Stephen T. Sweet

Bacteria and Archaea Physically Associated with Gulf of Mexico Gas Hydrates

ABSTRACT Although there is significant interest in the potential interactions of microbes with gas hydrate, no direct physical association between them has been demonstrated. We examined several intact samples of naturally occurring gas hydrate from the Gulf of Mexico for evidence of microbes. All samples were collected from anaerobic hemipelagic mud within the gas hydrate stability zone, at water depths in the ca. 540- to 2,000-m range. The δ13C of hydrate-bound methane varied from −45.1‰ Peedee belemnite (PDB) to −74.7‰ PDB, reflecting different gas origins. Stable isotope composition data indicated microbial consumption of methane or propane in some of the samples. Evidence of the presence of microbes was initially determined by 4,6-diamidino 2-phenylindole dihydrochloride (DAPI) total direct counts of hydrate-associated sediments (mean = 1.5 × 109 cells g−1) and gas hydrate (mean = 1.0 × 106 cells ml−1). Small-subunit rRNA phylogenetic characterization was performed to assess the composition of the microbial community in one gas hydrate sample (AT425) that had no detectable associated sediment and showed evidence of microbial methane consumption. Bacteria were moderately diverse within AT425 and were dominated by gene sequences related to several groups of Proteobacteria, as well asActinobacteria and low-G + C Firmicutes. In contrast, there was low diversity of Archaea, nearly all of which were related to methanogenic Archaea, with the majority specifically related to Methanosaeta spp. The results of this study suggest that there is a direct association between microbes and gas hydrate, a finding that may have significance for hydrocarbon flux into the Gulf of Mexico and for life in extreme environments.

Bacterial methane oxidation in sea-floor gas hydrate: Significance to life in extreme environments

Samples of thermogenic hydrocarbon gases, from vents and gas hydrate mounds within a sea-floor chemosynthetic community on the Gulf of Mexico continental slope at about 540 m depth, were collected by research submersible. The study area is characterized by low water temperature (mean = 7 C), high pressure (about 5,400 kPa), and abundant structure II gas hydrate. Bacterial oxidation of hydrate-bound methane (CH{sub 4}) is indicated by three isotopic properties of gas hydrate samples. Relative to the vent gas from which the gas hydrate formed, (1) methane-bound methane is enriched in {sup 13}C by as much as 3.8% PDB (Peedee belemnite), (2) hydrate-bound methane is enriched in deuterium (D) by as much as 37% SMOW (standard mean ocean water), and (3) hydrate-bound carbon dioxide (CO{sub 2}) is depleted in {sup 13}C by as much as 22.4% PDB. Hydrate-associated authigenic carbonate rock is also depleted in {sup 13}C. Bacterial oxidation of methane is a driving force in chemosynthetic communities, and in the concomitant precipitation of authigenic carbonate rock that modifies sea-floor geology. Bacterial oxidation of hydrate-bound methane expands the potential boundaries of life in extreme environments.

Organic geochemistry of sediments from chemosynthetic communities, Gulf of Mexico slope

We used a research submersible to obtain 33 sediment samples from chemosynthetic communities at 541–650 m water depths in the Green Canyon (GC) area of the Gulf of Mexico slope. Sediment samples from beneath an isolated mat of H2S-oxidizing bacteria at GC 234 contain oil (mean = 5650 ppm) and C1–C5 hydrocarbons (mean = 12,979 ppm) that are altered by bacterial oxidation. Control cores away from the mat contain lower concentrations of oil (mean = 2966 ppm) and C1–C5 hydrocarbons (mean = 83.6 ppm). Bacterial oxidation of hydrocarbons depletes O2 in sediments and triggers bacterial sulfate reduction to produce the H2S required by the mats. Sediment samples from GC 185 (Bush Hill) contain high concentrations of oil (mean = 24,775 ppm) and C1–C5 hydrocarbons (mean = 11,037 ppm) that are altered by bacterial oxidation. Tube worm communities requiring H2S occur at GC 185 where the sea floor has been greatly modified since the Pleistocene by accumulation of oil, thermogenic gas hydrates, and authigenic carbonate rock. Venting to the water column is suppressed by this sea-floor modification, enhancing bacterial activity in sediments. Sediments from an area with vesicomyid clams (GC 272) contain lower concentrations of oil altered by bacterial oxidation (mean = 1716 ppm) but C1–C5 concentrations are high (mean = 28,766 ppm). In contrast to other sampling areas, a sediment associated with the methanotrophic Seep Mytilid I (GC 233) is characterized by low concentration of oil (82 ppm) but biogenic methane (C1) is present (8829 ppm).

Distribution of phytoplankton pigments in the North Pacific Ocean in relation to physical and optical variability

Abstract To investigate phytoplankton distributions in the North Pacific Ocean, samples of suspended particulate material were collected from the upper 300 m during two cruises in 1985 for detailed analysis of algal pigments by high-performance liquid chromatography (HPLC). Transpacific Leg I along 24°N in April and May, crossed three prominent hydrographic features: the California Coastal Current, the North Pacific Central Gyre and Kuroshio Current. Transpacific Leg II, along 47°N in August and September, crossed the Kuroshio extension, the Subarctic Gyre and the North Pacific Current. Individual pigments were partitioned vertically in the water column, showing distinct spatial patterns across the Pacific Ocean which reflected the large-scale circulation. Vertical distributions of phytoplankton pigments displayed consistent patterns over spatial scales of thousands of kilometers. In near-surface, nitrate-rich waters, fucoxanthin was the dominant carotenoid. In nitrate-poor surface waters, zeaxanthin was the dominant carotenoid at the surface, and 19′-hexanoyloxyfucoxanthin and chlorophyll b concentrations were elevated near the base of the euphotic zone. Phacopigment concentrations greater than a few tens of nanograms per liter were never encountered. Based on Principal Component Analysis, station clustered into three general pigment categories which followed specific hydrographic characteristics of oligotrophic, highly productive and transitional regions.

Thermogenic vent gas and gas hydrate in the Gulf of Mexico slope: Is gas hydrate decomposition significant?

Samples of vent gas and gas hydrate on the Gulf of Mexico slope were collected by research submersible (∼540 m water depth) and by piston coring (∼1060–1070 m water depth). Although gas hydrate that crops out is transiently unstable, the larger volume of structure II gas hydrate in the gulf is stable or increasing in volume because gas from the subsurface petroleum system is venting prolifically within the gas hydrate stability zone. Vent gas from gas hydrate shows no meaningful molecular evidence of gas hydrate decomposition. Gas hydrate fabrics, mainly vein fillings, are typical of ongoing crystallization. Once crystallized, most hydrocarbons are protected from bacteria within the crystal lattice of gas hydrate. A leaky petroleum system is proposed to be the main source of thermogenic greenhouse gases in the central gulf. Stable gas hydrate sequesters large volumes of greenhouse gases, suggesting that gas hydrate may not be a significant factor in models of climate change at present.

Distribution and characteristics of PAHs in sediments from the Mediterranean coastal environment of Egypt

To assess the contamination of polycyclic aromatic hydrocarbons (PAHs) in the Mediterranean coastal environment of Egypt, 26 sediment samples from the coastline, harbours, estuaries and coastal lakes were collected and analyzed. The sediment PAH concentrations of thirty-nine 2-6 ring PAHs ranged from 13.5 to 22,600 ng/g. PAH profiles varied according to the nature of the site and its proximity to sources. Industrialized and urbanized region showed high level of PAHs contamination. In general, the contamination levels of PAHs were similar to those observed in contaminated and slightly contaminated sediments of the Mediterranean Sea. Molecular indices based on ratios of selected PAH concentrations were used to differentiate PAHs from pyrogenic and petrogenic and mixed origins. Good correlations were observed between the petrogenic index, MP/P, A-PAHs/P-PAHs and HMW/LMW. Finally, PAH levels in sediments were compared with Sediments Quality Guidelines (ERM-ERL) for evaluation of probable toxic effects on organism.

Gas venting and subsurface charge in the Green Canyon area, Gulf of Mexico continental slope: evidence of a deep bacterial methane source?

Abstract Questions as to the role of modern carbon in methanogenesis and the maximum depth of methane sources in the Gulf of Mexico continental slope remain unanswered. A research submersible was used to sample mixed bacterial and thermal gas ( δ 13 C of methane=−62.8‰, δD =−176‰) venting to the water column from the Gulf slope in Green Canyon (GC) 286. The Δ 14 C value of the methane (−998‰) is consistent with fossil carbon. Another gas vent on GC 185 is 100% methane ( δ 13 C =−62.9‰, δD =−155‰) and may be from a bacterial source. The Δ 14 C (−997‰) of this bacterial methane is also consistent with fossil carbon. Fossil bacterial methane and thermal hydrocarbons are present in Pliocene to Pleistocene reservoirs (∼3509–4184 m) of Genesis Field (GC 205, 161, 160). Oil in these reservoirs is biodegraded but gas is not, suggesting that gas charge to reservoirs continues presently at 3–4 km depth. Mixed thermal and bacterial methane may charge the deep reservoirs, and fossil methane from depth may ultimately vent on the sea floor at GC 286 and GC 185. Results of this study of Green Canyon suggest that bacterial methane in gas vents and in reservoirs is from deep fossil sources.

Temporal and spatial patterns of anthropogenic disturbance at McMurdo Station, Antarctica

Human visitations to Antarctica have increased in recent decades, raising concerns about preserving the continents environmental quality. To understand the spatial and temporal patterns of anthropogenic disturbances at the largest scientific station in Antarctica, McMurdo Station, a long-term monitoring program has been implemented. Results from the first nine years (1999?2007) of monitoring are reported. Most physical disturbance of land surfaces occurred prior to 1970 during initial establishment of the station. Hydrocarbons from fuel and anthropogenic metals occur in patches of tens to hundreds of square meters in areas of fuel usage and storage. Most soil contaminant concentrations are not expected to elicit biological responses. Past disposal practices have contaminated marine sediments with polychlorinated biphenyls (PCBs), petroleum hydrocarbons, and metals in close proximity to the station that often exceed concentrations expected to elicit biological responses. Chemical contamination and organic enrichment reduced marine benthic ecological integrity within a few hundred meters offshore of the station. Contaminants were detected in marine benthic organisms confirming bioavailability and uptake. PCBs in sediments are similar to suspected source materials, indicating minimal microbial degradation decades after release. Anthropogenic disturbance of the marine environment is likely to persist for decades. A number of monitoring design elements, indicators and methodologies used in temperate climates were effective and provide guidance for monitoring programs elsewhere in Antarctica.

Assessment of persistent organochlorine pollutants in sediments from Lake Manzala, Egypt

Organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) were determined in surface and core sediment samples collected from Manzala Lake, the largest of the Egyptian coastal lakes. Total concentrations of OCPs and ∑7 PCBs (ICES) in sediments ranged from 0.63 to 31.31 ng/g and 0.26 to 31.27 ng/g, respectively. Geographical distribution indicates that levels of contaminants were significantly higher in areas which are mainly influenced by municipal discharge, indicating significant sources of these compounds in urbanised areas. The composition of DDT and its metabolites suggest old input of DDT. The levels of contaminants in Manzala Lake were similar or lower than those observed in comparable areas worldwide. The profiles of ∑OCPs and ∑PCBs in a core from a site heavily impacted by sewage discharge have highest concentrations in the surface core section indicating recent inputs. Assessment of ecotoxicological risk indicated that sediments in two sites were likely to pose potential biological adverse impact.

Hydrocarbon contamination on the Antarctic Peninsula. III: The Bahia-Paraiso―Two years after the spill

Two years after the release of 600 000 l of diesel fuel arctic into Arthur Harbor, little spill-related contamination can be detected in intertidal limpets (Nacella concinna) and subtidal sediments. Periodic releases of small amounts of material from the ship oil nearby islands, in particular the intertidal areas of Christine, Limitrophe and Humble Islands. Subtidal sediment contamination is primarily due to other local inputs such as ship, boating and station activities. Beaches were unusually contaminated after 2 yr, but quiescent weather conditions, occasional releases from the wreck, and prevailing currents may concentrate hydrocarbon contamination in relatively low energy areas. Intertidal limpets (N. concinna) collected along these beaches were also contaminated. The volatility of the fluid, the amount spilled, and the dynamic weather and current conditions in Arthur Harbor tended to minimize long-term contamination of the area.