In conventional semiconductor solar cells, carriers are extracted at the band edges and the excess electronic energy (E*) is lost as heat. If E* is harvested, power conversion efficiency can be as ...high as twice the Shockley–Queisser limit. To date, materials suitable for hot carrier solar cells have not been found due to efficient electron/optical-phonon scattering in most semiconductors, but our recent experiments revealed long-lived hot carriers in single-crystal hybrid lead bromide perovskites. Here we turn to polycrystalline methylammonium lead iodide perovskite, which has emerged as the material for highly efficient solar cells. We observe energetic electrons with excess energy ⟨E*⟩ ≈ 0.25 eV above the conduction band minimum and with lifetime as long as ∼100 ps, which is 2–3 orders of magnitude longer than those in conventional semiconductors. The energetic carriers also give rise to hot fluorescence emission with pseudo-electronic temperatures as high as 1900 K. These findings point to a suppression of hot carrier scattering with optical phonons in methylammonium lead iodide perovskite. We address mechanistic origins of this suppression and, in particular, the correlation of this suppression with dynamic disorder. We discuss potential harvesting of energetic carriers for solar energy conversion.
Transcriptomics has emerged as a powerful approach for exploring physiological responses to the environment. However, like any other experimental approach, transcriptomics has its limitations. ...Transcriptomics has been criticized as an inappropriate method to identify genes with large impacts on adaptive responses to the environment because: (1) genes with large impacts on fitness are rare; (2) a large change in gene expression does not necessarily equate to a large effect on fitness; and (3) protein activity is most relevant to fitness, and mRNA abundance is an unreliable indicator of protein activity. In this review, these criticisms are re-evaluated in the context of recent systems-level experiments that provide new insight into the relationship between gene expression and fitness during environmental stress. In general, these criticisms remain valid today, and indicate that exclusively using transcriptomics to screen for genes that underlie environmental adaptation will overlook constitutively expressed regulatory genes that play major roles in setting tolerance limits. Standard practices in transcriptomic data analysis pipelines may also be limiting insight by prioritizing highly differentially expressed and conserved genes over those genes that undergo moderate fold-changes and cannot be annotated. While these data certainly do not undermine the continued and widespread use of transcriptomics within environmental physiology, they do highlight the types of research questions for which transcriptomics is best suited and the need for more gene functional analyses. Such information is pertinent at a time when transcriptomics has become increasingly tractable and many researchers may be contemplating integrating transcriptomics into their research programs.
Salinity stress occurs when salt concentration in the environment changes rapidly, for example because of tidal water flow, rainstorms, drought, or evaporation from small bodies of water. However, ...gradual changes in salt concentration can also cause osmotic stress in aquatic habitats if levels breach thresholds that reduce the fitness of resident organisms. The latter scenario is exemplified by climate change driven salinization of estuaries and by dilution of ocean surface salinity through changes in the water cycle. In this review, we discuss how fish employ the evolutionarily conserved cellular stress response (CSR) to cope with these different forms of salinity stress. Macromolecular damage is identified as the cause of impaired physiological performance during salinity stress and serves as the signal for inducing a CSR. Basic aspects of the CSR have been observed in fish exposed to salinity stress, including repair and protection of cellular macromolecules, reallocation of energy, cell cycle arrest, and in severe cases, programmed cell death. Osmosensing and signal transduction events that regulate these aspects of the CSR provide a link between environmental salinity and adaptive physiological change required for survival. The CSR has evolved to broaden the range of salinities tolerated by certain euryhaline fish species, but is constrained in stenohaline species that are sensitive to changes in environmental salinity. Knowledge of how the CSR diverges between euryhaline and stenohaline fish enables understanding of physiological mechanisms that underlie salt tolerance and facilitates predictions as to the relative vulnerabilities of different fish species to a rapidly changing hydrosphere.
Changes in intra‐ or extracellular osmolality (such as an increase in sodium ion concentration) can disrupt the structure of cellular macromolecules such as proteins, lipids, and DNA. Macromolecular damage serves as the trigger for the cellular stress response that acts to repair this damage through synthesis of molecular chaperones, programmed cell death (apoptosis), and changes in metabolism.
Research Highlights
This review summarizes current knowledge of cellular stress response (CSR) mechanisms used by fish to mitigate the effects of salinity stress. Osmosensing and signal transduction events that connect salinity change with the CSR are described.
Lead halide perovskite nanowires (NWs) have been demonstrated in pulsed lasing with high quantum yields, low thresholds, and broad tunability. However, continuous‐wave (CW) lasing, necessary for many ...optoelectronic applications, has not been achieved to date. This is thought to be due to many‐body screening, which reduces the excitonic resonance enhancement of the oscillator strength at high excitation densities necessary for population inversion. Here CW lasing in CsPbBr3 perovskite NWs is reported. Analysis of the cavity modes and their temperature dependence reveals that both CW and pulsed lasing originate from polariton modes near the bottleneck region on the lower polariton branch, with a vacuum Rabi splitting of 0.20 ± 0.03 eV. These findings suggest that lead halide perovskite NWs may serve as low‐power CW coherent light sources and as model systems for polaritonics in the strong‐coupling regime.
Optically pumped, continuous‐wave lasing in a CsPbBr3 nanowire is enabled by strong confinement‐enhanced light–matter coupling. Nanowire waveguide‐cavity photons are strongly coupled to the CsPbBr3 exciton resonance to give suboptical bandgap exciton–polariton states that undergo stimulated emission. The Rabi splitting is quantified by modeling the light leaking out at the end facets. This interaction leads to low‐threshold lasing and high effective Q‐factors.
Lead halide perovskites (LHPs) are solution processable semiconductors characterized by long carrier lifetimes. Recent studies have suggested that electrons and holes in LHPs interact with phonons to ...form large polarons on subpicosecond time-scales and polaron formation may also slow down hot carrier cooling. Using femtosecond time-resolved two-photon photoemission (TR-2PPE) and transient reflectance (TR) spectroscopies, we follow the initial electron cooling and polaron formation dynamics in single-crystal CsPbBr3 perovskite. We find that the hot electrons cool down initially (≤0.2 ps) with rates of −0.64 ± 0.06 eV/ps and −0.82 ± 0.08 eV/ps at 300 and 80 K, respectively. This weakly temperature-dependent rate is attributed to the initial relaxation of unscreened hot electrons by the emission of longitudinal optical (LO) phonons. On longer time scales, we observe dynamic changes in the photoemission cross-section and in the red-shift of the optical bandgap. We attribute these dynamic changes to large polaron formation from electron–LO phonon interaction, with temperature-dependent polaron formation time constants of τp = 0.7 ± 0.1 and 2.1 ± 0.2 ps at 300 and 80 K, respectively. The increase in polaron formation rate with temperature is correlated with the broadening in phonon resonances, suggesting that phonon disorder and dephasing facilitate large-polaron formation. The large polaron formation rate is not competitive with the cooling rate of unscreened hot electrons in CsPbBr3, in contrast to hybrid CH3NH3PbBr3 (or CH3NH3PbI3) where the two rates are similar. This contrast explains the observation of long-lived hot carriers in the latter but not the former.
Anthropogenic stressors, such as climate change, are driving fundamental shifts in the abiotic characteristics of marine ecosystems. As the environmental aspects of our world's oceans deviate from ...evolved norms, of major concern is whether extant marine species possess the capacity to cope with such rapid change. In what many scientists consider the post-genomic era, tools that exploit the availability of DNA sequence information are being increasingly recognized as relevant to questions surrounding ocean change and marine conservation. In this review, we highlight the application of high-throughput gene-expression profiling, primarily transcriptomics, to the field of marine conservation physiology. Through the use of case studies, we illustrate how gene expression can be used to standardize metrics of sub-lethal stress, track organism condition in natural environments and bypass phylogenetic barriers that hinder the application of other physiological techniques to conservation. When coupled with fine-scale monitoring of environmental variables, gene-expression profiling provides a powerful approach to conservation capable of informing diverse issues related to ocean change, from coral bleaching to the spread of invasive species. Integrating novel approaches capable of improving existing conservation strategies, including gene-expression profiling, will be critical to ensuring the ecological and economic health of the global ocean.
Knowledge of coastal groundwater flow is critical for managing coastal groundwater resources and quantifying submarine groundwater discharge (SGD), but this flow occurs over multiple scales that can ...be difficult to study in an integrated way. We designed a field and modeling study to investigate groundwater flow and the distribution of salinity during sea level rise in a domain that included beaches, salt marshes and the first major confined aquifer, which reached 10–15 km offshore. Numerical models were based on the flat‐lying, passive margin coastline of North Inlet, SC, and were constrained by field studies including subsurface resistivity surveys and hydraulic head observations. Simulations that included tidal fluctuations showed that the salt marsh generated more than three times as much SGD as the beach and inner shelf, per unit length of coastline. Groundwater exchange between scales was small, suggesting that physical fluxes of groundwater can be considered independently at different scales. However, salinization of the first major confined aquifer occurred by downward transport from overlying aquifers rather than intrusion from the seaward end, suggesting that studies of aquifer salinization should consider multiscale flow. During simulated sea level rise, fresh‐to‐brackish groundwater persisted in the first confined aquifer as far as the seaward end of the overlying confining unit, 10–20 km offshore. Total fluxes of SGD decreased significantly with future sea level rise, dominated by declining SGD in the salt marsh, and portending a marked decline in the flux of nutrients and carbon to estuaries and the coastal ocean.
Plain Language Summary
Knowledge of groundwater flow and transport in coastal areas is critical for managing water resources and for understanding the delivery of solutes to the ocean. Groundwater flow in coastal areas occurs at many spatial scales, but it has been difficult to judge the relative importance of flow at different scales. It has also been difficult to map the transition from fresh to salty groundwater. Our work compares groundwater flow at different scales to investigate controls on the freshwater‐saltwater interface in three stacked coastal aquifers in South Carolina. In simulations, confined aquifers can retain fresh water very near the seafloor 10 km offshore, if the aquifer is protected by a dense mud layer. The aquifer may not be entirely fresh between land and the seafloor, however. Salt can diffuse across confining layers into deeper aquifers. Fresh and saline groundwater also carry important dissolved constituents to the ocean. We found that the volume of groundwater discharge from salt marshes was much larger than discharge from the seafloor, suggesting that future studies should focus on salt marshes. Sea level rise will likely drown the marsh over the next 100 years, and the volume of groundwater that flows to the ocean will decline.
Key Points
Brackish to fresh groundwater may persist for 1,000s of years at the seafloor terminus of confined coastal aquifers
Surface water‐groundwater exchange was larger at the nearshore scale than at the embayment scale and was strongly affected by sea level rise
The first major confined aquifer showed significant salinization due to diffusion from overlying aquifers rather than inflow from the seafloor
The Olympia oyster (Ostrea lurida) is a foundation species inhabiting estuaries along the North American west coast. In California estuaries, O. lurida is adapted to local salinity regimes and ...populations differ in low salinity tolerance. In this study, oysters from three California populations were reared for two generations in a laboratory common garden and subsequently exposed to low salinity seawater. Comparative transcriptomics was then used to understand species‐level responses to hyposmotic stress and population‐level mechanisms underlying divergent salinity tolerances. Gene expression patterns indicate Olympia oysters are sensitive to hyposmotic stress: All populations respond to low salinity by up‐regulating transcripts indicative of protein unfolding, DNA damage and cell cycle arrest after sub‐lethal exposure. Among O. lurida populations, transcriptomic profiles differed constitutively and in response to low salinity. Despite two generations in common‐garden conditions, transcripts encoding apoptosis modulators were constitutively expressed at significantly different levels in the most tolerant population. Expression of cell death regulators may facilitate cell fate decisions when salinity declines. Following low salinity exposure, oysters from the more tolerant population expressed a small number of mRNAs at significantly higher levels than less tolerant populations. Proteins encoded by these transcripts regulate ciliary activity within the mantle cavity and may function to prolong valve closure and reduce mortality in low salinity seawater. Collectively, gene expression patterns suggest sub‐lethal impacts of hyposmotic stress in Olympia oysters are considerable and that even oysters with greater low salinity tolerance may be vulnerable to future freshwater flooding events.
•Upper saline plumes do not exist in all beaches.•Upper saline plumes are useful indicators of the groundwater flow rates in a sandy beach.•Higher beach slopes support stronger upper saline plumes ...and higher rates of TDR and DDR.
Current conceptual models for groundwater flow in beaches highlight an upper saline plume, which is separated from the lower salt wedge by a zone of brackish to fresh groundwater discharge. There is currently limited knowledge of what conditions allow an upper saline plume to exist and what factors control its formation. We used variable-density, saturated–unsaturated, transient groundwater flow models to investigate the configuration of the freshwater–saltwater interface in beaches with slopes varying from 0.1 to 0.01, in the absence of waves. We also varied hydraulic conductivity, dispersivity, tidal amplitude and inflow of fresh groundwater. The simulated salinity configuration of the freshwater–saltwater interfaces varied significantly. No upper saline plumes formed in any beach with hydraulic conductivities less than 10m/d. The slope of the beach was also a significant control. Steeper beach faces allowed stronger upper saline plumes to develop. Median sediment grain size of the beach is strongly correlated to both beach slope and permeability, and therefore the development of an upper saline plume. Prior studies of groundwater flow and salinity in beaches have used a range of theoretical dispersivities and the appropriate values of dispersivity to be used to represent real beaches remains unclear. We found the upper saline plume to weaken with the use of larger values of dispersivity. Our results suggest that upper saline plumes do not form in all beaches and may be less common than previously considered.
The radiationless recombination of electron-hole pairs in semiconductors is detrimental to optoelectronic technologies. A prominent mechanism is Auger recombination, in which nonradiative ...recombination occurs efficiently by transferring the released energy-momentum to a third charge carrier. Here we use femtosecond photoemission to directly detect Auger electrons as they scatter into energy and momentum spaces from Auger recombination in a model semiconductor, GaSb. The Auger rate is modulated by a coherent phonon mode at 2 THz, confirming phonon participation in momentum conservation. The commonly assumed Auger rate constant is found not to be a constant, but rather decreases by 4 orders of magnitude as hot electrons cool down by ∼90 meV. These findings provide quantitative guidance in understanding Auger recombination and in designing materials for efficient optoelectronics.
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