Roger Everett Summons

The Viking missions landed two spacecraft on Mars each equipped with a pyrolysis‐gas chromatograph‐mass spectrometer system capable of detecting signs of life. Although working as intended, detections of chloromethane and dichloromethane were initially attributed to terrestrial contamination. Decades later, the Phoenix Mars Lander identified perchlorate salts in the Martian regolith that are known to catalyze the oxidation and chlorination reactions of organics. Subsequent analyses by the Sample Analysis at Mars (SAM) instrument suite aboard the Curiosity rover have since confirmed a variety of syngenetic chlorinated hydrocarbons in Gale crater samples. However, the nature of the parent organic matter (exogenous, abiotic endogenous, or biological) remains unresolved. Here we evaluate the Murchison (CM2) carbonaceous chondrite as a proxy for organic matter delivered to the surface of Mars by meteoritic and interplanetary dust particle infall over geologic timescales. We characterize the generation of chlorinated hydrocarbons during the pyrolysis of Murchison in the presence of 1 and 5 wt.% calcium perchlorate. Experiments produced a suite of C ₁ ‐C ₇ chlorinated hydrocarbons dominated by aromatic species, including compounds detected by both Viking and SAM. Increasing the perchlorate content from 1 to 5 wt.% did not amplify the relative abundances or diversity of chlorinated products but rather preferentially oxidized parent organics. Results suggest that organics detected at Gale crater represent a fraction of molecules liberated from a macromolecular source that is compositionally consistent with meteoritic organic matter. Unambiguously resolving the origin and diversity of Martian organic matter may ultimately require the coordinated laboratory analysis of samples returned to Earth.

The search for organic matter on Mars has rapidly evolved in the past decade with simple aromatic, S-heterocycles, and aliphatic organic molecules detected in Gale crater. We report the in situ detection of >20 organic molecules from clay-bearing sandstones in the ~3.5-billion-year-old Knockfarrill Hill member of Glen Torridon, Gale crater, by the Sample Analysis at Mars instrument suite onboard the Curiosity rover. These molecules were liberated by the onboard tetramethylammonium hydroxide wet chemistry experiment. Diverse thermochemolysis products, including benzothiophene, methyl benzoate, and single and dicyclic aromatic molecules were released and detected by evolved gas analysis and gas chromatography-mass spectrometry. Results indicate the experiment successfully released molecules preserved in ancient macromolecular or free organic matter within Martian bedrock despite ~3.5 billion years of diagenesis and radiation exposure.

Throughout Earth’s history, organic molecules from both abiogenic and biogenic sources have been buried in sedimentary rocks. Most of these organic molecules have been significantly altered by geologic processes through deep time. Nonetheless, the nature and distribution of those ancient fragmentary organic remains have the potential to reveal diagnostic biomolecular information after billions of years of burial. Here, we analyzed 406 fossil, modern biological, meteoritic, and synthetic samples using pyrolysis gas chromatography and mass spectrometry. We explored these analytical data via supervised machine-learning methods to discriminate samples of biogenic vs. abiogenic origin, plant vs. animal phylogenetic affinity, and photosynthetic vs. nonphotosynthetic physiology. Dividing 272 samples with known phylogenetic affinity and physiology into 9 categories, each further divided into 75% training and 25% testing sets, our random forest models accurately predict pairwise assignments of modern vs. fossil or meteoritic organics (100% correct assignments), fossil plant tissues vs. meteoritic organics (97%), modern vs. fossil plant tissues (98%), and modern plants vs. animal tissues (95%). Pairwise comparisons between fossil biogenic samples vs. abiogenic samples resulted in 93% correct classifications, while analysis of modern and ancient photosynthetic vs. nonphotosynthetic samples also resulted in 93% correct assignments. Our analyses demonstrate that molecular biosignatures can survive in ancient fossils and allow for the identification of organismal origins and traits. Consistent with previous morphological and isotopic inferences, we present evidence for biogenic molecular assemblages in Paleoarchean rocks (3.33 Ga) and for photoautotrophy in Neoarchean rocks (2.52 Ga).

Putative metazoan body fossils from the Precambrian are curiously lacking morphological characteristics that link them unambiguously to extant animal phyla, including sponges. Chemical fossils such as the rare C ₃₀ hydrocarbons 24- iso -propylcholestane (24- i pc) and 26-methylstigmastane (26-mes), however, have been proposed as evidence for the Neoproterozoic emergence of the Demospongiae (Porifera) due to their prevalence in rocks of this age and the occurrence of their sterol precursors in contemporary demosponges. However, there are alternative hypotheses which posit that diagenetic alteration products of algal sterols, or those from Rhizaria or other protists, account for the enigmatic steroid distributions observed in these ancient sedimentary rocks. Here, we report additional support for the Neoproterozoic rise of demosponges through the chemical characterization of two previously unrecognized C ₃₁ hydrocarbons—24- n -butylcholestane (24- n bc) and 24- sec -butylcholestane (24- sec bc). Precursor C ₃₁ sterols from contemporary demosponges, as well as a suite of synthesized C ₃₁ sterol standards were reduced to their sterane counterparts. Gas chromatography-tandem mass spectrometry analysis and collisionally activated dissociation mass spectra confirmed the presence of 24- n bc and 24- sec bc in well-preserved early Ediacaran rocks, and the coelution of these compounds with synthetic standards enhances the robustness of these findings. Co-occurrence of abundant 24- i pc and 24- sec bc was found for numerous Neoproterozoic-Cambrian rock/oil samples, closely mimicking the abundance patterns and high structural selectivity of major C ₃₀ and C ₃₁ sterols detected in numerous species of modern demosponges. These findings support the hypothesized first emergence of sponges during the Neoproterozoic Era.

Weathering processes typically restrict the distance spilled oil travels in the ocean to a few hundred kilometers. Leveraging oiled marine debris as “drifters of opportunity”, we tested the hypothesis of the unprecedented long-range (thousands of kilometers) transequatorial transport of oil adhered to marine debris by surface currents. Physical oceanographic modeling provided the plausibility for this hypothesis, and molecular forensics provided the definitive evidence supporting it. Oil carried by marine debris arriving at Palm Beach, Florida in 2020 matched oil from the 2019 Brazil mystery oil spill, having traveled ~8,500 km in ~240 days. We demonstrate an additive contaminant effect whereby plastic pollution facilitates the long-range transport of oil pollution. These findings underscore that regional inputs into the global ocean can have transboundary impacts.

The ephemeral, supraglacial meltwater ponds of the McMurdo Ice Shelf’s undulating ice serve as analogues for refugia where eukaryotic organisms could have thrived during the Cryogenian period. The seafloor sediment and debris lined ponds support the growth of a diverse array of cyanobacterial mat communities and provide habitats for a variety of protists and meiofauna. Here, we show that these eukaryotic assemblages, assessed by steroid biomarker and 18S rRNA gene analyses, inform long-standing questions regarding the diversity of, and controls on, community composition in these environments. Sixteen photosynthetically active microbial mats from meltwater ponds, a 700-year-old relict microbial mat, and a microbial mat from the Bratina Lagoon were analysed for their sterol compositions. These sterols were subjected to simulated diagenesis via catalytic hydrogenation/hydrogenolysis affording their sterane hydrocarbon counterparts, facilitating comparisons with ancient settings. Pond salinity appeared to be a factor influencing the sterol distributions observed. Analyses of 18S rRNA gene sequences conducted on the modern mats independently confirm that the ponds host diverse eukaryotes, including many types of microalgae, protists, and an array of unclassifiable organisms. Our findings support the hypothesis that supraglacial meltwater ponds like those of the McMurdo ice are strong candidates for refugia that sheltered complex life during Snowball Earth episodes.

The Great Oxidation Event (GOE) represents a major shift in Earth’s surficial redox balance. Delineating the driver(s) and tempo of the GOE and its impact on microbial evolution and biogeochemistry can be aided by characterizing the cycling of redox-sensitive elements such as nitrogen. While previous studies have shown that the transition to a broadly aerobic marine nitrogen cycle occurred in step with the final phase of the GOE ~2.33 billion years ago (Ga), an evolving understanding of the GOE as a dynamic oscillatory process and the narrow spatial distribution of existing studies highlight ambiguity in the marine nitrogen cycle in the lead up to permanent atmospheric oxygenation. Here, we present stable carbon (δ ¹³ C) and nitrogen (δ ¹⁵ N) isotope ratios derived from the ~2.43 Ga Duitschland and ~2.33 Ga Rooihoogte formations in four drill cores separated by hundreds of kilometers. A significant negative carbon isotope excursion (6 to 8‰) in the Duitschland Formation indicates massive oxidation of organic carbon in close association with a putative snowball Earth event and an earlier pulse of atmospheric oxygen at 2.43 Ga. Further, consistently positive δ ¹⁵ N values (≤ +20.3‰) within the Duitschland Formation, combined with a broad temporal shift across global δ ¹⁵ N records to a distribution comparable to modern marine sediments, signify an aerobic nitrogen cycle ~100 My earlier than previously accepted. Our results update a key timepoint in the evolution of the marine nitrogen cycle and the oxidation of the Earth’s surface surrounding the GOE.

Organic molecules preserved in ancient Martian rocks provide a critical record of the past habitability of Mars and could be chemical biosignatures. Experiments conducted by the Sample Analysis at Mars instrument onboard the Curiosity rover have previously reported several classes of indigenous chlorinated and sulfur-containing organic compounds in Gale crater sedimentary rocks, with chemical structures of up to six carbons. Here, we report the detection of decane (C ₁₀ H ₂₂ ), undecane (C ₁₁ H ₂₄ ), and dodecane (C ₁₂ H ₂₆ ) at the tens of pmol level, released from the Cumberland drilled mudstone sample, using a modified SAM analytical procedure optimized for the detection of larger organic molecules. Laboratory experiments support the hypothesis that the alkanes detected were originally preserved in the mudstone as long-chain carboxylic acids. The origin of these molecules remains uncertain, as they could be derived from either abiotic or biological sources.

The dominant organisms in modern oxic ecosystems rely on respiratory quinones with high redox potential (HPQs) for electron transport in aerobic respiration and photosynthesis. The diversification of quinones, from low redox potential (LPQ) in anaerobes to HPQs in aerobes, is assumed to have followed Earth’s surface oxygenation ~2.3 billion years ago. However, the evolutionary origins of HPQs remain unresolved. Here, we characterize the structure and biosynthetic pathway of an ancestral HPQ, methyl-plastoquinone (mPQ), that is unique to bacteria of the phylum Nitrospirota . mPQ is structurally related to the two previously known HPQs, plastoquinone from Cyanobacteriota /chloroplasts and ubiquinone from Pseudomonadota /mitochondria, respectively. We demonstrate a common origin of the three HPQ biosynthetic pathways that predates the emergence of Nitrospirota , Cyanobacteriota , and Pseudomonadota . An ancestral HPQ biosynthetic pathway evolved ≥ 3.4 billion years ago in an extinct lineage and was laterally transferred to these three phyla ~2.5 to 3.2 billion years ago. We show that Cyanobacteriota and Pseudomonadota were ancestrally aerobic and thus propose that aerobic metabolism using HPQs significantly predates Earth’s surface oxygenation. Two of the three HPQ pathways were later obtained by eukaryotes through endosymbiosis forming chloroplasts and mitochondria, enabling their rise to dominance in modern oxic ecosystems.