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Guaymas Basin, located in the Gulf of California, is a hydrothermally active marginal basin. Due to steep geothermal gradients and localized heating by sill intrusions, microbial substrates like short-chain fatty acids and hydrocarbons are abiotically produced from sedimentary organic matter at comparatively shallow depths. We analyzed the effect of hydrocarbons on uptake of hydrocarbons by microorganisms via nano-scale secondary ion mass spectrometry (NanoSIMS) and microbial sulfate reduction rates (SRR), using samples from two drill sites sampled by IODP Expedition 385 (U1545C and U1546D). These sites are in close proximity of each other (ca. 1 km) and have very similar sedimentology. Site U1546D experienced the intrusion of a sill that has since then thermally equilibrated with the surrounding sediment. Both sites currently have an identical geothermal gradient, despite their different thermal history. The localized heating by the sill led to thermal cracking of sedimentary organic matter and formation of potentially bioavailable organic substrates. There were low levels of hydrocarbon and nitrogen uptake in some samples from both sites, mostly in surficial samples. Hydrocarbon and methane additions stimulated SRR in near-seafloor samples from Site U1545C, while samples from Site U1546D reacted positively only on methane. Our data indicate the potential of microorganisms to metabolize hydrocarbons even in the deep subsurface of Guaymas Basin. [ABSTRACT FROM AUTHOR]

The marine subsurface is a long-term sink of atmospheric carbon dioxide with significant implications for climate on geologic timescales. Subsurface microbial cells can either enhance or reduce carbon sequestration in the subsurface, depending on their metabolic lifestyle. However, the activity of subsurface microbes is rarely measured. Here, we used nanoscale secondary ion mass spectrometry (nanoSIMS) to quantify anabolic activity in 3,203 individual cells from the thermally altered deep subsurface in the Guaymas Basin, Mexico (3-75 m below the seafloor, 0-14°C). We observed that a large majority of cells were active (83%-100%), although the rates of biomass generation were low, suggesting cellular maintenance rather than doubling. Mean single-cell activity decreased with increasing sediment depth and temperature and was most strongly correlated with porewater sulfate concentrations. Intracommunity heterogeneity in microbial activity decreased with increasing sediment depth and age. Using a dual-isotope labeling approach, we determined that all active cells analyzed were heterotrophic, deriving the majority of their cellular carbon from organic sources. However, we also detected inorganic carbon assimilation in these heterotrophic cells, likely via processes such as anaplerosis, and determined that inorganic carbon contributes at least 5% of the total biomass carbon in heterotrophs in this community. Our results demonstrate that the deep marine biosphere at Guaymas Basin is largely active and contributes to subsurface carbon cycling primarily by not only assimilating organic carbon but also fixing inorganic carbon. Heterotrophic assimilation of inorganic carbon may be a small yet significant and widespread underappreciated source of labile carbon in the global subsurface. [ABSTRACT FROM AUTHOR]

The Guaymas Basin, Gulf of California is a hydrothermally active sediment-covered oceanic spreading center in the early stages of rifting. Hot-spring fluids were collected from its southern trough in 1998, 2003, and 2008 and analyzed for the concentration and isotopic composition of organic and inorganic aqueous species to assess subseafloor geochemical processes. Fluids discharged from the seafloor through hydrothermal edifices with measured temperatures of 91 to 317 °C. In general, fluids exiting larger structures were characterized by higher temperatures than fluids venting from smaller structures. Measured pH (25 °C) ranged from 5.7 to 7.1. Endmember fluids were characterized by near-zero concentrations of Mg and SO 4 , depletions in Na, and enrichments in K, Ca, and Cl relative to seawater, reflecting extensive interaction with sediment during circulation. Thermal maturation of sedimentary organic matter resulted in the addition of abundant ΣNH 4 +, ΣCO 2 , CH 4 , short-chain n -alkanes, and carboxylic acids to solution. Organic compounds added to fluids during recharge pass through high temperature deep-seated reaction zones and reach high levels of thermal maturity while others may be added by entrainment of lower temperature sedimentary pore fluids in hydrothermal upflow zones. The relative abundance of many organic compounds mobilized during hydrothermal alteration is consistent with metastable states of thermodynamic equilibrium that likely record subseafloor temperature conditions. The isotope composition of low molecular weight hydrocarbons and elevated concentrations of dissolved Cl suggest that temperatures may have exceeded the two-phase boundary for seawater and subsequently cooled before discharging at the seafloor. Periods of high temperature fluid-sediment or fluid-rock interaction followed by cooling may provide a mechanism for the aqueous mobilization and concentration of sulfide-forming metals in subseafloor mineral deposits. Fluid compositions have remained constant at the southern trough of Guaymas Basin for at least 26 years, indicating decadal stability in the physical and chemical conditions in subseafloor environments as well as the km-scale regional recharge zones containing unaltered sediment. [ABSTRACT FROM AUTHOR]

The high-resolution analysis of radiolarians and silicoflagellates in sediments from Holes U1545A and U1549A drilled during IODP Expedition 385 in the Guaymas Basin, in the Gulf of California provides detailed insights into the evolution of ocean circulation and water masses, and its relation to Eastern Tropical Pacific Ocean climate conditions, over the past 31,000 cal years BP (based on AMS radiocarbon dates). In the pre-Last Glacial Maximum, the Guaymas Basin experienced alternating circulation patterns of California Current Water (CCW) and Gulf of California Water (GCW), with an extended presence of the Pacific Intermediate Water (PIW) owing to: amplified jet streams; southern movement of the California Current System (CCS) and the incursion of CCW into the gulf; and increased North Pacific Intermediate Water (NPIW) formation. The Last Glacial Maximum witnessed the incursion of CCW due to the stronger CCS. The dominance of the PIW indicates the expansion and formation of NPIW. The Heinrich-I event as manifested in the core record, displays two distinct patterns, one suggesting GCW-like dominance and the other, the occurrence of CCW. The Bølling-Ållerød interstadial featured the entry of Tropical Surface Water (TSW), GCW, and CCW, linked with the northward migration of the Intertropical Convergence Zone. In the Younger Dryas, CCW dominated, transitioning to GCW as colder climatic conditions and more intense CCS. The Holocene displayed alternating periods of TSW and GCW, with a modern monsoon regime from 7,600 to 1,000 cal years BP. From 1,000 cal years BP to the present the ITCZ shifted to the south. [ABSTRACT FROM AUTHOR]

The Guaymas Basin in the Gulf of California is characterized by active seafloor spreading, the rapid deposition of organic-rich sediments, steep geothermal gradients, and abundant methane of mixed thermogenic and microbial origin. Subsurface sediment samples from eight drilling sites with distinct geochemical and thermal profiles were selected for DNA extraction and PCR amplification to explore the diversity of methane-cycling archaea in the Guaymas Basin subsurface. We performed PCR amplifications with general (mcrIRD), and ANME-1 specific primers that target the alpha (α) subunit of methyl coenzyme M reductase (mcrA). Diverse ANME-1 lineages associated with anaerobic methane oxidation were detected in seven out of the eight drilling sites, preferentially around the methane-sulfate interface, and in several cases, showed preferences for specific sampling sites. Phylogenetically, most ANME-1 sequences from the Guaymas Basin subsurface were related to marine mud volcanoes, seep sites, and the shallow marine subsurface. The most frequently recovered methanogenic phylotypes were closely affiliated with the hyperthermophilic Methanocaldococcaceae, and found at the hydrothermally influenced Ringvent site. The coolest drilling site, in the northern axial trough of Guaymas Basin, yielded the greatest diversity in methanogen lineages. Our survey indicates the potential for extensive microbial methane cycling within subsurface sediments of Guaymas Basin. [ABSTRACT FROM AUTHOR]

The high-resolution analysis of radiolarians and silicoflagellates in sediments from Holes U1545A and U1549A drilled during IODP Expedition 385 in the Guaymas Basin, in the Gulf of California provides detailed insights into the evolution of ocean circulation and water masses, and its relation to Eastern Tropical Pacific Ocean climate conditions, over the past 31,000 cal years BP (based on AMS radiocarbon dates). In the pre-Last Glacial Maximum, the Guaymas Basin experienced alternating circulation patterns of California Current Water (CCW) and Gulf of California Water (GCW), with an extended presence of the Pacific Intermediate Water (PIW) owing to: amplified jet streams; southern movement of the California Current System (CCS) and the incursion of CCW into the gulf; and increased North Pacific Intermediate Water (NPIW) formation. The Last Glacial Maximum witnessed the incursion of CCW due to the stronger CCS. The dominance of the PIW indicates the expansion and formation of NPIW. The Heinrich-I event as manifested in the core record, displays two distinct patterns, one suggesting GCW-like dominance and the other, the occurrence of CCW. The Bølling-Ållerød interstadial featured the entry of Tropical Surface Water (TSW), GCW, and CCW, linked with the northward migration of the Intertropical Convergence Zone. In the Younger Dryas, CCW dominated, transitioning to GCW as colder climatic conditions and more intense CCS. The Holocene displayed alternating periods of TSW and GCW, with a modern monsoon regime from 7,600 to 1,000 cal years BP. From 1,000 cal years BP to the present the ITCZ shifted to the south.

Unsubstituted aromatic hydrocarbons (UAHs) are recalcitrant molecules abundant in crude oil, which is accumulated in subsurface reservoirs and occasionally enters the marine environment through natural seepage or human-caused spillage. The challenging anaerobic degradation of UAHs by microorganisms, in particular under thermophilic conditions, is poorly understood. Here, we established benzeneand naphthalene-degrading cultures under sulfate-reducing conditions at 50°C and 70°C from Guaymas Basin sediments. We investigated the microorganisms in the enrichment cultures and their potential for UAH oxidation through shortread metagenome sequencing and analysis. Dependent on the combination of UAH and temperature, different microorganisms became enriched. A Thermoplasmatota archaeon was abundant in the benzene-degrading culture at 50°C, but catabolic pathways remained elusive, because the archaeon lacked most known genes for benzene degradation. Two novel species of Desulfatiglandales bacteria were strongly enriched in the benzene-degrading culture at 70°C and in the naphthalene-degrading culture at 50°C. Both bacteria encode almost complete pathways for UAH degradation and for downstream degradation. They likely activate benzene via methylation, and naphthalene via direct carboxylation, respectively. The two species constitute the first thermophilic UAH degraders of the Desulfatiglandales. In the naphthalene-degrading culture incubated at 70°C, a Dehalococcoidia bacterium became enriched, which encoded a partial pathway for UAH degradation. Comparison of enriched bacteria with related genomes from environmental samples indicated that pathways for benzene degradation are widely distributed, while thermophily and capacity for naphthalene activation are rare. Our study highlights the capacities of uncultured thermophilic microbes for UAH degradation in petroleum reservoirs and in contaminated environments. [ABSTRACT FROM AUTHOR]

Unsubstituted aromatic hydrocarbons (UAHs) are recalcitrant molecules abundant in crude oil, which is accumulated in subsurface reservoirs and occasionally enters the marine environment through natural seepage or human-caused spillage. The challenging anaerobic degradation of UAHs by microorganisms, in particular under thermophilic conditions, is poorly understood. Here, we established benzene- and naphthalene-degrading cultures under sulfate-reducing conditions at 50°C and 70°C from Guaymas Basin sediments. We investigated the microorganisms in the enrichment cultures and their potential for UAH oxidation through short-read metagenome sequencing and analysis. Dependent on the combination of UAH and temperature, different microorganisms became enriched. A Thermoplasmatota archaeon was abundant in the benzene-degrading culture at 50°C, but catabolic pathways remained elusive, because the archaeon lacked most known genes for benzene degradation. Two novel species of Desulfatiglandales bacteria were strongly enriched in the benzene-degrading culture at 70°C and in the naphthalene-degrading culture at 50°C. Both bacteria encode almost complete pathways for UAH degradation and for downstream degradation. They likely activate benzene via methylation, and naphthalene via direct carboxylation, respectively. The two species constitute the first thermophilic UAH degraders of the Desulfatiglandales. In the naphthalene-degrading culture incubated at 70°C, a Dehalococcoidia bacterium became enriched, which encoded a partial pathway for UAH degradation. Comparison of enriched bacteria with related genomes from environmental samples indicated that pathways for benzene degradation are widely distributed, while thermophily and capacity for naphthalene activation are rare. Our study highlights the capacities of uncultured thermophilic microbes for UAH degradation in petroleum reservoirs and in contaminated environments.

The Guaymas Basin in the Gulf of California is characterized by active seafloor spreading, hydrothermal activity, and organic matter accumulation on the seafloor due to high sedimentation rates. In the hydrothermal sediments of Guaymas Basin, microbial community compositions and coexistence patterns change across steep gradients of temperature, potential carbon sources, and electron acceptors. Nonmetric multidimensional scaling and guanine-cytosine percentage analyses reveal that the bacterial and archaeal communities adjust compositionally to their local temperature regime. Functional inference using PICRUSt shows that microbial communities consistently maintain their predicted biogeochemical functions in different sediments. Phylogenetic profiling shows that microbial communities retain distinct sulfate-reducing, methane-oxidizing, or heterotrophic lineages within specific temperature windows. The preservation of similar biogeochemical functions across microbial lineages with different temperature adaptations stabilizes the hydrothermal microbial community in a highly dynamic environment. IMPORTANCE Hydrothermal vent sites have been widely studied to investigate novel bacteria and archaea that are adapted to these extreme environments. However, community-level analyses of hydrothermal microbial ecosystems look beyond the presence and activity of particular types of microbes and examine to what extent the entire community of bacteria and archaea is adapted to hydrothermal conditions; these include elevated temperatures, hydrothermally generated carbon sources, and inorganic electron donors and acceptors that are characteristic for hydrothermal environments. In our case study of bacterial and archaeal communities in hydrothermal sediments of Guaymas Basin, we found that sequence-inferred microbial function was maintained in differently structured bacterial and archaeal communities across different samples and thermal regimes. The resulting preservation of biogeochemical functions across thermal gradients is an important factor in explaining the consistency of the microbial core community in the dynamic sedimentary environment of Guaymas Basin. [ABSTRACT FROM AUTHOR]

The Guaymas Basin in the Gulf of California is characterized by active seafloor spreading, the rapid deposition of organic-rich sediments, steep geothermal gradients, and abundant methane of mixed thermogenic and microbial origin. Subsurface sediment samples from eight drilling sites with distinct geochemical and thermal profiles were selected for DNA extraction and PCR amplification to explore the diversity of methane-cycling archaea in the Guaymas Basin subsurface. We performed PCR amplifications with general (mcrIRD), and ANME-1 specific primers that target the alpha (α) subunit of methyl coenzyme M reductase (mcrA). Diverse ANME-1 lineages associated with anaerobic methane oxidation were detected in seven out of the eight drilling sites, preferentially around the methane-sulfate interface, and in several cases, showed preferences for specific sampling sites. Phylogenetically, most ANME-1 sequences from the Guaymas Basin subsurface were related to marine mud volcanoes, seep sites, and the shallow marine subsurface. The most frequently recovered methanogenic phylotypes were closely affiliated with the hyperthermophilic Methanocaldococcaceae, and found at the hydrothermally influenced Ringvent site. The coolest drilling site, in the northern axial trough of Guaymas Basin, yielded the greatest diversity in methanogen lineages. Our survey indicates the potential for extensive microbial methane cycling within subsurface sediments of Guaymas Basin.

The example of the Guaymas Basin in the Gulf of California (DSDP 478 borehole) shows the change in the chemical composition of the Upper Pleistocene sediments in specific short-term hydrothermal systems that arise when basalt sills intrude into nonlithified cold highly porous sediments with sea water. Changes in the chemical composition of the sediments (turbidites interbedded with hemipelagic sediments) in contact zones with basalt sills in Guaymas Basin in the Gulf of California are studied by X-ray fluorescence (XRF) analysis and inductively coupled plasma mass spectrometry (ICP-MS). [ABSTRACT FROM AUTHOR]

In the Guaymas Basin of the Gulf of California, changes in the mineral and chemical composition of sills of different thicknesses were studied in hydrothermal systems, which originated during the intrusion of the hot sills into young (Pleistocene) cold seawater-saturated sediments. Holes DSDP 477 and 477A were drilled in a high heat flow area, and holes 478 and 481/481A, in a low heat flow area. Thick sills from holes 477, 477A, and 478 (47, 30, and 112 m, respectively) remained fresh or very weakly altered because water released from the host Pleistocene sediments upon heating practically did not penetrate them. Olivine (a few wt %) in the sills remains unaltered, whereas mesostasis also amounting to a few percent is replaced by clay minerals. The high heat flow alone, without water input, could not provide favorable conditions for the changes in the mineral and chemical composition of thick sills in holes 477 and 477A (1250 mW/m2). The thin (from 0.1 to 0.65 m thick) and relatively thin (from 1.2 to 4.5 m) sills recovered in holes 478 and 481/481A were altered. They are fractured and therefore favorable for the penetration of water released from heated host sediments. In addition, a large volume of these sills is occupied by mesostasis (mainly from 18 to 45%, reaching in some samples from 53 to 70%) variably replaced by clay minerals. Olivine is frequently completely replaced by clay minerals. The alteration of thin basalt sills is accompanied by a sharp decrease in the K content, which in some cases is accompanied by an insignificant decrease in Mn, Mg, P, Na, and Ti. The low heat flow measured in the areas of holes 478 and 481/481A (150 and 165 mW/m2) indicates that the established mineral and chemical changes in the thin sills were caused by auto-hydrothermal processes, without heat and mass impact of convective long-lived hydrothermal systems. [ABSTRACT FROM AUTHOR]

Hydrocarbons are degraded by specialized types of bacteria, archaea, and fungi. Their occurrence in marine hydrocarbon seeps and sediments prompted a study of their role and their potential interactions, using the hydrocarbon-rich hydrothermal sediments of Guaymas Basin in the Gulf of California as a model system. This sedimented vent site is characterized by localized hydrothermal circulation that introduces seawater sulfate into methane- and hydrocarbon-rich sediments, and thus selects for diverse hydrocarbon-degrading communities of which methane, alkane- and aromatics-oxidizing sulfate-reducing bacteria and archaea have been especially well-studied. Current molecular and cultivation surveys are detecting diverse fungi in Guaymas Basin hydrothermal sediments, and draw attention to possible fungal-bacterial interactions. In this Hypothesis and Theory article, we report on background, recent results and outcomes, and underlying hypotheses that guide current experiments on this topic in the Edgcomb and Teske labs in 2021, and that we will revisit during our ongoing investigations of bacterial, archaeal, and fungal communities in the deep sedimentary subsurface of Guaymas Basin. [ABSTRACT FROM AUTHOR]

Sulfate reduction is the quantitatively most important process to degrade organic matter in anoxic marine sediment and has been studied intensively in a variety of settings. Guaymas Basin, a young marginal ocean basin, offers the unique opportunity to study sulfate reduction in an environment characterized by organic-rich sediment, high sedimentation rates, and high geothermal gradients (100–958°C km−1). We measured sulfate reduction rates (SRR) in samples taken during the International Ocean Discovery Program (IODP) Expedition 385 using incubation experiments with radiolabeled 35SO42− carried out at in situ pressure and temperature. The highest SRR (387 nmol cm−3 d−1) was recorded in near-surface sediments from Site U1548C, which had the steepest geothermal gradient (958°C km−1). At this site, SRR were generally over an order of magnitude higher than at similar depths at other sites (e.g., 387–157 nmol cm−3 d−1 at 1.9 mbsf from Site U1548C vs. 46–1.0 nmol cm−3 d−1 at 2.1 mbsf from Site U1552B). Site U1546D is characterized by a sill intrusion, but it had already reached thermal equilibrium and SRR were in the same range as nearby Site U1545C, which is minimally affected by sills. The wide temperature range observed at each drill site suggests major shifts in microbial community composition with very different temperature optima but awaits confirmation by molecular biological analyses. At the transition between the mesophilic and thermophilic range around 40°C–60°C, sulfate-reducing activity appears to be decreased, particularly in more oligotrophic settings, but shows a slight recovery at higher temperatures. [ABSTRACT FROM AUTHOR]

Sulfate reduction is the quantitatively most important process to degrade organic matter in anoxic marine sediment and has been studied intensively in a variety of settings. Guaymas Basin, a young marginal ocean basin, offers the unique opportunity to study sulfate reduction in an environment characterized by organic-rich sediment, high sedimentation rates, and high geothermal gradients (100–958°C km−1). We measured sulfate reduction rates (SRR) in samples taken during the International Ocean Discovery Program (IODP) Expedition 385 using incubation experiments with radiolabeled 35SO42− carried out at in situ pressure and temperature. The highest SRR (387 nmol cm−3 d−1) was recorded in near-surface sediments from Site U1548C, which had the steepest geothermal gradient (958°C km−1). At this site, SRR were generally over an order of magnitude higher than at similar depths at other sites (e.g., 387–157 nmol cm−3 d−1 at 1.9 mbsf from Site U1548C vs. 46–1.0 nmol cm−3 d−1 at 2.1 mbsf from Site U1552B). Site U1546D is characterized by a sill intrusion, but it had already reached thermal equilibrium and SRR were in the same range as nearby Site U1545C, which is minimally affected by sills. The wide temperature range observed at each drill site suggests major shifts in microbial community composition with very different temperature optima but awaits confirmation by molecular biological analyses. At the transition between the mesophilic and thermophilic range around 40°C–60°C, sulfate-reducing activity appears to be decreased, particularly in more oligotrophic settings, but shows a slight recovery at higher temperatures.

Hydrocarbons are degraded by specialized types of bacteria, archaea, and fungi. Their occurrence in marine hydrocarbon seeps and sediments prompted a study of their role and their potential interactions, using the hydrocarbon-rich hydrothermal sediments of Guaymas Basin in the Gulf of California as a model system. This sedimented vent site is characterized by localized hydrothermal circulation that introduces seawater sulfate into methane- and hydrocarbon-rich sediments, and thus selects for diverse hydrocarbon-degrading communities of which methane, alkane- and aromatics-oxidizing sulfate-reducing bacteria and archaea have been especially well-studied. Current molecular and cultivation surveys are detecting diverse fungi in Guaymas Basin hydrothermal sediments, and draw attention to possible fungal-bacterial interactions. In this Hypothesis and Theory article, we report on background, recent results and outcomes, and underlying hypotheses that guide current experiments on this topic in the Edgcomb and Teske labs in 2021, and that we will revisit during our ongoing investigations of bacterial, archaeal, and fungal communities in the deep sedimentary subsurface of Guaymas Basin.

The tubeworm Riftia pachyptila is a key primarily producer in hydrothermal vent communities due to the symbiosis with sulphur-oxidizing bacteria, which provide nourishment to the worm from sulphides, oxygen and carbon dioxide. These substances diffuse from the vent water into the bloodstream of the worm through their tentacular crowns, and then to the bacteria, hosted in a specialized organ of the worm, called a trophosome. The uptake rates of these substances depend on the surface/volume relationship of the tentacles. We here describe two morphotypes, 'fat' and 'slim', respectively, from the basalt sulphide-rich vents at 9 °N and 21 °N at the East Pacific Rise, and the highly sedimented, sulphide-poor vents at 27 °N in the Guaymas Basin. The 'fat' morphotype has a thicker body and tube, longer trunk and smaller tentacular crowns, whereas the 'slim' morphotype has shorter trunk, thinner body and tube, and presents longer tentacular crowns and has a higher number of tentacular lamellae. Given the dependence on sulphides for the growth of R. pachyptila , as well as high genetic connectivity of the worm's populations along the studied localities, we suggest that such morphological differences are adaptive and selected to keep the sulphide uptake near to the optimum values for the symbionts. 'Fat' and 'slim' morphotypes are also found in the vestimentiferan Ridgeia piscesae in similar sulphide-rich and poor environments in the northern Pacific. [ABSTRACT FROM AUTHOR]

The surficial hydrothermal sediments of Guaymas Basin harbor complex microbial communities where oxidative and reductive nitrogen, sulfur, and carbon-cycling populations and processes overlap and coexist. Here, we resolve microbial community profiles in hydrothermal sediment cores of Guaymas Basin on a scale of 2 millimeters, using Denaturing Gradient Gel Electrophoresis (DGGE) to visualize the rapid downcore changes among dominant bacteria and archaea. DGGE analysis of bacterial 16S rRNA gene amplicons identified free-living and syntrophic deltaproteobacterial sulfate-reducing bacteria, fermentative Cytophagales, members of the Chloroflexi (Thermoflexia), Aminicenantes, and uncultured sediment clades. The DGGE pattern indicates a gradually changing downcore community structure where small changes on a 2-millimeter scale accumulate to significantly changing populations within the top 4 cm sediment layer. Functional gene DGGE analyses identified anaerobic methane-oxidizing archaea (ANME) based on methyl-coenzyme M reductase genes, and members of the Betaproteobacteria and Thaumarchaeota based on bacterial and archaeal ammonia monooxygenase genes, respectively. The co-existence and overlapping habitat range of aerobic, nitrifying, sulfate-reducing and fermentative bacteria and archaea, including thermophiles, in the surficial sediments is consistent with dynamic redox and thermal gradients that sustain highly complex microbial communities in the hydrothermal sediments of Guaymas Basin.

The surficial hydrothermal sediments of Guaymas Basin harbor complex microbial communities where oxidative and reductive nitrogen, sulfur, and carbon-cycling populations and processes overlap and coexist. Here, we resolve microbial community profiles in hydrothermal sediment cores of Guaymas Basin on a scale of 2 millimeters, using Denaturing Gradient Gel Electrophoresis (DGGE) to visualize the rapid downcore changes among dominant bacteria and archaea. DGGE analysis of bacterial 16S rRNA gene amplicons identified free-living and syntrophic deltaproteobacterial sulfate-reducing bacteria, fermentative Cytophagales, members of the Chloroflexi (Thermoflexia), Aminicenantes, and uncultured sediment clades. The DGGE pattern indicates a gradually changing downcore community structure where small changes on a 2-millimeter scale accumulate to significantly changing populations within the top 4 cm sediment layer. Functional gene DGGE analyses identified anaerobic methane-oxidizing archaea (ANME) based on methyl-coenzyme M reductase genes, and members of the Betaproteobacteria and Thaumarchaeota based on bacterial and archaeal ammonia monooxygenase genes, respectively. The co-existence and overlapping habitat range of aerobic, nitrifying, sulfate-reducing and fermentative bacteria and archaea, including thermophiles, in the surficial sediments is consistent with dynamic redox and thermal gradients that sustain highly complex microbial communities in the hydrothermal sediments of Guaymas Basin. [ABSTRACT FROM AUTHOR]

Cold seeps and hydrothermal vents are seafloor habitats fueled by subsurface energy sources. Both habitat types coexist in Guaymas Basin in the Gulf of California, providing an opportunity to compare microbial communities with distinct physiologies adapted to different thermal regimes. Hydrothermally active sites in the southern Guaymas Basin axial valley, and cold seep sites at Octopus Mound, a carbonate mound with abundant methanotrophic cold seep fauna at the Central Seep location on the northern off-axis flanking regions, show consistent geochemical and microbial differences between hot, temperate, cold seep, and background sites. The changing microbial actors include autotrophic and heterotrophic bacterial and archaeal lineages that catalyze sulfur, nitrogen, and methane cycling, organic matter degradation, and hydrocarbon oxidation. Thermal, biogeochemical, and microbiological characteristics of the sampling locations indicate that sediment thermal regime and seep-derived or hydrothermal energy sources structure the microbial communities at the sediment surface.

Clay minerals in basalt sills from the northern East Pacific Rise, covered by Upper Pleistocene clayey–sandy–silty sediments (turbidites, hemipelagites, and diatom oozes) sampled from DSDP Holes 477, 478, and 481A, were studied by X-ray methods based on the modeling of diffraction patterns. Trioctahedral smectites formed in thin (0.1–0.5 and 1.8–4 m) fissured sills that are well permeable for water delivered from the water-saturated host sediments heated by the sills. Smectites in basalts are found in the interstices (20–40 vol % in the rock and up to 50–80 vol % in hyalobasalts). They replace olivine and fill cracks and vesicles. Plagioclase and clinopyroxene are generally unaltered. The structure of smectites is characterized by different height of layers depending on the composition of cations in the interlayers and the degree of their hydration. The different-height layers either make up individual smectite phases or alternate in different proportions and with a high degree of segregation in the mixed-layer structures. Under conditions of a prolonged cooling, thick (43 m) sills are distinguished from the thin varieties by the formation of trioctahedral minerals (smectite–chlorite, chlorite, defective chlorite, and talc). They are found mainly in the interstices that make up 3–7 vol % of the rock. [ABSTRACT FROM AUTHOR]

The removal of exchangeable oxygen from diatom opal prior to δ18Odiatom analysis is a crucial first step before analyzing frustule oxygen isotopes for paleoceanographic applications. We present the results of experiments that quantify the temperature‐dependent reactivity of biogenic silica with water under laboratory conditions. We demonstrate that controlled exchange between rinse water and diatom opal at room temperature results in predictable alteration of δ18Odiatom, after vacuum dehydroxylation. Diatom samples equilibrated with an 18O‐enriched δ18Oequil. water solution of +94.4‰ at ~21 °C for 70 hr prior to dehydroxylation yield δ18Odiatom data that can be directly interpreted with existing empirical δ18Odiatom versus temperature relationships. We compare sediment trap‐based δ18Odiatom temperature data with modern δ18Oforam temperatures. Finally, we present an ~220 year record of δ18Odiatom data from an eastern Pacific Guaymas Basin boxcore, analyzed using the microfluorination technique, that indicates diatom chlorophyll maximum (Chlmax) temperatures have shifted from a period during the late eighteenth and nineteenth centuries that was 8 °C cooler during the fall bloom relative to mean annual alkenone U37k′ sea surface temperature, to the late twentieth century when we observe no difference between alkenone sea surface temperature and diatom Chlmax. These data suggest a twentieth century seasonal shift in the timing of fall upwelling and Guaymas Basin stratification breakdown that could be due to reduced upwelling efficiency and/or increased summer mixed layer thermal stratification. Key Points: Diatom δ18O data show predictable alteration at ambient laboratory temperatures due to exchange with rinse water but is correctableGuaymas Basin diatom sediments biased toward the fall bloom and track well with existing foraminifera data setsDiatom δ18O temperatures record chlorophyll maximum temperatures warming by ~10°C over a 220 year time frame [ABSTRACT FROM AUTHOR]

Abstract—Chemical composition of the Upper Pleistocene sediments (turbidites interbedded with hemipelagic sediments) from contact zones with basalt sills in the Guaymas Basin, Gulf of the California (DSDP Hole 481A), was studied by the methods of XRF and ICP-MS. Alteration for most macroelements in these sediments is not or slightly. This is not indicates immobility or slightly mobility of these elements, as a process of a distribution of elements occurs inside of sediments without a change their bulk chemical compositions at dissolution/deposition process, in which primary diatoms, opal CT, quartz, and clay minerals. In results, as a whole there is no change in the content of macroelements in sediments. Whereas, the contents Corg, Li, Cu, As, Mo, Ag, Cd, Sb, Ta, W, Tl, Pb, Bi strongly decreasing in altered sediments from contact zones with sills. [ABSTRACT FROM AUTHOR]

Interannual oceanic oscillations, climate change, and extreme events present a significant and complex challenge to management of pelagic fisheries. In recent years, anomalous oceanographic and atmospheric conditions have been reported across the northeast Pacific, yet research results concerning the biophysical mechanisms impacting specific organisms, populations, and fishery systems remain scarce. Here we discuss trends within the Gulf of California's jumbo squid fishery in the context of relevant climate drivers, ecological dynamics, and mesoscale oceanographic processes. Following the 2009–2010 El Niño event, the jig-based fishery collapsed as squid adopted the small size-at-maturity phenotype characteristic of this species in the equatorial portion of its range. Analysis of regional sea surface temperatures and heights indicates a pronounced shift in the oceanographic trajectory of the system between the 2009–2010 and 2015–2016 El Niño events, whereas in situ hydrographic sampling reveals coincident changes in subsurface temperature and salinity. With persistently tropical oceanic conditions present across critical portions of their pelagic habitat, jumbo squid have lost access to previously productive coastal foraging zones, which in turn has limited opportunities for fishery recovery. Given the importance of jumbo squid to the region's pelagic marine foodwebs, the long-term persistence of such phenomena could have significant implications for local fisheries, economies, and livelihoods. [ABSTRACT FROM AUTHOR]

The geochemical characteristics of nascent oceanic crust play a crucial role in unraveling the mantle dynamics of the initial opening of an ocean. The Gulf of California, located at the East Pacific margin, is one of the best examples of an active and obliquely rifted continental margin. However, the mantle source composition of the nascent oceanic crust in the central part of the Gulf has not yet been comprehensively investigated. Here, we report in situ major and trace element contents as well as B Sr isotope compositions for basaltic glass samples from off-axis sills drilled by the International Ocean Discovery Program (IODP) Expedition 385 at Sites U1547 and U1548 in the Guaymas Basin in the central part of the Gulf of California. These glassy samples represent tholeiites and predominantly show trace element patterns akin to those of enriched mid-ocean ridge basalts (E-MORBs) but with distinctive enrichments in Ba and K and depletions in Nb, Ta, and Ti. In addition, these samples have high B contents (3.07–3.67 ppm) with enriched Sr isotopes (87Sr/86Sr = 0.7032–0.7037) and heavy B isotopes (δ11B = −5.52‰ to 1.20‰). The mixing model based on B/Nb values and Sr B isotopes shows that the nascent oceanic crust in the Guaymas Basin might be generated through partial melting of a depleted MORB mantle (DMM) source metasomatized by melts from subducted slab materials referring to partially dehydrated sediment and altered oceanic crust components. The magmas in the Gulf of California show a systematic decline in their enrichment in fluid-mobile elements (Ba) and depletion in fluid-immobile elements (Nb, Ta, and Ti) from the northern (e.g., Isla San Luis volcanic center) to the central part (Guaymas Basin) and southward to the mouth (e.g., Alarcón Basin) of the Gulf. This suggests that the enriched (recycled) components in their mantle source were gradually extracted and exhausted during Gulf opening and oceanic crustal accretion that advanced in a northward direction. Our results indicate that the Guaymas Basin magmas were derived from a mantle that was fertilized by subduction components. The subduction signature corroborates the Gulf opening as a process that started in response to long-term oblique convergence at the eastern Pacific plate margin without any influence from a mantle plume. This evolution is different from nascent ocean basins that evolved from intraplate rifting, such as the Red Sea, leading to vastly different mantle source characteristics. • Basaltic glasses were sampled from the nascent oceanic crust of Guaymas Basin. • Mantle source beneath the Guaymas Basin was fertilized by subduction components. • The Gulf of California started opening in response to long-term oblique convergence. • Enriched mantle was purged gradually by melt extraction during the Gulf opening. [ABSTRACT FROM AUTHOR]

Guaymas Basin is a young marginal rift basin in the Gulf of California characterized by active seafloor spreading, rapid deposition of organic-rich sediments, and steep geothermal gradients where sedimentary organic material of photosynthetic origin turns into hydrocarbons - potential microbial substrates. After introducing the sources of hydrocarbons in the subsurface, this short overview discusses the phylogenetic diversity, temperature range and occurrence patterns of novel alkane-oxidizing bacteria and archaea in the hydrothermal sediments of Guaymas Basin. Many of these alkane-degrading specialists function as syntrophic consortia, where sulfate-reducing bacteria accept electron from methane- or alkane-oxidizing archaea. As a source of metagenomes, enrichments, and cultures of these methodologically and conceptually challenging microorganisms, Guaymas Basin stands out as an indispensable field site that has consistently catalyzed new insights into the microbiology of anaerobic hydrocarbon oxidation and assimilation. • Phylogenetic and physiological diversity of hydrocarbon-oxidizing bacteria and archaea. • Hydrothermal sediments of Guaymas Basin. • Recent microbial discoveries of hydrocarbon-oxidizing bacteria and archaea in Guaymas Basin. • Complementing extensive genome-based and physiological overviews on hydrocarbon-oxidizing bacteria and archaea. [ABSTRACT FROM AUTHOR]

Orange filamentous Beggiatoaceae form massive microbial mats on hydrothermal sediments in Guaymas Basin; these bacteria are considered to oxidize sulfide with nitrate and nitrite as electron acceptors. From a previously analyzed genome of an orange Beggiatoaceae filament, three candidate genes for enzymes with nitrite-reducing function – an orange octaheme cytochrome, a nirS nitrite reductase, and a nitrite/tetrathionate-reducing octaheme cytochrome – were cloned and expressed in Escherichia coli. The expressed and purified orange cytochrome showed reduced nitrite-reducing activity compared to the multifunctional native protein obtained from microbial mats. The nirS gene product showed in vitro but no in-gel nitrite-reducing activity; and the nitrite/tetrathionate-reducing octaheme cytochrome was capable of reducing both nitrite and tetrathionate in vitro. Phylogenetic analysis shows that the orange Beggiatoaceae nirS, in contrast to the other candidate nitrite reductases, does not form monophyletic lineages with its counterparts in other large sulfur-oxidizing bacteria, and most likely represents a recent acquisition by lateral gene transfer. The nitrite/tetrathionate-reducing enzyme of the orange Beggiatoaceae is related to nitrite- and tetrathionate reductases harbored predominantly by Gammaproteobacteria, including obligate endosymbionts of hydrothermal vent tubeworms. Thus, the orange Guaymas Basin Beggiatoaceae have a repertoire of at least three different functional enzymes for nitrite reduction. By demonstrating the unusual diversity of enzymes with a potential role in nitrite reduction, we show that bacteria in highly dynamic, sulfide-rich hydrothermal vent habitats adapt to these conditions that usually prohibit nitrate and nitrite reduction. In the case of the orange Guaymas Beggiatoaceae, classical denitrification appears to be replaced by different multifunctional enzymes for nitrite and tetrathionate reduction; the resulting ecophysiological flexibility provides a new key to the dominance of these Beggiatoaceae in hydrothermal hot spots.

The clay minerals in the Upper Pleistocene sediments from DSDP Hole 478 (a drilled depth of 464 m) located in the central area of the Guaymas Basin, Gulf of California, are studied. The composition of the clay minerals and their quantitative proportions in natural mixtures are determined by modeling of X-ray diffraction patterns for oriented specimens of finely dispersed particles. The terrigenous clay minerals are mainly dioctahedral (mixed-layer smectite–illite is dominant, the amount of illite is smaller) with admixtures of chlorite and kaolinite. They are distributed throughout the entire sedimentary section, except for thermally altered sediments from the contact zones with the top of the sills, where terrigenous dioctahedral clay minerals disappear, except for illite, and are substituted by trioctahedral mixed-layer chlorite–smectite and corrensite–chlorite; trioctahedral smectite also appears. Terrigenous illite is missing in the sediments at the base of the sedimentary cover exposed by the hole, on the top of the sill complex with a thickness of no less than 125 m. [ABSTRACT FROM AUTHOR]

A single specimen of the anemone Paraphelliactis pabista was recovered from the Southern Trough of Guaymas Basin during the deep-sea expedition Extreme 2008 conducted onboard the R/V Atlantis/DSRV-2 ALVIN. We studied the bioaccumulation capacity of heavy metals in various tissues of the anemone (oral disk–columella–pedal disk), and retention or adhesion of mineral particles in the epidermis, mesoglea, and gastrodermis. The digested tissues were analyzed for As, Ba, Co, Cu, Cr, Fe, Mn, Ni, Pb, Se, Sb, Sr, Ti, V, and Zn by inductively coupled plasma mass spectrometry. This analysis revealed the capacity of P. pabista for accumulating heavy metals. The predominant mineral particles identified in tissue samples was barite followed by Fe, aluminum-silicates, Sr, and with less presence Cr, Ti, and pyrite. Of the three body compartments analyzed of this anemone, the oral and pedal disks show a greater capacity of bioaccumulation of heavy metals than the columella. [ABSTRACT FROM AUTHOR]

Orange filamentous Beggiatoaceae form massive microbial mats on hydrothermal sediments in Guaymas Basin; these bacteria are considered to oxidize sulfide with nitrate and nitrite as electron acceptors. From a previously analyzed genome of an orange Beggiatoaceae filament, three candidate genes for enzymes with nitrite-reducing function – an orange octaheme cytochrome, a nirS nitrite reductase, and a nitrite/tetrathionate-reducing octaheme cytochrome – were cloned and expressed in Escherichia coli. The expressed and purified orange cytochrome showed reduced nitrite-reducing activity compared to the multifunctional native protein obtained from microbial mats. The nirS gene product showed in vitro but no in-gel nitrite-reducing activity; and the nitrite/tetrathionate-reducing octaheme cytochrome was capable of reducing both nitrite and tetrathionate in vitro. Phylogenetic analysis shows that the orange Beggiatoaceae nirS , in contrast to the other candidate nitrite reductases, does not form monophyletic lineages with its counterparts in other large sulfur-oxidizing bacteria, and most likely represents a recent acquisition by lateral gene transfer. The nitrite/tetrathionate-reducing enzyme of the orange Beggiatoaceae is related to nitrite- and tetrathionate reductases harbored predominantly by Gammaproteobacteria, including obligate endosymbionts of hydrothermal vent tubeworms. Thus, the orange Guaymas Basin Beggiatoaceae have a repertoire of at least three different functional enzymes for nitrite reduction. By demonstrating the unusual diversity of enzymes with a potential role in nitrite reduction, we show that bacteria in highly dynamic, sulfide-rich hydrothermal vent habitats adapt to these conditions that usually prohibit nitrate and nitrite reduction. In the case of the orange Guaymas Beggiatoaceae , classical denitrification appears to be replaced by different multifunctional enzymes for nitrite and tetrathionate reduction; the resulting ecophysiological flexibility provides a new key to the dominance of these Beggiatoaceae in hydrothermal hot spots. [ABSTRACT FROM AUTHOR]

Volatile fatty acids (VFAs) and alcohols are key intermediates of anaerobic carbon metabolism, yet their biogeochemical cycling remains poorly constrained in hydrothermal systems. We investigated the abundance, stable carbon isotopic composition, and metabolic cycling of VFAs and alcohols to elucidate their generation and utilization pathways in hydrothermally influenced sediments (4 °C to 90 °C) from the Guaymas Basin. Acetate (up to 229 μM) and methanol (up to 37 μM) were abundant in porewaters. The δ13C values of acetate varied between −35.6‰ and −18.1‰. Carbon isotopic signatures, thermodynamic predictions, and experimental incubations suggested biological sources such as fermentation and acetogenesis for acetate. Acetate and methanol were predominantly consumed by nonmethanogenic processes (e.g., sulfate reduction), as reflected in high oxidation rates versus low methanogenesis rates, and further evidenced through inhibition experiments with molybdate. These results reveal an important role for VFAs and alcohols as energy sources for diverse chemoheterotrophs in organic‐rich hydrothermally influenced sediments. Plain Language Summary: Hydrothermal systems are unique seafloor habitats that host abundant and diverse microbial communities, but questions remain regarding their energy strategy and metabolic activity. We found that low molecular weight organic compounds such as acetate and methanol were abundant in the hydrothermal sediments of Guaymas Basin. Multiple lines of evidence suggested that these substrates were produced largely via biological pathways. We further investigated the microbial metabolism of acetate and methanol and found that both compounds could be used as an energy source to support various microbial processes in the hydrothermal systems. Key Points: Volatile fatty acids (VFAs) and alcohols were abundant in the hydrothermal sediment of the Guaymas BasinIsotopic signatures, thermodynamics, and sediment incubations suggest that the source of VFAs is biological fermentation and acetogenesisHigh oxidation rates of acetate and methanol document utilization predominantly by nonmethanogenic chemoheterotrophs as an energy source [ABSTRACT FROM AUTHOR]

We investigated the impact of temperature on the microbial turnover of organic matter (OM) in a hydrothermal vent system in Guaymas Basin, by calculating microbial bio- and necromass turnover times based on the culture-independent D:L-amino acid model. Sediments were recovered from two stations near hydrothermal mounds (<74°C) and from one cold station (<9°C). Cell abundance at the two hydrothermal stations dropped from 108 to 106 cells cm-3 within ~5 m of sediment depth resulting in a 100-fold lower cell number at this depth than at the cold site where numbers remained constant at 108 cells cm-3 throughout the recovered sediment. There were strong indications that the drop in cell abundance was controlled by decreasing OM quality. The quality of the sedimentary OM was determined by the diagenetic indicators %TAAC (percentage of total organic carbon present as amino acid carbon), %TAAN (percentage of total nitrogen present as amino acid nitrogen), aspartic acid:b-alanine ratios, and glutamic acid:g-amino butyric acid ratios. All parameters indicated that the OM became progressively degraded with increasing sediment depth, and the OM in the hydrothermal sediment was more degraded than in the uniformly cold sediment. Nonetheless, the small community of microorganisms in the hydrothermal sediment demonstrated short turnover times. The modeled turnover times of microbial bio- and necromass in the hydrothermal sediments were notably faster (biomass: days to months; necromass: up to a few hundred years) than in the cold sediments (biomass: tens of years; necromass: thousands of years), suggesting that temperature has a significant influence on the microbial turnover rates. We suggest that short biomass turnover times are necessary for maintance of essential cell funtions and to overcome potential damage caused by the increased temperature. The reduced OM quality at the hyrothemal sites might thus only allow for a small population size of microorganisms. [ABSTRACT FROM AUTHOR]