Transient Storage Research Articles

The significance of model structure in one-dimensional stream solute transport models with multiple transient storage zones – competing vs. nested arrangements

Transient storage models are commonly used to simulate solute transport in streams to characterize hydrologic controls on biogeochemical cycling. Recently, 2-storage zone (2-SZ) models have been developed to represent in-channel surface transient storage (STS) and hyporheic transient storage (HTS) separately to overcome the limitations of single storage zone (1-SZ) models. To advance biogeochemical models, we seek to separate the effects of these storage zones on solute fate and cycling in streams. Here we compare and contrast the application and interpretations from two model structures that include STS and HTS storage: a competing model structure, where both zones are connected to the stream at the same location and the stream interacts with the STS and HTS separately, and a nested model structure, where STS is an intermediary between the stream and HTS. We adapt common residence time metrics used to compare single transient storage models for the competing and nested 2-SZ models. As a test case, we investigated the transient storage characteristics of a first-order stream in Pennsylvania, using 1-SZ, nested 2-SZ, and competing 2-SZ model configurations at several different flow conditions. While both 2-SZ models fit the observed STS and in-stream breakthrough curves well, calibrated model parameters and solute molecule travel paths differ, as evident by the faster exchange rate displayed by the nested model, and therefore so does the interpretation of associated transient storage metrics and its relationship with biogeochemical cycling processes. In addition, a study of hypothetical zone-specific reaction rates was very illustrative of the differences in discrimination characterized by each model structure, particularly for the case where reactions would predominantly occur in the STS (i.e. photochemical reactions), because of the compounding effects to the HTS for the nested 2-SZ; however, for the case where reactions would predominantly occur in the HTS, the influence of model structure was found to be relegated only to the HTS.

A mean residence time relationship for lateral cavities in gravel-bed rivers and streams: Incorporating streambed roughness and cavity shape

[2] Accurate estimates of mass-exchange parameters in transient storage zones are needed to better understand and quantify solute transport and dispersion in riverine systems. Currently, the predictive mean residence time relies on an empirical entrainment coefficient with a range in variance due to the absence of hydraulic and geomorphic quantities driving mass exchange. Two empirically derived relationships are presented for the mean residence time of lateral cavities—a prevalent and widely recognized type of transient storage—in gravel-bed rivers and streams that incorporates hydraulic and geomorphic parameters. The relationships are applicable for gravel-bed rivers and streams with a range of cavity width to length (W/L) aspect ratios (0.2–0.75), shape, and Reynolds numbers (Re, ranging from 1.0 × 104 to 1.0 × 107). The relationships equate normalized mean residence time to nondimensional quantities: Froude number, Re, W/L, depth ratio (ratio of cavity to shear layer depth), roughness factor (ratio of shear to channel velocity), and shape factor (representing degree of cavity equidimensionality). One relationship excludes bed roughness (equation (13)) and the other includes bed roughness (equation (14)). The empirically derived relationships have been verified for conservative tracers (R2 of 0.83) within a range of flow and geometry conditions. Topics warranting future research are testing the empirical relationship that includes the roughness factor using parameters measured in the vicinity of the cavity to reduce the variance in the correlation, and further development of the relationship for nonconservative transport.

Modeling and inverting reactive stream tracers undergoing two-site sorption and decay in the hyporheic zone

[1] Performing stream-tracer experiments is an accepted technique to assess transport characteristics of streams undergoing hyporheic exchange. Recently, combining conservative and reactive tracers, in which the latter presumably undergoes degradation exclusively within the hyporheic zone, has been suggested to study in-stream transport, hyporheic exchange, and the metabolic activity of the hyporheic zone. The combined quantitative analysis to adequately describe such tests, however, has been missing. In this paper, we present mathematical methods to jointly analyze breakthrough curves of a conservative tracer (fluorescein), a linearly degrading tracer (resazurin), and its daughter compound (resorufin), which are synchronously introduced into the stream as pulses. In-stream transport is described by the one-dimensional advection-dispersion equation, amended with a convolution term to account for transient storage within the hyporheic zone over a distribution of travel times, transformation of the reactive tracer in the hyporheic zone, and two-site sorption of the parent and daughter compounds therein. We use a shape-free approach of describing the hyporheic travel-time distribution, overcoming the difficulty of identifying the best functional parameterization for transient storage. We discuss how this model can be fitted to the breakthrough curves of all three compounds and demonstrate the method by an application to a tracer test in the third-order stream Goldersbach in Southern Germany. The entire river water passes once through the hyporheic zone over a travel distance of about 200 m with mean hyporheic residence times ranging between 16 and 23 min. We also observed a secondary peak in the transfer functions at about 1 h indicating a second hyporheic flow path. We could jointly fit the breakthrough curves of all compounds in three monitoring stations and evaluated the parameter uncertainty of the individual and joint fits by a method based on conditional realizations of the hyporheic travel-time distribution. The approach gives insight into in-stream transport, hyporheic exchange, metabolic activity, and river-bed sorption of the stream under investigation.

Transient molten salt two-tank thermal storage modeling for CSP performance simulations

Solar thermal power plants are a promising way of providing renewable electricity. Unlike other renewable energy technologies, solar thermal power plants have the ability to store thermal energy, which decouples the electric power generation from hours of available solar irradiation. Today, the only applied commercial thermal storage system is a two-tank sensible heat thermal storage based on molten salts. This work focuses on the transient heat loss modeling of such molten salt thermal energy storage tanks. More precisely, this paper proposes a degree of physical modeling which describes the governing heat transfer mechanisms sufficiently accurate, obtaining a suitable tool for concentrating solar power (CSP) performance simulations, applicable for parabolic trough collector, as well as for central receiver plants. A fully transient storage tank model has been developed and simulated over 3 sets of 6 reference days, providing typical weather conditions of a solar thermal power plant location as model input. The model has been successfully compared to heat loss data that was observed at real application molten salt storage tanks. Several modeling assumptions, used heat transfer correlations and simplifications are outlined. Modelica has been used as modeling language.

Evaluating the fate of six common pharmaceuticals using a reactive transport model: Insights from a stream tracer test

Quantitative information regarding the capacity of rivers to self-purify pharmaceutical residues is limited. To bridge this knowledge gap, we present a methodology for quantifying the governing processes affecting the fate of pharmaceuticals in streaming waters and, especially, to evaluate their relative significance for tracer observations. A tracer test in Säva Brook, Sweden was evaluated using a coupled physical–biogeochemical model framework containing surface water transport together with a representation of transient storage in slow/immobile zones of the stream, which are presumably important for the retention and attenuation of pharmaceuticals. To assess the key processes affecting the environmental fate of the compounds, we linked the uncertainty estimates of the reaction rate coefficients to the relative influence of transformation and sorption that occurred in different stream environments. The hydrological and biogeochemical contributions to the fate of the pharmaceuticals were decoupled, and the results indicate a moderate hydrological retention in the hyporheic zone as well as in the densely vegetated parts of the stream. Biogeochemical reactions in these transient storage zones further affected the fate of the pharmaceuticals, and we found that sorption was the key process for bezafibrate, metoprolol, and naproxen, while primary transformation was the most important process for clofibric acid and ibuprofen. Conversely, diclofenac was not affected by sorption or transformation.

Concurrent conservative and reactive tracer tests in a stream undergoing hyporheic exchange

Key Points concurrent tests of conservative and reactive tracers in streams joint analysis of the tracers improved the estimation of hyporheic parameters Markov chain Monte Carlo methods used to infer parameter distributions

Lack of anodic capacitance causes power overshoot in microbial fuel cells

Power overshoot commonly makes the performance evaluation of microbial fuel cells (MFCs) inaccurate. Here, three types of carbon with different capacitance (ultracapacitor activated carbon (UAC), plain activated carbon (PAC) and carbon black (CB)) rolled on stainless steel mesh (SSM) as anodes to investigate the relationship between overshoot and anodic capacitance. It was not observed in all cycles of UAC-MFCs (from Cycle 2 to 4) due to the largest abiotic capacitance (Cm(abiotic)) of 2.1F/cm(2), while this phenomenon was eliminated in PAC-MFCs (Cm(abiotic)=1.6 F/cm(2)) from Cycle 3 and in CB-MFCs (Cm(abiotic)=0.5F/cm(2)) from Cycle 4, indicated that the Cm(abiotic) of the anode stored charges and functioned as electron shuttle to overcome the power overshoot. With bacterial colonization, the transient charge storage in biofilm resulted in a 0.1-0.4F/cm(2) increase in total capacitance for anodes, which was the possible reason for the elimination of power overshoot in PAC/CB-MFCs after multi cycle acclimation.

Vegetation and wildfire controls on sediment yield in bedrock landscapes

AbstractSteep, rocky landscapes commonly exhibit high sediment yields and are especially sensitive to climate, tectonics, and wildfire. Predicting landscape response to these perturbations demands a quantitative understanding of erosion processes. However, existing models for hillslope sediment production and transport do not apply to landscapes with patchy soil and slopes that exceed the angle for sediment stability. Here we present field measurements in southern California, USA, which indicate that sediment storage on steep slopes is enabled by vegetation that traps sediment upslope. We find that the storage capacity of unburned vegetation dams follows a geometric scaling model with a cubic dependence on effective plant width and an inverse dependence on local slope. Measured sediment volumes behind burned vegetation dams indicate a loss of at least 75% relative to unburned dams, and when expanded to the catchment scale, our measurements match records of postfire sediment yield from nearby retention basins. Contrary to existing models, our observations indicate that wildfire‐induced sediment yield is driven by transient storage and release of sediment by vegetation dams, rather than increased bedrock‐to‐soil conversion rates. Without a feedback between soil production and wildfire, fire may play little role in long‐term landscape evolution, and increasing fire frequency in response to climate change may not result in heightened sedimentation hazards due to supply limitations.

Prospects for flash flood forecasting in mountainous regions – An investigation of Tropical Storm Fay in the Southern Appalachians

Summary The sensitivity of Quantitative flash-Flood Estimates (QFEs) and Quantitative flash-Flood Forecasts (QFFs) to Quantitative Precipitation Estimates (QPEs) and Quantitative Precipitation Forecasts (QPFs) was investigated in three headwater catchments with different topographic and hydro-geomorphic characteristics during the passage of Tropical Storm Fay, 2008 over the Southern Appalachian Mountains in North Carolina, USA. QFEs and QFFs were generated by a high-resolution hydrologic model (250 × 250 m 2 ) with coupled surface–subsurface physics and rainfall forcing from the Next Generation Multi-sensor QPE (Q2) spatial rainfall (1 × 1 km 2 ) product, and from National Digital Forecast Database (NDFD) operational QPF product (5 × 5 km 2 ). Optimal QPE products (Q2+) were derived by assimilating rainfall observations from a high density raingauge network through adaptive bias correction. Deterministic QFEs simulated by the hydrologic model agree well with streamgauge observations (15-min intervals) regarding total water volume and peak flow with Nash–Sutcliffe (NS) coefficients 0.8–0.9, thus suggesting that the model without calibration captures well the dominant flash-flood physics. The propagation of uncertainty in storm rainfall to rainfall–runoff response was subsequently evaluated through model simulations forced by Monte Carlo replications of the QPEs to generate QFE distributions. Analysis of the joint QPE–QFE distributions shows that flood response at the catchment scale is highly non-linear, and exhibits strong dependence on basin physiography, initial soil moisture conditions (transient basin storage capacity), the space–time organization of runoff generation and conveyance mechanisms, and in particular interflow dynamics, with respect to the space–time structure of rainfall. QFFs for 6- to 1-h lead times using precipitation composites of Q2 QPE and NDFD QPF to drive the hydrology model in operational mode exhibited ubiquitous lack of skill yielding consistently negative NS scores. An experiment consisting of merging satellite-like observations into operational QPE/QPF showed significant improvement in QFF performance (e.g. 5–50% relative NS increases), especially when the timing of satellite overpass is such that it captures transient episodes of heavy rainfall during the event. Future advances in QFF remain principally constrained by progress in QPE and QPF at the spatial resolution necessary to resolve rainfall-interflow dynamics in mountainous regions.

Continuous monitoring reveals multiple controls on ecosystem metabolism in a suburban stream

Summary1. Primary production and respiration in streams, collectively referred to as stream ecosystem metabolism, are fundamental processes that determine trophic structure, biomass and nutrient cycling. Few studies have used high‐frequency measurements of gross primary production (GPP) and ecosystem respiration (ER) over extended periods to characterise the factors that control stream ecosystem metabolism at hourly, daily, seasonal and annual scales.2. We measured ecosystem metabolism at 5‐min intervals for 23 months in Shepherd Creek, a small suburban stream in Cincinnati, Ohio (U.S.A.).3. Daily GPP was best predicted by a model containing light and its synergistic interaction with water temperature. Water temperature alone was not significantly related to daily GPP, rather high temperatures enhanced the capacity of autotrophs to use available light.4. The relationship between GPP and light was further explored using photosynthesis–irradiance curves (P–I curves). Light saturation of GPP was evident throughout the winter and spring and the P–I curve frequently exhibited strong counterclockwise hysteresis. Hysteresis occurred when water temperatures were greater in the afternoon than in the morning, although light was similar, further suggesting that light availability interacts synergistically with water temperature.5. Storm flows strongly depressed GPP in the spring while desiccation arrested aquatic GPP and ER in late summer and autumn.6. Ecosystem respiration was best predicted by GPP, water temperature and the rate of water exchange between the surface channel and transient storage zones. We estimate that c. 70% of newly fixed carbon was immediately respired by autotrophs and closely associated heterotrophs.7. Interannual, seasonal, daily and hourly variability in ecosystem metabolism was attributable to a combination of light availability, water temperature, storm flow dynamics and desiccation. Human activities affect all these factors in urban and suburban streams, suggesting stream ecosystem processes are likely to respond in complex ways to changing land use and climate.

Modeling transport of Escherichia coli in a creek during and after artificial high-flow events: Three-year study and analysis

Escherichia coli is the leading indicator of microbial contamination of natural waters, and so its in-stream fate and transport needs to be understood to eventually minimize surface water contamination by microorganisms. To better understand mechanisms of E. coli release and transport from soil sediment in a creek the artificial high-water flow events were created by releasing 60–80 m3 of city water on a tarp-covered stream bank in four equal allotments in July 2008, 2009 and 2010. A conservative tracer difluorobenzoic acid (DFBA) was added to the released water in 2009 and 2010. Water flow rate, E. coli and DFBA concentrations as well as water turbidity were monitored with automated samplers at three in-stream weirs. A one-dimensional model was applied to simulate water flow, and E. coli and DFBA transport during these experiments. The Saint–Venant equations were used to calculate water depth and discharge while a stream solute transport model accounted for release of bacteria by shear stress from bottom sediments, advection-dispersion, and exchange with transient storage (TS). Reach-specific model parameters were estimated by evaluating observed time series of flow rates and concentrations of DFBA and E. coli at all three weir stations. Observed DFBA and E. coli breakthrough curves (BTC) exhibited long tails after the water pulse and tracer peaks had passed indicating that transient storage (TS) might be an important element of the in-stream transport process. Comparison of simulated and measured E. coli concentrations indicated that significant release of E. coli continued when water flow returned to the base level after the water pulse passed and bottom shear stress was small. The mechanism of bacteria continuing release from sediment could be the erosive boundary layer exchange enhanced by changes in biofilm properties by erosion and sloughing detachment.

Twisting and subunit rotation in single FOF1-ATP synthase

F(O)F(1)-ATP synthases are ubiquitous proton- or ion-powered membrane enzymes providing ATP for all kinds of cellular processes. The mechanochemistry of catalysis is driven by two rotary nanomotors coupled within the enzyme. Their different step sizes have been observed by single-molecule microscopy including videomicroscopy of fluctuating nanobeads attached to single enzymes and single-molecule Förster resonance energy transfer. Here we review recent developments of approaches to monitor the step size of subunit rotation and the transient elastic energy storage mechanism in single F(O)F(1)-ATP synthases.

Divergence and flow direction as indicators of subsurface heterogeneity and stage-dependent storage in alluvial floodplains

Assuming homogeneity in alluvial aquifers is convenient, but limits our ability to accurately predict stream-aquifer interactions. Research is needed on (i) identifying the presence of focused, as opposed to diffuse, groundwater discharge/recharge to streams and (ii) the magnitude and role of large-scale bank and transient storage in alluvial floodplains relative to changes in stream stage. The objective of this research was to document and quantify the effect of stage-dependent aquifer heterogeneity and bank storage relative to changes in stream stage using groundwater flow divergence and direction. Monitoring was performed in alluvial floodplains adjacent to the Barren Fork Creek and Honey Creek in northeastern Oklahoma. Based on results from subsurface electrical resistivity mapping, observation wells were installed in high and low electrical resistivity subsoils. Water levels in the wells were recorded real time using pressure transducers (August to October 2009). Divergence was used to quantify heterogeneity (i.e. variation in hydraulic conductivity, porosity, and/or aquifer thickness), and flow direction was used to assess the potential for large-scale (100 m) bank or transient storage. Areas of localized heterogeneity appeared to act as divergence zones allowing stream water to quickly enter the groundwater system, or as flow convergence zones draining a large groundwater area. Maximum divergence or convergence occurred with maximum rates of change in flow rates or stream stage. Flow directions in the groundwater changed considerably between base and high flows, suggesting that the floodplains acted as large-scale bank storage zones, rapidly storing and releasing water during passage of a storm hydrograph. During storm events at both sites, the average groundwater direction changed by at least 90° from the average groundwater direction during baseflow. Aquifer heterogeneity in floodplains yields hyporheic flows that are more responsive and spatially and temporally complex than would be expected compared to more common assumptions of homogeneity. Copyright © 2012 John Wiley & Sons, Ltd.

Do transient storage parameters directly scale in longer, combined stream reaches? Reach length dependence of transient storage interpretations

Little work has been done to assess parameterizations and related interpretations (i.e., metrics of exchange) of transient storage modeling (TSM) over multiple spatial scales in streams. In this paper, we simulate conservative solute transport in a small mountain stream over combinations of five consecutive sub-reaches (38 m, 105 m, 281 m, 433 m, and 619 m below injection point) to (1) determine how optimized parameter estimates vary with reach length and reach combination, and (2) evaluate whether equally well-optimized simulations of solute transport in the channel result in varying interpretations of tracer exchange with the storage zone. Each simulated stream solute concentration breakthrough curve (BTC) showed consistently accurate fits to observations. However, our results indicate approximate equifinality (similar fits from different parameter sets) in the simulations of concentrations of stream tracer across individual sub-reaches and combined reaches leading to varying interpretations of transient storage exchange parameterization (i.e., variable optimized parameter estimates) and concentration time series of tracer in the storage zone. These results suggest strong reach-length dependence in simulated exchange. Based on stream BTCs alone, the TSM is useful in characterizing the influence of transient storage on in-stream solute transport, though it does not consistently reproduce storage zone dynamics. Characterization of the solute exchange between the stream and the storage zones remains problematic, and the effects of transient storage cannot be directly compared within overlapping stream reaches, an important consideration in designing and interpreting stream solute transport experiments.

Phoenix: Storage Using an Autonomous Mobile Infrastructure

We propose a system that makes opportunistic use of mobile computing devices and ad hoc networking to provide a transient storage service to clients in a localized geographical region. The main challenge is to offset the potential data loss caused by node mobility with internode communication. We first argue on the basis of simulation and theory that such a service is feasible, given a sufficiently high density of mobile devices. A distributed communication and storage protocol is then presented for situations where all mobile devices are within communication range of each other, and it is shown through testbed experiments and simulation that the protocol operates correctly and makes efficient use of storage space and communication bandwidth, while maximizing the longevity of stored data.

Estrogen regulation of spine density and excitatory synapses in rat prefrontal and somatosensory cerebral cortex

The steroid hormone, 17β-estradiol (E2) has been reported to enhance executive functions that are known to be mediated by the prefrontal cortex (PFC), although the underlying mechanisms remain unclear. To shed light on the potential mechanisms, we examined the effect of E2 in vivo upon spine density in the rat PFC and the somatosensory cortex (SSC), which has been implicated to be a transient storage site for information that can also contribute to working memory. The results revealed that E2 significantly enhanced the number of dendritic spines in both the SSC and PFC, as well as the expression of spinophilin. In vitro studies revealed further mechanistic insights by demonstrating that E2 enhanced AMPA GluR1 receptor expression and excitatory glutamatergic synapse formation in rat cortical neurons, without an effect upon inhibitory GABAergic synapse formation. Furthermore, E2 rapidly enhanced ERK and Akt activation in cortical neurons, and inhibitors of ERK and Akt activation significantly attenuated E2 induction of excitatory glutamatergic synapses. Administration of E2-BSA likewise significantly enhanced excitatory glutamatergic synapses in cortical neurons, and administration of an ER antagonist, ICI182,780 and a non-NMDA receptor antagonist (NBQX) significantly attenuated the effect of E2 upon enhancement of excitatory glutamatergic synapses, suggesting mediation by extranuclear estrogen receptors and involvement of non-NMDA receptor activation and signaling. As a whole, the studies demonstrate that E2 enhances spine density in both the PFC and SSC, and that E2 enhances excitatory glutamatergic synapse formation in cortical neurons via a rapid extranuclear ER-mediated signaling mechanism that involves up-regulation of GluR1 and mediation by Akt and ERK signaling pathways.

Phosphorus Uptake and Release from Submerged Sediments in a Simulated Stream Channel Inundated with a Poultry Litter Source

Northwest Arkansas, similar to many regions around the world, is home to intensive poultry production, which concentrates large amounts of nutrients in localized areas. Concerns over phosphorus (P) continue in these regions despite extensive conservation management efforts. Part of the concerns relates to the legacy effect of P in streams and the role of fluvial sediments in confounding land conservation measures. Sediment substrate was collected from five streams containing a variety of land uses in the Upper Illinois River Watershed (UIRW) to assess the buffering capacity of sediments on water column P. A purpose built fluvarium was used to determine sediment–P relationships during three flow phases: (I) baseflow (<0.005 mg P L−1), (II) uptake-enriched (1.8 mg P L−1), and (III) re-equilibration (<0.005 mg P L−1) where water was circulated over the sediment for 48 h at 0.001 m3 s−1 (1 L s−1). During each phase, flow was monitored and water sampled for determination of dissolved reactive P (DRP). In phase I, DRP reached equilibrium concentrations, which closely mimicked stream DRP at the time of sediment collection (R 2 = 0.77), and the highest concentration measured was 0.080 mg P L−1 and the lowest 0.016 mg P L−1. Sediments rapidly bound P (40 % within 1 h) during phase II. During phase II, 84 to 96 % of added P was removed from solution. Of this bound P, 1 to 7 % was released during phase III. Results indicate that fluvial sediments in the UIRW act as transient storage sites for P during high P events. Finally, streams that bound the most P during nutrient-rich flow released the least when returned to low P flow, indicating a greater ability to buffer P in streams.

Nutrient uptake in a stream affected by hydropower plants: comparison between stream channels and diversion canals

Small hydropower plants divert part of the water from wide and physically complex stream channels with active hyporheic areas to narrow and hydraulically simple concrete canals, and thus, might affect nutrient dynamics. We compared nutrient uptake in diversion canals and in stream channels in the Leitzaran Stream (Basque Country, northern Spain). We predicted that simple morphology in diversion canals will result in lower nutrient uptake in canals than in stream channels. Periphytic chlorophyll and biomass did not differ significantly between reach types. Water was significantly deeper and faster in canals than in stream channels, but the transient storage zone did not differ significantly between reach types. There were no significant differences between uptake length for neither phosphate nor ammonium between reach types. Uptake length in both stream channels and diversion canals decreased with discharge, in a pattern similar to that previously described for pristine rivers across the world. Uptake velocity and uptake rate for phosphate did not differ significantly between reach types, but in the case of ammonium both retention metrics were significantly larger in the diversion canals. Results suggest that although hydropower schemes have minor effects on nutrient retention, these depend on the proportion of flow diverted.

Lipid Droplets Control the Maternal Histone Supply of Drosophila Embryos

Histones are essential for chromatin packing, yet free histones not incorporated into chromatin are toxic. While in most cells multiple regulatory mechanisms prevent accumulation of excess histones, early Drosophila embryos contain massive extranuclear histone stores, thought to be essential for development. Excess histones H2A, H2B, and H2Av are bound to lipid droplets, ubiquitous fat storage organelles especially abundant in embryos. It has been proposed that sequestration on lipid droplets allows safe transient storage of supernumerary histones. Here, we critically test this sequestration hypothesis. We find that histones are anchored to lipid droplets via the previously uncharacterized protein Jabba: Jabba localizes to droplets, coimmunoprecipitates with histones, and is necessary to recruit histones to droplets. Jabba mutants lack the maternal H2A, H2B, and H2Av deposits altogether; presumably, these deposits are eliminated unless sequestered on droplets. Jabba mutant embryos compensate for this histone deficit by translating maternal histone mRNAs. However, when histone expression is mildly compromised, the maternal histone protein deposits are essential for proper early mitoses and for viability. A growing number of proteins from other cellular compartments have been found to transiently associate with lipid droplets. Our studies provide the first insight into mechanism and functional relevance of this sequestration. We conclude that sequestration on lipid droplets allows embryos to build up extranuclear histone stores and provides histones for chromatin assembly during times of high demand. This work reveals a novel aspect of histone metabolism and establishes lipid droplets as functional storage sites for unstable or detrimental proteins.

Proteome analysis of chlorotic leaves of the Arabidopsis mex1 mutant defective in the mobilization of starch degradation products

Leaf starch synthesized during the day for transient storage of photoassimilated carbon is degraded the following night to support respiration and growth in plants. Maltose is a major product of starch degradation, and is exported to the cytosol through the maltose transporter (MEX1). The Arabidopsis mex1 mutant displays growth retardation and an exceptional chlorotic phenotype that is not observed in other mutants demonstrating defective starch synthesis or degradation. Consistent with the chlorotic phenotype, proteomic analysis revealed degeneration of the photosynthetic machinery in mex1, and the down-regulation of essential components for photosynthesis was also observed. The chlorosis observed in mex1 occurs during vegetative growth period under normal growth conditions, which is distinct from general senescence-induced chlorosis. No up-regulation of senescence-related genes was found in the proteomic analysis of mex1, suggesting that the chlorotic process occurring in mex1 is likely distinct from senescence-dependent processes. On the other hand, cellular processes needed to survive stress situations caused by the blocking of maltose export are induced in mex1 by up-regulation of stress-related proteins, such as a germin-like protein and glutathione S-transferase. The increased abundance of heat shock protein 93-V participating in chloroplast biogenesis and rubisco activase, a regulatory protein of photosynthesis, likely reflects an attempt by the mex1 mutant to maintain chloroplast function to survive stress conditions.