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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 thePorticoccaceaefamily (Candidatus Reddybacter) dominated the microbial community. Using metagenome‐assembled genomes, we reconstructed the central metabolism ofCandidatus 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 thePorticoccaceaefamily and found evidence suggesting the acquisition ofpmoand 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, DongJoo; Leonte, Mihai; Valentine, David L.; Sparrow, Katy J.; Weber, Thomas; Kessler, John D. (October 2020, Geophysical Research Letters)

   Abstract Geological sources of methane (CH₄), such as hydrocarbon seeps, are significant yet poorly constrained sources of CH₄to seawater and the overlying atmosphere_.We investigate the radiocarbon content (¹⁴C) and concentrations of dissolved CH₄in 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 CH₄concentration were populated with seep‐CH₄and that lower concentrations of CH₄were well explained by mixing with the regional background of nongeological CH₄. Substantial differences in concentration and¹⁴C‐CH₄were observed over distances <5 km, demonstrating that surface currents mix background‐CH₄into 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 CH₄in the surface layer but is a significant driver of patchiness in oceanic CH₄biogeochemistry. 

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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. Heterogeneity of methane seep biomes in the Northeast Pacific

   https://doi.org/10.1016/j.dsr2.2017.10.016  

   Seabrook, S. (January 2018, Deep sea research. Part II, Topical studies in oceanography)

   Methane seeps provide biogeochemical and microbial heterogeneity in deep-sea habitats. In the Northeast (NE) Pacific Ocean recent studies have found an abundance of seeps at varying spatial separations and within distinct biogeochemical environments ranging in oxygen, depth, and temperature. Here, we examine eight newly discovered seeps and two known seeps covering 800 km and varying across 2000 m water depth to identify: (1) novel megafaunal communities in this geographical region; (2) variations in the microbiome of seep habitats across the margin; (3) spatial and biogeochemical drivers of microbial diversity at seeps. In addition to authigenic carbonates, clam beds, microbial mats, and exposed hydrates - we also observed Siboglinidae tube worm bushes and an anomalous deep-sea barnacle adding to the overall habitats known from the NE Pacific. The microbial communities showed high variability in their spatial distribution and community structure. The seep communities formed distinct groups that included multiple groups of anaerobic methane oxidizing Archaea (ANME; 1, 2ab, 2c, and 3), often co-occurring within one site – however, there were also other sites with clearly dominant members (e.g. ANME-1s at Nehalem Bank). Sulfide oxidizers were dominated by the non-mat forming Campylobacterales and even though vertical gradients in redox potential typify seep sediments, in two cases there was not a significant change in community structure across the top five cm of sediment. We posit that these patterns were driven by ‘bubble-turbation,’ and bioirrigation by megafauna. A surprising latitudinal trend was observed in species diversity and richness with increasing richness significantly correlated to increasing latitude. Overall, our results demonstrate that heterogeneity is ubiquitous in the seep biome, spanning all faunal classes, and that the understanding of seeps and the drivers of the community structure can be improved by studying seeps at a range of spatial scales. 

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