional and global estimates of lake OC burial

Phosphorylation of the actin binding protein Drebrin at S647 and is regulated by neuronal activity and PTEN

Defects in actin dynamics affect activity-dependent modulation of synaptic transmission and neuronal plasticity, and can cause cognitive impairment. A salient candidate actin-binding protein linking synaptic dysfunction to cognitive deficits is Drebrin (DBN). However, the specific mode of how DBN is regulated at the central synapse is largely unknown. In this study we identify and characterize the interaction of the PTEN tumor suppressor with DBN. Our results demonstrate that PTEN binds DBN and that this interaction results in the dephosphorylation of a site present in the DBN C-terminus - serine 647. PTEN and pS647-DBN segregate into distinct and complimentary compartments in neurons, supporting the idea that PTEN negatively regulates DBN phosphorylation at this site. We further demonstrate that neuronal activity increases phosphorylation of DBN at S647 in hippocampal neurons in vitro and in ex vivo hippocampus slices exhibiting seizure activity, potentially by inducing rapid dissociation of the PTEN:DBN complex. Our results identify a novel mechanism by which PTEN is required to maintain DBN phosphorylation at dynamic range and signifies an unusual regulation of an actin-binding protein linked to cognitive decline and degenerative conditions at the CNS synapse

Understanding the transfer of contemporary temperature signals into lake sediments via paired oxygen isotope ratios in carbonates and diatom silica: problems and potential

Although the oxygen isotope composition (δ18O) of calcite (δ18Ocalcite) and, to a lesser extent, diatom silica (δ18Odiatom) are widely used tracers of past hydroclimates (especially temperature and surface water hydrology), the degree to which these two hosts simultaneously acquire their isotope signals in modern lacustrine environments, or how these are altered during initial sedimentation, is poorly understood. Here, we present a unique dataset from a natural limnological laboratory to explore these issues. This study compares oxygen and hydrogen isotope data (δ18O, δ2H) of contemporary lake water samples at ~2-weekly intervals over a 2-year period (2010–12) with matching collections of diatoms (δ18Odiatom) and calcite (δ18Ocalcite) from sediment traps (at 10 m and 25 m) at Rostherne Mere (maximum depth 30 m), a well-monitored, eutrophic, seasonally stratified monomictic lake in the UK. The epilimnion shows a seasonal pattern of rising temperature and summer evaporative enrichment in 18O, and while there is a temperature imprint in both δ18Odiatom and δ18Ocalcite, there is significant inter-annual variability in both of these signals. The interpretation of δ18Odiatom and δ18Ocalcite values is complicated due to in-lake processes (e.g. non-equilibrium calcite precipitation, especially in spring, leading to significant 18Ocalcite depletion), and for δ18Odiatom, by post-mortem, depositional and possibly dissolution or diagenetic effects. For 2010 and 2011 respectively, there is a strong temperature dependence of δ18Ocalcite and δ18Odiatom in fresh trap material, with the fractionation slope for δ18Odiatom of ca. −0.2‰/°C, in agreement with several other studies. The δ18Odiatom data indicate the initiation of rapid post-mortem secondary alteration of fresh diatom silica (within ~6 months), with some trap material undergoing partial maturation in situ. Diatom δ18O of the trap material is also influenced by resuspension of diatom frustules from surface sediments (notably in summer 2011), with the net effect seen as an enrichment of deep-trap 18Odiatom by about +0.7‰ relative to shallow-trap values. Contact with anoxic water and anaerobic bacteria are potentially key to initiating this silica maturation process, as deep-trap samples that were removed prior to anoxia developing do not show enrichment. Dissolution (perhaps enhanced by anaerobic bacterial communities) may also be responsible for changes to δ18Odiatom that lead to increasing, but potentially predictable, error in inferred temperatures using this proxy. High resolution, multi-year monitoring can shed light on the complex dynamics affecting δ18Odiatom and δ18Ocalcite and supports the careful use of sedimentary δ18Odiatom and δ18Ocalcite as containing valuable hydroclimatic signals especially at a multi-annual resolution, although there remain substantial challenges to developing a reliable geothermometer on paired δ18Odiatom and δ18Ocalcite. In particular, δ18Odiatom needs cautious interpretation where silica post-mortem secondary alteration is incomplete and diatom preservation is not perfect, and we recommend dissolution be routinely assessed on diatom samples used for isotopic analyses

Anthropogenic alteration of nutrient supply increases the global freshwater carbon sink

Lakes have a disproportionate effect on the global carbon (C) cycle relative to their area, mediating C transfer from land to atmosphere, and burying organic-C in their sediments. The magnitude and temporal variability of C burial is, however, poorly constrained, and the degree to which humans have influenced lake C cycling through landscape alteration has not been systematically assessed. Here, we report global and biome specific trajectories of lake C sequestration based on 516 lakes and show that some lake C burial rates (i.e., those in tropical forest and grassland biomes) have quadrupled over the last 100 years. Global lake C-sequestration (~0.12 Pg year-1) has increased by ~72 Tg year-1 since 1900, offsetting 20% of annual CO2 freshwater emissions rising to ~30% if reservoirs are included and contributing to the residual continental C sink. Nutrient availability explains ~70% of the observed increase, while rising temperatures have a minimal effect

Diel surface temperature range scales with lake size

Ecological and biogeochemical processes in lakes are strongly dependent upon water temperature. Long-term surface warming of many lakes is unequivocal, but little is known about the comparative magnitude of temperature variation at Diel timescales, due to a lack of appropriately resolved data. Here we quantify the pattern and magnitude of Diel temperature variability of surface waters using high-frequency data from 100 lakes. We show that the near-surface Diel temperature range can be substantial in summer relative to long-term change and, for lakes smaller than 3 km2, increases sharply and predictably with decreasing lake area. Most small lakes included in this study experience average summer Diel ranges in their near-surface temperatures of between 4 and 7°C. Large Diel temperature fluctuations in the majority of lakes undoubtedly influence their structure, function and role in biogeochemical cycles, but the full implications remain largely unexplored

Genetic Deficiency of Glycogen Synthase Kinase-3β Corrects Diabetes in Mouse Models of Insulin Resistance

Despite treatment with agents that enhance β-cell function and insulin action, reduction in β-cell mass is relentless in patients with insulin resistance and type 2 diabetes mellitus. Insulin resistance is characterized by impaired signaling through the insulin/insulin receptor/insulin receptor substrate/PI-3K/Akt pathway, leading to elevation of negatively regulated substrates such as glycogen synthase kinase-3β (Gsk-3β). When elevated, this enzyme has antiproliferative and proapoptotic properties. In these studies, we designed experiments to determine the contribution of Gsk-3β to regulation of β-cell mass in two mouse models of insulin resistance. Mice lacking one allele of the insulin receptor (Ir+/−) exhibit insulin resistance and a doubling of β-cell mass. Crossing these mice with those having haploinsufficiency for Gsk-3β (Gsk-3β+/−) reduced insulin resistance by augmenting whole-body glucose disposal, and significantly reduced β-cell mass. In the second model, mice missing two alleles of the insulin receptor substrate 2 (Irs2−/−), like the Ir+/− mice, are insulin resistant, but develop profound β-cell loss, resulting in early diabetes. We found that islets from these mice had a 4-fold elevation of Gsk-3β activity associated with a marked reduction of β-cell proliferation and increased apoptosis. Irs2−/− mice crossed with Gsk-3β+/− mice preserved β-cell mass by reversing the negative effects on proliferation and apoptosis, preventing onset of diabetes. Previous studies had shown that islets of Irs2−/− mice had increased cyclin-dependent kinase inhibitor p27kip1 that was limiting for β-cell replication, and reduced Pdx1 levels associated with increased cell death. Preservation of β-cell mass in Gsk-3β+/−Irs2−/− mice was accompanied by suppressed p27kip1 levels and increased Pdx1 levels. To separate peripheral versus β-cell–specific effects of reduction of Gsk3β activity on preservation of β-cell mass, mice homozygous for a floxed Gsk-3β allele (Gsk-3F/F) were then crossed with rat insulin promoter-Cre (RIP-Cre) mice to produce β-cell–specific knockout of Gsk-3β (βGsk-3β−/−). Like Gsk-3β+/− mice, βGsk-3β−/− mice also prevented the diabetes of the Irs2−/− mice. The results of these studies now define a new, negatively regulated substrate of the insulin signaling pathway specifically within β-cells that when elevated, can impair replication and increase apoptosis, resulting in loss of β-cells and diabetes. These results thus form the rationale for developing agents to inhibit this enzyme in obese insulin-resistant individuals to preserve β-cells and prevent diabetes onset

Diel surface temperature range scales with lake size

Ecological and biogeochemical processes in lakes are strongly dependent upon water temperature. Long-term surface warming of many lakes is unequivocal, but little is known about the comparative magnitude of temperature variation at diel timescales, due to a lack of appropriately resolved data. Here we quantify the pattern and magnitude of diel temperature variability of surface waters using high-frequency data from 100 lakes. We show that the near-surface diel temperature range can be substantial in summer relative to long-term change and, for lakes smaller than 3 km2, increases sharply and predictably with decreasing lake area. Most small lakes included in this study experience average summer diel ranges in their near-surface temperatures of between 4 and 7°C. Large diel temperature fluctuations in the majority of lakes undoubtedly influence their structure, function and role in biogeochemical cycles, but the full implications remain largely unexplored

Lithium chloride therapy fails to improve motor function in a transgenic mouse model of Machado-Joseph disease

The accumulation of misfolded proteins in neurons, leading to the formation of cytoplasmic and nuclear aggregates, is a common theme in age-related neurodegenerative diseases, possibly due to disturbances of the proteostasis and insufficient activity of cellular protein clearance pathways. Lithium is a well-known autophagy inducer that exerts neuroprotective effects in different conditions and has been proposed as a promising therapeutic agent for several neurodegenerative diseases. We tested the efficacy of chronic lithium 10.4 mg/kg) treatment in a transgenic mouse model of Machado-Joseph disease, an inherited neurodegenerative disease, caused by an expansion of a polyglutamine tract within the protein ataxin-3. A battery of behavioral tests was used to assess disease progression. In spite of activating autophagy, as suggested by the increased levels of Beclin-1, Atg7, and LC3II, and a reduction in the p62 protein levels, lithium administration showed no overall beneficial effects in this model concerning motor performance, showing a positive impact only in the reduction of tremors at 24 weeks of age. Our results do not support lithiumchronic treatment as a promising strategy for the treatment of Machado-Joseph disease (MJD).FCT -Fundação para a Ciência e a Tecnologia(SFRH/BD/51059/2010