The emergence of alternative stable states in forest systems has significant implications for the functioning and structure of the terrestrial biosphere, yet empirical evidence remains scarce. Here, we combine global forest biodiversity observations and simulations to test for alternative stable states in the presence of evergreen and deciduous forest types. We reveal a bimodal distribution of forest leaf types across temperate regions of the Northern Hemisphere that cannot be explained by the environment alone, suggesting signatures of alternative forest states. Moreover, we empirically demonstrate the existence of positive feedbacks in tree growth, recruitment and mortality, with trees having 4-43% higher growth rates, 14-17% higher survival rates and 4-7 times higher recruitment rates when they are surrounded by trees of their own leaf type. Simulations show that the observed positive feedbacks are necessary and sufficient to generate alternative forest states, which also lead to dependency on history (hysteresis) during ecosystem transition from evergreen to deciduous forests and vice versa. We identify hotspots of bistable forest types in evergreen-deciduous ecotones, which are likely driven by soil-related positive feedbacks. These findings are integral to predicting the distribution of forest biomes, and aid to our understanding of biodiversity, carbon turnover, and terrestrial climate feedbacks.

Reservoirs exert a profound influence on the cycling of dissolved organic matter (DOM) in inland waters by altering flow regimes. Biological incubations can help to disentangle the role that microbial processing plays in the DOM cycling within reservoirs. However, the complex DOM composition poses a great challenge to the analysis of such data. Here we tested if the interpretable machine learning (ML) methodologies can contribute to capturing the relationships between molecular reactivity and composition. We developed time-specific ML models based on 7-day and 30-day incubations to simulate the biogeochemical processes in the Three Gorges Reservoir over shorter and longer water retention periods, respectively. Results showed that the extended water retention time likely allows the successive microbial degradation of molecules, with stochasticity exerting a non-negligible effect on the molecular composition at the initial stage of the incubation. This study highlights the potential of ML in enhancing our interpretation of DOM dynamics over time.

A bright (m(F150W,AB) = 24 mag), z = 1.95 supernova (SN) candidate was discovered in JWST/NIRCam imaging acquired on 2023 November 17. The SN is quintuply imaged as a result of strong gravitational lensing by a foreground galaxy cluster, detected in three locations, and remarkably is the second lensed SN found in the same host galaxy. The previous lensed SN was called "Requiem," and therefore the new SN is named "Encore." This makes the MACS J0138.0-2155 cluster the first known system to produce more than one multiply imaged SN. Moreover, both SN Requiem and SN Encore are Type Ia SNe (SNe Ia), making this the most distant case of a galaxy hosting two SNe Ia. Using parametric host fitting, we determine the probability of detecting two SNe Ia in this host galaxy over a similar to 10 yr window to be approximate to 3%. These observations have the potential to yield a Hubble constant (H-0) measurement with similar to 10% precision, only the third lensed SN capable of such a result, using the three visible images of the SN. Both SN Requiem and SN Encore have a fourth image that is expected to appear within a few years of similar to 2030, providing an unprecedented baseline for time-delay cosmography.

Fixed nitrogen species generated by the early Earth's atmosphere are thought to be critical to the emergence of life and the sustenance of early metabolisms. A previous study estimated nitrogen fixation in the Hadean Earth's N2/CO2-dominated atmosphere; however, that previous study only considered a limited chemical network that produces NOx species (i.e., no HCN formation) via the thermochemical dissociation of N2 and CO2 in lightning flashes, followed by photochemistry. Here, we present an updated model of nitrogen fixation on Hadean Earth. We use the Chemical Equilibrium with Applications (CEA) thermochemical model to estimate lightning-induced NO and HCN formation and an updated version of KINETICS, the 1-D Caltech/JPL photochemical model, to assess the photochemical production of fixed nitrogen species that rain out into the Earth's early ocean. Our updated photochemical model contains hydrocarbon and nitrile chemistry, and we use a Geant4 simulation platform to consider nitrogen fixation stimulated by solar energetic particle deposition throughout the atmosphere. We study the impact of a novel reaction pathway for generating HCN via HCN2, inspired by the experimental results which suggest that reactions with CH radicals (from CH4 photolysis) may facilitate the incorporation of N into the molecular structure of aerosols. When the HCN2 reactions are added, we find that the HCN rainout rate rises by a factor of five in our 1-bar case and is about the same in our 2- and 12-bar cases. Finally, we estimate the equilibrium concentration of fixed nitrogen species under a kinetic steady state in the Hadean ocean, considering loss by hydrothermal vent circulation, photoreduction, and hydrolysis. These results inform our understanding of environments that may have been relevant to the formation of life on Earth, as well as processes that could lead to the emergence of life elsewhere in the universe.

Geological evidence and atmospheric and climate models suggest habitable conditions occurred on early Mars, including in a lake in Gale crater. Instruments aboard the Curiosity rover measured organic compounds of unknown provenance in sedimentary mudstones at Gale crater. Additionally, Curiosity measured nitrates in Gale crater sediments, which suggests that nitrate-dependent Fe2+ oxidation (NDFO) may have been a viable metabolism for putative martian life. Here, we perform the first quantitative assessment of an NDFO community that could have existed in an ancient Gale crater lake and quantify the long-term preservation of biological necromass in lakebed mudstones. We find that an NDFO community would have the capacity to produce cell concentrations of up to 10(6) cells mL(-1), which is comparable to microbes in Earth's oceans. However, only a concentration of < 10(4) cells mL(-1), due to organisms that inefficiently consume less than 10% of precipitating nitrate, would be consistent with the abundance of organics found at Gale. We also find that meteoritic sources of organics would likely be insufficient as a sole source for the Gale crater organics, which would require a separate source, such as abiotic hydrothermal or atmospheric production or possibly biological production from a slowly turning over chemotrophic community.

The Columbia River Basalt Group (CRBG) is the youngest continental flood basalt province, proposed to be sourced from the deep-seated plume that currently resides underneath Yellowstone National Park. If so, the earliest erupted basalts from this province, such as those in the Picture Gorge Basalt (PGB), aid in understanding and modeling plume impingement and the subsequent evolution of basaltic volcanism. Using geochemical and isotopic data, this study explores potential mantle sources and magma evolution of the PGB. Long known geochemical signatures of the PGB include overall large ion lithophile element (LILE) enrichment and relative depletion of high field strength elements (HFSE) typical of other CRBG main-phase units. Basaltic samples of the PGB have Sr-87/Sr-86 ratios on the low end of the range displayed by other CRBG lavas and mantle-like delta O-18 values. The relatively strong enrichment of LILE and depletion of HFSE coupled with depleted isotopic signatures suggest a metasomatized upper mantle as the most likely magmatic source for the PGB. Previous geochemical modeling of the PGB utilized the composition of two high-MgO primitive dikes exposed in the northern portion of the Monument Dike swarm as parental melt. However, fractionation of these dike compositions cannot generate the compositional variability illustrated by basaltic lavas and dikes of the PGB. This study identifies a second potential parental PGB composition best represented by basaltic flows in the extended spatial distribution of the PGB. This composition also better reflects the lowest stratigraphic flows identified in the previously mapped extent of the PGB. Age data reveal that PGB lavas erupted first and throughout eruptions of main-phase CRBG units (Steens, Imnaha, Grande Ronde Basalt). Combining geochemical signals with these age data indicates cyclical patterns in the amounts of contributing mantle components. Eruption of PGB material occurred in two pulses, demonstrated by a similar to 0.4 Ma temporal gap in reported ages, 16.62 to 16.23 Ma. Coupling ages with observed geochemical signals, including relative elemental abundances of LILE, indicates increased influence of a more primitive, potentially plume-like source with time.

Dinoflagellate genomes often are very large and difficult to assemble, which has until recently precluded their analysis with modern functional genomic tools. Here, we present a protocol for mapping three-dimensional (3D) genome organization in dinoflagellates and using it for scaffolding their genome assemblies. We describe steps for crosslinking, nuclear lysis, denaturation, restriction digest, ligation, and DNA shearing and purification. We then detail procedures sequencing library generation and computational analysis, including initial Hi -C read mapping and 3D -DNA scaffolding/assembly correction. For complete details on the use and execution of this protocol, please refer to Marinov et al.1

The enormous increase in mid-IR sensitivity and spatial and spectral resolution provided by the JWST spectrographs enables, for the first time, detailed extragalactic studies of molecular vibrational bands. This opens an entirely new window for the study of the molecular interstellar medium in luminous infrared galaxies (LIRGs). We present a detailed analysis of rovibrational bands of gas-phase CO, H2O, C2H2, and HCN toward the heavily obscured eastern nucleus of the LIRG VV 114, as observed by NIRSpec and the medium resolution spectrograph on the Mid-InfraRed Instrument (MIRI MRS). Spectra extracted from apertures of 130 pc in radius show a clear dichotomy between the obscured active galactic nucleus (AGN) and two intense starburst regions. We detect the 2.3 mu m CO bandheads, characteristic of cool stellar atmospheres, in the star-forming regions, but not toward the AGN. Surprisingly, at 4.7 mu m, we find highly excited CO (T ex approximate to 700-800 K out to at least rotational level J = 27) toward the star-forming regions, but only cooler gas (T ex approximate to 200 K) toward the AGN. We conclude that only mid-infrared pumping through the rovibrational lines can account for the equilibrium conditions found for CO and H2O in the deeply embedded starbursts. Here, the CO bands probe regions with an intense local radiation field inside dusty young massive star clusters or near the most massive young stars. The lack of high-excitation molecular gas toward the AGN is attributed to geometric dilution of the intense radiation from the bright point source. An overview of the relevant excitation and radiative transfer physics is provided in an appendix.