Nonthermal hydrogen loss at Mars: Contributions of photochemical mechanisms to escape and identification of key processes

Hydrogen loss to space is a key control on the evolution of the Martian atmosphere and the desiccation of the red planet. Thermal escape is thought to be the dominant loss process, but both forward modeling studies and remote sensing observations have indicated the presence of a second, higher-temperature "nonthermal" or "hot" hydrogen component, some fraction of which also escapes. Exothermic reactions and charge/momentum exchange processes produce hydrogen atoms with energy above the escape energy, but H loss via many of these mechanisms has never been studied, and the relative importance of thermal and nonthermal escape at Mars remains uncertain. Here we estimate hydrogen escape fluxes via 47 mechanisms, using newly-developed escape probability profiles. We find that HCO$^+$ dissociative recombination is the most important of the mechanisms, accounting for 30-50% of the nonthermal escape. The reaction CO$_2^+$ + H$_2$ is also important, producing roughly as much escaping H as momentum exchange between hot O and H. Total nonthermal escape from the mechanisms considered amounts to 39% (27%) of thermal escape, for low (high) solar activity. Our escape probability profiles are applicable to any thermospheric hot H production mechanism and can be used to explore seasonal and longer-term variations, allowing for a deeper understanding of desiccation drivers over various timescales. We highlight the most important mechanisms and suggest that some may be important at Venus, where nonthermal escape dominates and much of the literature centers on charge exchange reactions, which do not result in significant escape in this study.Comment: 47 pages, 4 figures, 3 tables. Accepted manuscript. An edited version of this paper was published by AG

HCO+ Dissociative Recombination: A Significant Driver of Nonthermal Hydrogen Loss at Mars

Hydrogen escape to space has shaped Mars' atmospheric evolution, driving significant water loss. An unknown fraction of atmospheric H lost acquires its escape energy from photochemical processes, with multiple observational studies suggesting much higher densities of such &ldquo;hot&rdquo; H than models predict. Here, we show that a previously unconsidered mechanism, HCO + dissociative recombination, produces more escaping hot H than any previously studied process, potentially accounting for more than 50% of escape during solar minimum aphelion conditions and &sim;5% of the expected long-term average loss. This hot H is predicted to impact observed brightness profiles negligibly, posing a significant challenge to the interpretation of spacecraft remote sensing observations. This mechanism's efficiency is largely due to the high (63%&ndash;83%) albedo ofthe planet to H at 1&ndash;10 eV energies, indicating the likely importance of dozens of similar photochemical mechanisms for the desiccation of Mars, Venus and planets throughout the universe.</p

Metatranscriptomic Sequencing of Winter and Spring Planktonic Communities from Lake Erie, a Laurentian Great Lake

Previous reports suggest planktonic and under-ice winter microbial communities in Lake Erie are dominated by diatoms. Here, we report the assembled metatranscriptomes of 79 Lake Erie surface water microbial communities spanning both the winter (28 samples) and spring (51 samples) months over spatial, temporal, and climatic gradients in 2019 through 2020

Sulfolipid substitution ratios of Microcystis aeruginosa and planktonic communities as an indicator of phosphorus limitation in Lake Erie

Phosphorus (P) availability frequently limits primary production in lakes, influences the physiology of phytoplankton, shapes community structure, and can stimulate or constrain the formation of cyanobacterial blooms. Given the importance of P, numerous methods are available to assess P stress in phytoplankton communities. Marine phytoplankton are known to substitute sulfolipids for phospholipids in response to P limitation. We asked whether sulfolipid substitution might serve as an additional indicator of P stress in freshwater phytoplankton communities. The question was addressed using cultures of Microcystis aeruginosa, Lake Erie microcosms, and surveys of lipid profiles in Lake Erie during a Microcystis spp. bloom. Peak area response ratios of the intact polar lipids sulfoquinovosyldiacylglycerol (SQDG) to phosphatidylglycerol (PG) were used as the metric of lipid substitution. In cultures of M. aeruginosa NIES-843, the SQDG : PG ratio increased from ~ 0.9 to ~ 3.3 with decreasing P concentration. In P-limited communities, the SQDG : PG ratio increased from ~ 6 to ~ 11 after 48 h in microcosm controls, while P amendments reduced the ratio to ~ 3. In Lake Erie surveys, the SQDG : PG ratio ranged from ~ 0.4 to ~ 7.4 and was negatively correlated (Pearson r = −0.62) with total dissolved P. The SQDG : PG ratio was not correlated with concentrations of chlorophyll a, soluble reactive P, or N : P molar ratios. These results demonstrated that M. aeruginosa and Microcystis-dominated communities remodel lipid profiles in response to P scarcity, providing a potential short-term, time-integrated biomarker of nutrient history and P stress in fresh waters

Single-cell meta-analysis of SARS-CoV-2 entry genes across tissues and demographics

Angiotensin-converting enzyme 2 (ACE2) and accessory proteases (TMPRSS2 and CTSL) are needed for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) cellular entry, and their expression may shed light on viral tropism and impact across the body. We assessed the cell-type-specific expression of ACE2, TMPRSS2 and CTSL across 107 single-cell RNA-sequencing studies from different tissues. ACE2, TMPRSS2 and CTSL are coexpressed in specific subsets of respiratory epithelial cells in the nasal passages, airways and alveoli, and in cells from other organs associated with coronavirus disease 2019 (COVID-19) transmission or pathology. We performed a meta-analysis of 31 lung single-cell RNA-sequencing studies with 1,320,896 cells from 377 nasal, airway and lung parenchyma samples from 228 individuals. This revealed cell-type-specific associations of age, sex and smoking with expression levels of ACE2, TMPRSS2 and CTSL. Expression of entry factors increased with age and in males, including in airway secretory cells and alveolar type 2 cells. Expression programs shared by ACE2+TMPRSS2+ cells in nasal, lung and gut tissues included genes that may mediate viral entry, key immune functions and epithelial-macrophage cross-talk, such as genes involved in the interleukin-6, interleukin-1, tumor necrosis factor and complement pathways. Cell-type-specific expression patterns may contribute to the pathogenesis of COVID-19, and our work highlights putative molecular pathways for therapeutic intervention

Contrasting Sources and Mobility of Trace Metals in Recent Sediments of Western Lake Erie

Concentrations of the major and trace metals varied considerably in the western basin of Lake Erie, ranging from 0.9 to 25.3 mg/g for aluminum, from 2.9 to 36.5 mg/g for iron, from 6.4 to 74.8 mg/g for calcium, from 1.2 to 13.5 mu g/g for cobalt, from 2.8 to 61.6 mu g/g for copper, from 2.7 to 83.0 mu g/g for lead, from 0.1 to 2.9 mu g/g for cadmium, and from 7.1 to 127.3 mu g/g for strontium. Distinct patterns of sediment metal variability allowed the identification of two major fluvial sources and some active in-lake biogeochemical processes. The inputs of Sr were largely from the Maumee River, the inputs of Cu, Pb, Cd, and Co were dominated by the Detroit River, and the inputs of Fe and Al were roughly evenly from the two rivers. The removal of Sr and Ca from the water column was mainly through coprecipitation with calcite. In contrast, the transfer of Cu, Pb, Cd, and Co was largely attributed to the removal of fine sediment particles from the Detroit River mouth and adjacent nearshore areas and the deposition of the metals scavenged by settling organic materials in the basin\u27s central deeper areas. The mobility of the trace metals was different during the in-lake mass transfer, with Co being the most mobile and Cd being the least mobile. Furthermore, the trace metal mobility differences have decreased significantly during the past half-century due to a substantial increase in organic matter from eutrophication in the basin. (C) 2018 International Association for Great Lakes Research

Contrasting Sources and Mobility of Trace Metals in Recent Sediments of Western Lake Erie

Concentrations of the major and trace metals varied considerably in the western basin of Lake Erie, ranging from 0.9 to 25.3 mg/g for aluminum, from 2.9 to 36.5 mg/g for iron, from 6.4 to 74.8 mg/g for calcium, from 1.2 to 13.5 mu g/g for cobalt, from 2.8 to 61.6 mu g/g for copper, from 2.7 to 83.0 mu g/g for lead, from 0.1 to 2.9 mu g/g for cadmium, and from 7.1 to 127.3 mu g/g for strontium. Distinct patterns of sediment metal variability allowed the identification of two major fluvial sources and some active in-lake biogeochemical processes. The inputs of Sr were largely from the Maumee River, the inputs of Cu, Pb, Cd, and Co were dominated by the Detroit River, and the inputs of Fe and Al were roughly evenly from the two rivers. The removal of Sr and Ca from the water column was mainly through coprecipitation with calcite. In contrast, the transfer of Cu, Pb, Cd, and Co was largely attributed to the removal of fine sediment particles from the Detroit River mouth and adjacent nearshore areas and the deposition of the metals scavenged by settling organic materials in the basin\u27s central deeper areas. The mobility of the trace metals was different during the in-lake mass transfer, with Co being the most mobile and Cd being the least mobile. Furthermore, the trace metal mobility differences have decreased significantly during the past half-century due to a substantial increase in organic matter from eutrophication in the basin. (C) 2018 International Association for Great Lakes Research

Forecasting microcystin concentrations in Lake Erie using an Eulerian tracer model

Cyanobacteria biomass models are routinely used in Lake Erie to predict the occurrence and location of algal blooms. However, current forecasts do not predict the microcystin toxins produced by these blooms. In this study, we used an extensive dataset of microcystin concentrations to generate weekly distribution maps in Lake Erie for the summers of 2018 and 2019. Using a 3D Eulerian tracer model (ETM) initialized with these maps, we simulated microcystin transport over 7 days, under two conditions: (1) the initial microcystin is mixed within the surface-mixed layer; (2) the initial microcystin is distributed throughout the entire water column. Two scenarios were tested for each condition: one incorporating microcystin production rates into hydrodynamic transport and one excluding them. Model performance was evaluated against weekly sample data in predicting whether microcystin concentrations surpassed specific thresholds (0.3, 1.0, 5.0, 10.0, and 20.0 µg/L), and in predicting trend directionality over each week. Overall, the ETM with hydrodynamics alone captured the transport of microcystins and predicted microcystin concentrations in 69% of the simulations. Incorporating microcystin production into the model increased the accuracy of forecasts by an additional 10%. Moreover, models with microcystin production successfully predicted microcystin concentrations greater than 5 μg/L during a large bloom, high-microcystin year (2019), while incorrectly forecasting concentrations above 5 μg/L during a small bloom year (2018). With limited data to initialize the ETM, no single model configuration consistently outperformed others. It is necessary to consider the full range of model configurations when utilizing their outputs for making management decisions

Nanobubble ozone treatment effects on cyanobacterial biomass and cyanotoxins in a small eutrophic lake

Cyanobacterial harmful algal blooms (CHABs) fueled by excessive nutrient runoff are becoming an increasing problem worldwide. Nanobubble ozone technology (NBOT) is an emerging treatment to remediate lakes from CHABs. NBOT was tested in Lake Sylvan, a 42-acre manmade eutrophic lake that experiences annual CHABs. NBOTs were deployed from 7 July to 17 September 2021, and dosage was tripled on 9 August. CHAB chlorophyll, microcystins, and saxitoxins (among other parameters) were measured June to October 2021, spanning pre-NBOT, NBOT deployment, and post NBOT treatment. CHAB chlorophyll increased throughout July (peaking at 100 μg/L) then sharply declined following the increased dose (25-35 μg/L) but increased again 3 weeks (51-78 μg/L) after increased dose. CHAB chlorophyll spiked (80-95 μg/L) in early October after NBOT treatment. Microcystins steadily increased from \u3c0.15 to 2.5 μg/L throughout June and July and remained between 0.4 to 1.0 μg/L August – October. No contact advisories were posted during 2021 unlike previous years. Total saxitoxins rapidly increased in mid-July peaking at 4 μg/L, and then declined. Several large rainstorms (10 cm/d) occurred during NBOT treatment which may contributed nutrients. These storms complicated the evaluation of NBOT efficacy. Lake Sylvan will be monitored in 2022 without NBOT treatment for baseline conditions

Science meets policy: A framework for determining impairment designation criteria for large waterbodies affected by cyanobacterial harmful algal blooms

Toxic cyanobacterial harmful algal blooms (cyanoHABs) are one of the most significant threats to the security of Earth\u27s surface freshwaters. In the United States, the Federal Water Pollution Control Act of 1972 (i.e., the Clean Water Act) requires that states report any waterbody that fails to meet applicable water quality standards. The problem is that for fresh waters impacted by cyanoHABs, no scientifically-based framework exists for making this designation. This study describes the development of a data-based framework using the Ohio waters of western Lake Erie as an exemplar for large lakes impacted by cyanoHABs. To address this designation for Ohio\u27s open waters, the Ohio Environmental Protection Agency (EPA) assembled a group of academic, state and federal scientists to develop a framework that would determine the criteria for Ohio EPA to consider in deciding on a recreation use impairment designation due to cyanoHAB presence. Typically, the metrics are derived from on-lake monitoring programs, but for large, dynamic lakes such as Lake Erie, using criteria based on discrete samples is problematic. However, significant advances in remote sensing allows for the estimation of cyanoHAB biomass of an entire lake. Through multiple years of validation, we developed a framework to determine lake-specific criteria for designating a waterbody as impaired by cyanoHABs on an annual basis. While the criteria reported in this manuscript are specific to Ohio\u27s open waters, the framework used to determine them can be applied to any large lake where long-term monitoring data and satellite imagery are available