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Nitrous oxide (N2O) -- a product of microbial nitrogen metabolism -- is acompelling exoplanet biosignature gas with distinctive spectral features in thenear- and mid-infrared, and only minor abiotic sources on Earth. Previousinvestigations of N2O as a biosignature have examined scenarios using EarthlikeN2O mixing ratios or surface fluxes, or those inferred from Earth's geologicrecord. However, biological fluxes of N2O could be substantially higher, due toa lack of metal catalysts or if the last step of the denitrification metabolismthat yields N2 from N2O had never evolved. Here, we use a global biogeochemicalmodel coupled with photochemical and spectral models to systematically quantifythe limits of plausible N2O abundances and spectral detectability for Earthanalogs orbiting main-sequence (FGKM) stars. We examine N2O buildup over arange of oxygen conditions (1%-100% present atmospheric level) and N2O fluxes(0.01-100 teramole per year; Tmol = 10^12 mole) that are compatible withEarth's history. We find that N2O fluxes of 10 [100] Tmol yr$^{-1}$ would leadto maximum N2O abundances of ~5 [50] ppm for Earth-Sun analogs, 90 [1600] ppmfor Earths around late K dwarfs, and 30 [300] ppm for an Earthlike TRAPPIST-1e.We simulate emission and transmission spectra for intermediate and maximum N2Oconcentrations that are relevant to current and future space-based telescopes.We calculate the detectability of N2O spectral features for high-flux scenariosfor TRAPPIST-1e with JWST. We review potential false positives, includingchemodenitrification and abiotic production via stellar activity, and identifykey spectral and contextual discriminants to confirm or refute the biogenicityof the observed N2O.

ObjectivesTo determine if urinary biomarkers of effect and potential harm are elevated in electronic cigarette users compared with non-smokers and if elevation correlates with increased concentrations of metals in urine.Study design and settingThis was a cross-sectional study of biomarkers of exposure, effect and potential harm in urine from non-smokers (n=20), electronic cigarette users (n=20) and cigarette smokers (n=13). Participant's screening and urine collection were performed at the Roswell Park Comprehensive Cancer Center, and biomarker analysis and metal analysis were performed at the University of California, Riverside.ResultsMetallothionein was significantly elevated in the electronic cigarette group (3761±3932 pg/mg) compared with the non-smokers (1129±1294 pg/mg, p=0.05). 8-OHdG (8-hydroxy-2'-deoxyguanosine) was significantly elevated in electronic cigarette users (442.8±300.7 ng/mg) versus non-smokers (221.6±157.8 ng/mg, p=0.01). 8-Isoprostane showed a significant increase in electronic cigarette users (750.8±433 pg/mg) versus non-smokers (411.2±287.4 pg/mg, p=0.03). Linear regression analysis in the electronic cigarette group showed a significant correlation between cotinine and total metal concentration; total metal concentration and metallothionein; cotinine and oxidative DNA damage; and total metal concentration and oxidative DNA damage. Zinc was significantly elevated in the electronic cigarette users (584.5±826.6 µg/g) compared with non-smokers (413.6±233.7 µg/g, p=0.03). Linear regression analysis showed a significant correlation between urinary zinc concentration and 8-OHdG in the electronic cigarette users.ConclusionsThis study is the first to investigate biomarkers of potential harm and effect in electronic cigarette users and to show a linkage to metal exposure. The biomarker levels in electronic cigarette users were similar to (and not lower than) cigarette smokers. In electronic cigarette users, there was a link to elevated total metal exposure and oxidative DNA damage. Specifically, our results demonstrate that zinc concentration was correlated to oxidative DNA damage.

Abstract. The Southern California Bight is adjacent to one of the world'slargest urban areas, Los Angeles. As a consequence, anthropogenic impactscould disrupt local marine ecosystems due to municipal and industrial wastedischarge, pollution, flood control measures, and global warming. SantaMonica Basin (SMB), due to its unique setting in a low-oxygen and high-sedimentation environment, can provide an excellent sedimentary paleorecordof these anthropogenic changes. This study examined 10 sediment cores,collected from different parts of the SMB between spring and summer 2016,and compared them to existing cores in order to document changes insedimentary dynamics during the last 250 years, with an emphasis on the last40 years. The 210Pb-based mass accumulation rates (MARs) for the deepest and lowest oxygen-containing parts of the SMB basin (900–910 m) have been remarkablyconsistent during the past century, averaging 17.1±0.6 mg cm−2 yr−1. At slightly shallower sites (870–900 m), accumulation ratesshowed more variation but yield the same accumulation rate, 17.9±1.9 mg cm−2 yr−1. Excess 210Pb sedimentation rates were consistentwith rates established using bomb test 137Cs profiles. We also examined14C profiles from two cores collected in the deepest part of the SMB,where fine laminations are present up to about 450 yr BP. These dataindicate that the MAR was slower prior to ∼1900 CE (ratesobtained were 9 and 12 mg cm−2 yr−1). The δ13Corg profilesshow a relatively constant value where laminations are present, suggestingthat the change in sediment accumulation rate is not accompanied by a changein organic carbon sources to the basin. The increase in sedimentation ratetowards the Recent occurs at about the time previous studies predicted anincrease in siltation and the demise of a shelly shelf benthic fauna on theSMB shelf. X-radiographs show finely laminated sediments in the deepest part of thebasin only, with centimeter-scale layering of sediments or no layering whatsoever inshallower parts of the SMB basin. The absence of finely laminated sedimentsin cores MUC 10 (893 m) and MUC 3 (777 m) suggests that the rate at whichanoxia is spreading has not increased appreciably since cores were lastanalyzed in the 1980s. Based on core top data collected during the past halfcentury, sedimentary dynamics within SMB have changed minimally during the last40 years. Specifically, mass accumulation rates, laminated sediment fabric,extent of bioturbation and % Corg have not changed. The onlyparameter that appeared to have changed in the last 450 years was the MAR,with an apparent > 50 % increase occurring between ∼1850 CE and the early 1900s. The post-1900 CE constancy ofsedimentation through a period of massive urbanization in Los Angeles issurprising.

Abstract The habitable zone (HZ) is commonly defined as the range of distances from a host star within which liquid water, a key requirement for life, may exist on a planet’s surface. Substantially more CO2 than present in Earth’s modern atmosphere is required to maintain clement temperatures for most of the HZ, with several bars required at the outer edge. However, most complex aerobic life on Earth is limited by CO2 concentrations of just fractions of a bar. At the same time, most exoplanets in the traditional HZ reside in proximity to M dwarfs, which are more numerous than Sun-like G dwarfs but are predicted to promote greater abundances of gases that can be toxic in the atmospheres of orbiting planets, such as carbon monoxide (CO). Here we show that the HZ for complex aerobic life is likely limited relative to that for microbial life. We use a 1D radiative-convective climate and photochemical models to circumscribe a Habitable Zone for Complex Life (HZCL) based on known toxicity limits for a range of organisms as a proof of concept. We find that for CO2 tolerances of 0.01, 0.1, and 1 bar, the HZCL is only 21%, 32%, and 50% as wide as the conventional HZ for a Sun-like star, and that CO concentrations may limit some complex life throughout the entire HZ of the coolest M dwarfs. These results cast new light on the likely distribution of complex life in the universe and have important ramifications for the search for exoplanet biosignatures and technosignatures.

We describe how environmental context can help determine whether oxygen (O2) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in-depth, interdisciplinary example of O2 biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O2 is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life. However, as a biosignature, O2 faces two major challenges: (1) it was only present at high abundance for a relatively short period of Earth's history and (2) we now know of several potential planetary mechanisms that can generate abundant O2 without life being present. Consequently, our ability to interpret both the presence and absence of O2 in an exoplanetary spectrum relies on understanding the environmental context. Here we examine the coevolution of life with the early Earth's environment to identify how the interplay of sources and sinks may have suppressed O2 release into the atmosphere for several billion years, producing a false negative for biologically generated O2. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. We review the most recent knowledge of false positives for O2, planetary processes that may generate abundant atmospheric O2 without a biosphere. We provide examples of how future photometric, spectroscopic, and time-dependent observations of O2 and other aspects of the planetary environment can be used to rule out false positives and thereby increase our confidence that any observed O2 is indeed a biosignature. These insights will guide and inform the development of future exoplanet characterization missions. Key Words: Biosignatures-Oxygenic photosynthesis-Exoplanets-Planetary atmospheres. Astrobiology 18, 630-662.

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets-Biosignatures-Habitability markers-Photosynthesis-Planetary surfaces-Atmospheres-Spectroscopy-Cryptic biospheres-False positives. Astrobiology 18, 663-708.

Current investigations of exoplanet biosignatures have focused on static evidence of life, such as the presence of biogenic gases like O2 or CH4. However, the expected diversity of terrestrial planet atmospheres and the likelihood of both "false positives" and "false negatives" for conventional biosignatures motivate exploration of additional life detection strategies, including time-varying signals. Seasonal variation in atmospheric composition is a biologically modulated phenomenon on Earth that may occur elsewhere because it arises naturally from the interplay between the biosphere and time-variable insolation. The search for seasonality as a biosignature would avoid many assumptions about specific metabolisms and provide an opportunity to directly quantify biological fluxes - allowing us to characterize, rather than simply recognize, biospheres on exoplanets. Despite this potential, there have been no comprehensive studies of seasonality as an exoplanet biosignature. Here, we provide a foundation for further studies by reviewing both biological and abiological controls on the magnitude and detectability of seasonality of atmospheric CO2, CH4, O2, and O3 on Earth. We also consider an example of an inhabited world for which atmospheric seasonality may be the most notable expression of its biosphere. We show that life on a low O2 planet like the weakly oxygenated mid-Proterozoic Earth could be fingerprinted by seasonal variation in O3 as revealed in its UV Hartley-Huggins bands. This example highlights the need for UV capabilities in future direct-imaging telescope missions (e.g., LUVOIR/HabEx) and illustrates the diagnostic importance of studying temporal biosignatures for exoplanet life detection/characterization.

Here we review how environmental context can be used to interpret whether O2is a biosignature in extrasolar planetary observations. This paper builds onthe overview of current biosignature research discussed in Schwieterman et al.(2017), and provides an in-depth, interdisciplinary example of biosignatureidentification and observation that serves as a basis for the development ofthe general framework for biosignature assessment described in Catling et al.,(2017). O2 is a potentially strong biosignature that was originally thought tobe an unambiguous indicator for life at high-abundance. We describe thecoevolution of life with the early Earth's environment, and how the interplayof sources and sinks in the planetary environment may have resulted insuppression of O2 release into the atmosphere for several billion years, afalse negative for biologically generated O2. False positives may also bepossible, with recent research showing potential mechanisms in exoplanetenvironments that may generate relatively high abundances of atmospheric O2without a biosphere being present. These studies suggest that planetarycharacteristics that may enhance false negatives should be considered whenselecting targets for biosignature searches. Similarly our ability to interpretO2 observed in an exoplanetary atmosphere is also crucially dependent onenvironmental context to rule out false positive mechanisms. We describe futurephotometric, spectroscopic and time-dependent observations of O2 and theplanetary environment that could increase our confidence that any observed O2is a biosignature, and help discriminate it from potential false positives. Byobserving and understanding O2 in its planetary context we can increase ourconfidence in the remote detection of life, and provide a model forbiosignature development for other proposed biosignatures.

In the coming years and decades, advanced space- and ground-basedobservatories will allow an unprecedented opportunity to probe the atmospheresand surfaces of potentially habitable exoplanets for signatures of life. Lifeon Earth, through its gaseous products and reflectance and scatteringproperties, has left its fingerprint on the spectrum of our planet. Aided bythe universality of the laws of physics and chemistry, we turn to Earth'sbiosphere, both in the present and through geologic time, for analog signaturesthat will aid in the search for life elsewhere. Considering the insights gainedfrom modern and ancient Earth, and the broader array of hypothetical exoplanetpossibilities, we have compiled a state-of-the-art overview of our currentunderstanding of potential exoplanet biosignatures including gaseous, surface,and temporal biosignatures. We additionally survey biogenic spectral featuresthat are well-known in the specialist literature but have not yet been robustlyvetted in the context of exoplanet biosignatures. We briefly review advances inassessing biosignature plausibility, including novel methods for determiningchemical disequilibrium from remotely obtainable data and assessment tools fordetermining the minimum biomass required for a given atmospheric signature. Wefocus particularly on advances made since the seminal review by Des Marais etal. (2002). The purpose of this work is not to propose new biosignaturesstrategies, a goal left to companion papers in this series, but to review thecurrent literature, draw meaningful connections between seemingly disparateareas, and clear the way for a path forward.

Ocean-atmosphere chemistry on Earth has undergone dramatic evolutionary changes throughout its long history, with potentially significant ramifications for the emergence and long-term stability of atmospheric biosignatures. Though a great deal of work has centered on refining our understanding of false positives for remote life detection, much less attention has been paid to the possibility of false negatives, that is, cryptic biospheres that are widespread and active on a planet's surface but are ultimately undetectable or difficult to detect in the composition of a planet's atmosphere. Here, we summarize recent developments from geochemical proxy records and Earth system models that provide insight into the long-term evolution of the most readily detectable potential biosignature gases on Earth-oxygen (O2), ozone (O3), and methane (CH4). We suggest that the canonical O2-CH4 disequilibrium biosignature would perhaps have been challenging to detect remotely during Earth's ∼4.5-billion-year history and that in general atmospheric O2/O3 levels have been a poor proxy for the presence of Earth's biosphere for all but the last ∼500 million years. We further suggest that detecting atmospheric CH4 would have been problematic for most of the last ∼2.5 billion years of Earth's history. More broadly, we stress that internal oceanic recycling of biosignature gases will often render surface biospheres on ocean-bearing silicate worlds cryptic, with the implication that the planets most conducive to the development and maintenance of a pervasive biosphere will often be challenging to characterize via conventional atmospheric biosignatures. Key Words: Biosignatures-Oxygen-Methane-Ozone-Exoplanets-Planetary habitability. Astrobiology 17, 287-297.