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1. Microbial production and consumption of hydrocarbons in the global ocean

   Love, C.R.; Arrington, E.C.; Gosselin, K.M.; Reddy, C.M.; Van Mooy, B.A.S.; Nelson, R.K.; Valentine, D.L. (February 2020, Nature microbiology)

   null (Ed.)

   Seeps, spills and other oil pollution introduce hydrocarbons into the ocean. Marine cyanobacteria also produce hydrocarbons from fatty acids, but little is known about the size and turnover of this cyanobacterial hydrocarbon cycle. We report that cyanobacteria in an oligotrophic gyre mainly produce n-pentadecane and that microbial hydrocarbon production exhibits stratification and diel cycling in the sunlit surface ocean. Using chemical and isotopic tracing we find that pentadecane production mainly occurs in the lower euphotic zone. Using a multifaceted approach, we estimate that the global flux of cyanobacteria-produced pentadecane exceeds total oil input in the ocean by 100- to 500-fold. We show that rapid pentadecane consumption sustains a population of pentadecane-degrading bacteria, and possibly archaea. Our findings characterize a microbial hydrocarbon cycle in the open ocean that dwarfs oil input. We hypothesize that cyanobacterial hydrocarbon production selectively primes the ocean’s microbiome with long-chain alkanes whereas degradation of other petroleum hydrocarbons is controlled by factors including proximity to petroleum seepage. 

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2. Methylated cycloalkanes fuel a novel genus in the Porticoccaceae family (Ca. Reddybacter gen. nov)

   https://doi.org/10.1111/1462-2920.16474  

   Arrington, Eleanor C.; Tarn, Jonathan; Kittner, Hailie E.; Kivenson, Veronika; Liu, Rachel M.; Valentine, David L. (August 2023, Environmental Microbiology)

   Abstract Cycloalkanes are abundant and toxic compounds in subsurface petroleum reservoirs and their fate is important to ecosystems impacted by natural oil seeps and spills. This study focuses on the microbial metabolism of methylcyclohexane (MCH) and methylcyclopentane (MCP) in the deep Gulf of Mexico. MCH and MCP are often abundant cycloalkanes observed in petroleum and will dissolve into the water column when introduced at the seafloor via a spill or natural seep. We conducted incubations with deep Gulf of Mexico (GOM) seawater amended with MCH and MCP at four stations. Within incubations with active respiration of MCH and MCP, we found that a novel genus of bacteria belonging to the Porticoccaceae family ( Candidatus Reddybacter ) dominated the microbial community. Using metagenome‐assembled genomes, we reconstructed the central metabolism of Candidatus Reddybacter , identifying a novel clade of the particulate hydrocarbon monooxygenase ( pmo ) that may play a central role in MCH and MCP metabolism. Through comparative analysis of 174 genomes, we parsed the taxonomy of the Porticoccaceae family and found evidence suggesting the acquisition of pmo and other genes related to the degradation of cyclic and branched hydrophobic compounds were likely key events in the ecology and evolution of this group of organisms. 

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3. Radiocarbon in marine methane reveals patchy impact of seeps on surface waters

   https://doi.org/10.1029/2020GL089516  

   Joung, Dong Joo; Leonte, Mihai; Valentine, David L.; Sparrow, Katy; Weber, Thomas; Kessler, John D. (January 2020, Geophysical Research Letters)

   Geological sources of methane (CH4), such as hydrocarbon seeps, are significant yet poorly constrained sources of CH4 to seawater and the overlying atmosphere. We investigate the radiocarbon content (14C) and concentrations of dissolved CH4 in surface waters from the Coal Oil Point seep field to test the hypothesis that geological sources can dominate the regional background signal of CH4. We find that surface waters with elevated CH4 concentration were populated with seep-CH4 and that lower concentrations of CH4 were well explained by mixing with the regional background of non-geological CH4. Substantial differences in concentration and 14C-CH4 were observed over distances <5 km, demonstrating that surface currents mix background-CH4 into the seep field. These results indicate that even a prolific seep region like the Santa Barbara Basin exerts limited influence on the regional background of CH4 in the surface layer but is a significant driver of patchiness in oceanic CH4 biogeochemistry. 

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4. Variations and gradients between methane seep and off-seep microbial communities in a submarine canyon system in the Northeast Pacific

   https://doi.org/10.7717/peerj.15119  

   Cummings, Susie; Ardor Bellucci, Lila M.; Seabrook, Sarah; Raineault, Nicole A.; McPhail, Kerry L.; Thurber, Andrew R. (January 2023, PeerJ)

   Methane seeps are highly abundant marine habitats that contribute sources of chemosynthetic primary production to marine ecosystems. Seeps also factor into the global budget of methane, a potent greenhouse gas. Because of these factors, methane seeps influence not only local ocean ecology, but also biogeochemical cycles on a greater scale. Methane seeps host specialized microbial communities that vary significantly based on geography, seep gross morphology, biogeochemistry, and a diversity of other ecological factors including cross-domain species interactions. In this study, we collected sediment cores from six seep and non-seep locations from Grays and Quinault Canyons (46–47°N) off Washington State, USA, as well as one non-seep site off the coast of Oregon, USA (45°N) to quantify the scale of seep influence on biodiversity within marine habitats. These samples were profiled using 16S rRNA gene sequencing. Predicted gene functions were generated using the program PICRUSt2, and the community composition and predicted functions were compared among samples. The microbial communities at seeps varied by seep morphology and habitat, whereas the microbial communities at non-seep sites varied by water depth. Microbial community composition and predicted gene function clearly transitioned from on-seep to off-seep in samples collected from transects moving away from seeps, with a clear ecotone and high diversity where methane-fueled habitats transition into the non-seep deep sea. Our work demonstrates the microbial and metabolic sphere of influence that extends outwards from methane seep habitats. 

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5. Examining inputs of biogenic and oil-derived hydrocarbons in surface waters following the Deepwater Horizon oil spill

   https://doi.org/10.1021/acsearthspacechem.9b00090  

   White, H.K.; Marx, C.T.; Valentine, D.L.; Sharpless, C.; Aeppli, C.; Gosselin, K.M.; Kivenson, V.; Liu, R.; Nelson, R. K.; Sylva, S.P.; et al (January 2019, ACS earth and space chemistry)

   null (Ed.)

   Aliphatic hydrocarbons in the surface ocean are derived from phytoplankton and oil, but the coexistence and cycling of these two sources is not well-defined. Moreover, phytoplankton and oil can create thin layers of a nonmiscible phase that appear as sheens on surface water and are visually difficult to distinguish. Here, we examine the co-occurrence of hydrocarbon compounds in surface water samples to determine the inputs from phytoplankton and oil using pentadecane (C15-n-alkane) and heptadecane (C17-n-alkane) as molecular markers. Surface water sheens collected from a 2015 field survey in the Northern Gulf of Mexico contained hydrocarbons from natural oil seepage, hytoplankton blooms (i.e., biogenic), and mixtures of the two. Microbial communities examined in surface water sheen samples were dominated by cyanobacteria of the Genus Trichodesmium. The hydrocarbon content of the field-collected surface sheens was used to inform the categorization of 2171 samples collected in 2010 during the Deepwater Horizon (DWH) oil spill. Of the water samples categorized, a small fraction (<1%) ontained only biogenic hydrocarbons, and ∼10% contained a biogenic-hydrocarbon input mixed with oil. This study provides a method for identifying biogenic inputs to oil slicks and surface sheens, and highlights a molecular approach to distinguish the two sources. KEYWORDS: Hydrocarbon, Pentadecane 

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