David L. Valentine

Nitrogen regulates primary productivity across much of the biosphere, yet fixed nitrogen loss remains poorly constrained because existing methods rely on indirect proxies or ex situ experiments. In this study, we show that natural abundances of the rare ¹⁵ N ¹⁵ N isotopologue of dinitrogen (N ₂ ) provide a direct tracer of biological N ₂ production across diverse aquatic environments. N ₂ produced by denitrification and anammox has a near-stochastic ¹⁵ N ¹⁵ N distribution [0 per mil (‰)], whereas atmospheric N ₂ carries a distinct ¹⁵ N ¹⁵ N excess (19‰), allowing the two sources to be quantitatively distinguished. Across aquifers, stratified lakes, coastal basins, oxygen minimum zones, and marine sediments, ¹⁵ N ¹⁵ N measurements reveal widespread nitrogen loss previously obscured by physical gas accumulation and nitrogen fixation. Natural ¹⁵ N ¹⁵ N abundances therefore provide a general framework for directly constraining fixed nitrogen loss across the aquatic nitrogen cycle.

Dissolved oxygen (O ₂ ) has been declining in coastal and open‐ocean regions, including the Santa Barbara Channel (SBC) in the Southern California Borderland. While previous studies have investigated the biogeochemical consequences of declining O ₂ in the SBC, its spatial distribution and temporal variability in the region remain poorly characterized. Because O ₂ dynamics shape the cycles of redox‐sensitive elements such as iron (Fe), improved characterization is essential for understanding regional biogeochemistry. We integrate observations of dissolved O ₂ spanning 1985–2023 using Autonomous Underwater Vehicle Sentry and the Remotely Operated Vehicle Jason . This new data set provides the first spatially resolved characterization of dissolved O ₂ across the SBC and Santa Barbara Basin. Above the Basin's sill, O ₂ exhibits a vigorous seasonal cycle, with low concentrations during springtime upwelling superimposed on a long‐term declining trend linked to decadal variability. This contrasts with the seasonal cycle below the sill, where higher O ₂ concentrations are observed in spring, reflecting reoxygenation by intermittent flushing events, which have weakened and become less frequent over time. Severe hypoxia and anoxia are widespread in the deep Basin, punctuated by coherent pockets of oxygenated waters, likely the remnants of flushing events. By combining reconstructions of the O ₂ distribution with a benthic Fe flux parameterization, we estimate the release of ∼24,000 tons of Fe annually from the sediment—far exceeding the Fe requirements of net primary production in the region. Our study provides new insights into O ₂ dynamics and their implications for biogeochemistry in the SBC, with relevance to global coastal deoxygenation.

Weathering processes typically restrict the distance spilled oil travels to a few hundred kilometers in the ocean. 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. Dispersion modeling backed by historical drift bottle experiments supported the plausibility for this hypothesis, and molecular forensics provided the definitive evidence proving it. Oil carried by marine debris arriving at Palm Beach, Florida in 2020 matched oil from the 2019 Brazil mystery oil spill, having traveled ∼8500 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.

Historical ocean disposal of industrial DDT (dichloro-diphenyl-trichloroethane) waste off Southern California is implicated in extensive ecological impacts extending well beyond this region, but details around disposal, transport, and fate are poorly characterized for deep ocean settings such as this. Here, we present results from an 814 km² survey of a primary disposal area, San Pedro Basin, by mapping the spatial distribution and depositional history of DDT and its immediate daughter products DDD (dichloro-diphenyl-dichloroethane) and DDE (dichloro-diphenyl-dichloroethylene). We find highly elevated concentrations for all three compounds throughout the study area, with "hotspots" from primary deposition and secondary migration processes. The majority of DDT and DDD is buried within a thin sediment layer consistent with peak deposition in the 1950s, whereas substantial amounts of DDE still linger in overlying sediments with an apparent contribution from the Palos Verdes Shelf. We characterize intense spatial variability across distance scales and develop an approach to address the resulting uncertainty toward estimating a total modern burden of ∼30-36 tonnes (DDT, DDD and DDE) in this area. These results are foundational for informing DDT's deep-sea transport and transformation processes and for defining the linkage of offshore disposal to current ecological problems in this region and beyond.

Methane is a major greenhouse gas and a key component of global biogeochemical cycles. Microbial methane often deviates from isotope and isotopolog equilibrium in surface environments but approaches equilibrium in deep subsurface sediments. The origin of this near-equilibrium isotopic signature in methane, whether directly produced by methanogens or achieved through anaerobic oxidation of methane (AOM), remains uncertain. Here, we show that, in the absence of AOM, microbial methane produced from deep-sea sediments exhibits isotopolog compositions approaching thermodynamic equilibrium due to energy limitation. In contrast, microbial methane from salt marsh and thermokarst lakes exhibits significant hydrogen and clumped isotopic disequilibrium due to high free-energy availability. We propose that clumped isotopologs of methane provide a proxy for characterizing the bioenergetics of environments for methane production. Together, these observations demonstrate methane clumped isotopes as a powerful tool to better understand the relation between methane metabolisms and the energy landscape in natural environments.

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.

Heterotrophic feeding on plankton and particulate organic matter (POM) by tropical Scleractinian corals is known to aid in the resistance and recovery from thermally induced bleaching. However, the relative importance of heterotrophy in promoting bleaching resistance and recovery is likely to vary based on ecological context and the severity of heat stress. In 2019, the Pacific Island of Mo'orea experienced mass coral mortality during a widespread marine heatwave. Many Acropora hyacinthus colonies on the shallow reef slope (5 m) were resistant to bleaching, while colonies on the deeper fore reef (14 m) often bleached and subsequently recovered, despite similar thermal stress. The role of heterotrophy in this divergent bleaching response was investigated using fatty acid (FA), isotopic, and elemental biomarkers. Multiple complementary lines of evidence, including feeding proxies, isotopic niche overlap, and putative POM FA biomarkers, indicated that bleaching resistant colonies were likely consuming more POM than their bleached and recovered counterparts. Additionally, although visually recovered, host energetics in recovered colonies remained compromised and exhibited proportionally less monounsaturated and polyunsaturated FAs and less relative nitrogen than resistant colonies. We show that corals that rely more on heterotrophic nutrient acquisition can better resist thermally induced bleaching. Our results also revealed the long‐term energetic costs of bleaching even after visual recovery. Ultimately, these results underscore the vital role of coral nutrition in shaping coral bleaching response and recovery.

Aqueous-soluble hydrocarbons dissolve into the ocean’s interior and structure deep-sea microbial populations influenced by natural oil seeps and spills. Among these hydrocarbons, n-pentane, is a seawater-soluble, volatile compound abundant in petroleum products and reservoirs, which partially partitions to the deep-water column following release from the seafloor. In this study, we explore the ecology and niche partitioning of two free-living Cycloclasticus strains recovered from seawater incubations with n-pentane and distinguish them as an open ocean variant and a seep-proximal variant, each with distinct capabilities for hydrocarbon catabolism. Comparative metagenomic analysis indicates the variant more frequently observed further from natural seeps encodes more general pathways for hydrocarbon consumption, including short-chain alkanes, aromatics, and long-chain alkanes, and also possesses redox versatility in the form of respiratory nitrate reduction and thiosulfate oxidation; in contrast, the seep variant specializes in short-chain alkanes and relies strictly on oxygen as the terminal electron acceptor. Both variants observed in our work were dominant ecotypes of Cycloclasticus observed during the Deepwater Horizon disaster, a conclusion supported by 16S rRNA gene analysis and read-recruitment of sequences collected from the submerged oil plume during active flow. A comparative genomic analysis of Cycloclasticus across various ecosystems suggests distinct strategies for hydrocarbon transformations among each clade. Our findings suggest Cycloclasticus is a versatile and opportunistic consumer of hydrocarbons and may have a greater role in the cycling of sulfur and nitrogen, thus contributing broad ecological impact to various ecosystems globally.

Aqueous-soluble hydrocarbons dissolve into the ocean interior and structure deep-sea microbial populations influenced by natural oil seeps and spills. n-Pentane is a seawater-soluble, volatile compound abundant in petroleum products and reservoirs and will partially partition to the deep-water column following release from the seafloor. In this study, we explore the ecology and niche partitioning of two free-living Cycloclasticus strains recovered from seawater incubations with n-pentane using and distinguish them as an open ocean variant and a seep-proximal variant, each with distinct capabilities for hydrocarbon catabolism. Comparative metagenomic analysis indicates the open ocean adapted variant encodes more general pathways for hydrocarbon consumption, including short-chain alkanes, aromatics, and long-chain alkanes, and also possesses redox versatility in the form of respiratory nitrate reduction and thiosulfate oxidation; in contrast, the seep variant specializes in short-chain alkanes and relies strictly on oxygen as the terminal electron acceptor. Both variants observed in our work were dominant ecotypes of Cycloclasticus observed during the Deepwater Horizon disaster, a conclusion supported by 16S rRNA analysis and read-recruitment of sequences from the submerged oil plume during active flow. A comparative genomic analysis of Cycloclasticus across various ecosystems suggests distinct strategies for hydrocarbon transformations among each clade. Our findings suggest Cycloclasticus is a versatile and opportunistic consumer of hydrocarbons and may have a greater role in the cycling of sulfur and nitrogen, thus contributing broad ecological impact to various ecosystems globally.