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This contribution presents the first study focused on the analysis of microbial mats in the Los Molles Formation (Toarcian – Early Callovian), Neuquén Basin, Argentina. This unit mainly represents offshore‐to‐shelf environments affected by storms and density currents. The Los Molles Formation is one of the oldest source rocks in the Neuquén Basin and constitutes an unconventional shale gas reservoir of great economic importance. The aim of this work was to identify the microbial activity from the description and interpretation of microbially induced sedimentary structures (MISS), to determine the paleoenvironmental and paleoecological conditions under which they formed, and to establish a possible relationship between these structures and the trace fossil Trichichnus. Samples from the levels with MISS were analyzed and described from macroscopic and binocular observations, petrographic microscope thin sections, and SEM samples with EDS analyses. The results showed several levels of microbial mats presenting diverse MISS, including biolaminations and Kinneyia‐like wrinkles structures that were described at the macroscopic level. In thin sections, biolaminations, filament‐like microstructures with different degrees of development, oriented grains and pyrite were observed. SEM images and EDS analyses showed different types of filaments, coccoids and EPS with high concentrations of carbon. These results revealed that the studied levels fulfill the established biogenicity criteria, guaranteeing that they have a bacterial origin. The abundance of the trace fossil Trichichnus sp. throughout the section and the proximity to some Kinneyia‐like wrinkle structures levels suggests that the same organisms may have generated them. Furthermore, they revealed that the Los Molles Formation, at the time of its deposition, experienced paleoecological and paleoenvironmental conditions appropriate for the establishment and development of microbial mats. The extensive levels of microbial mats in the study area suggest that they may have been a source of organic matter for the generation of hydrocarbons from the Los Molles Formation. [ABSTRACT FROM AUTHOR]

Discovery of Thiomargarita magnifica – an exceptionally large giant sulfur bacterium – urges us to pay additional attention to the giant sulfur bacteria and to revisit our recent bioinformatic finding of lipoxygenases in the representatives of the genus Beggiatoa. These close relatives of Thiomargarita magnifica meet the similar size requirements by forming multicellular structures. We hypothesize that their lipoxygenases are a part of the oxylipin signaling system that provides high level of cell-to-cell signaling complexity which, in turn, enables them to reach large sizes. [ABSTRACT FROM AUTHOR]

Sedimentary pyrite records are essential for reconstructing paleoenvironmental conditions, but these records may be affected by seasonal fluctuations in oxygen concentration and temperature, which can impact bioturbation, sulfide fluxes, and distributions of sulfide oxidizing microbes (SOMs). To investigate how seasonal oxygen stress influences surficial (<2 cm) pyrite formation, we measured time‐series concentrations and sulfur isotope (δ34S) compositions of pyrite sulfur along with those of potential precursor compounds at a bioturbated shoal site and an oxygen‐deficient channel site in Chesapeake Bay. We also measured radioisotope depth profiles to estimate sedimentation rates and bioturbation intensities. Results show that net pyrite precipitation was restricted to summer and early autumn at both sites. Pyrite concentration was higher and apparently more responsive to precursor compound concentration at the mildly bioturbated site than at the non‐bioturbated site. This disparity may be driven by differences in the dominant SOM communities between the two sites. Despite this, the sites' similar pyrite δ34S values imply that changes in SOM communities have limited effects on surficial pyrite δ34S values here. However, we found that pyrite δ34S values are consistently and anomalously lower than coeval precursor compounds at both sites. A steady‐state model demonstrates that equilibrium position‐specific isotope fractionation (PSIF) effects in the S8‐polysulfide pool can create a 4.3–7.3‰ gap between δ34S values of pyrite and zero‐valent sulfur. This study suggests that SOM communities may have distinct effects on pyrite accumulation in seasonally dynamic systems, and that PSIF in the polysulfide pool may leave an imprint in pyrite isotope records. Plain Language Summary: Formation of the iron sulfide mineral pyrite in sediments has contributed to long‐term oxygen accumulation on Earth, and pyrite's sulfur isotope composition in ancient sediments offers a lens onto low‐oxygen intervals in Earth's past. However, seasonal environmental changes in low‐oxygen waters can lead to complex relationships between animal burrowing activity, microbial processing of sulfur compounds, and the formation of pyrite. To understand controls on pyrite accumulation and sulfur isotope composition under seasonally dynamic, low‐oxygen waters, we analyzed shallow sediments at two Chesapeake Bay sites that are subject to these conditions. We found that a shallow site that was burrowed by small animals consistently contained more pyrite than a deeper, non‐burrowed site, even though the deeper site usually contained more pyrite‐forming compounds. This may result from distinct chemical effects of the different sulfur‐cycling microbes that dominate each site, implying that changes in the sulfur‐cycling microbial community composition might influence rates of shallow pyrite formation. We also propose that rapid redistribution of sulfur isotopes in polysulfides, a reactive pool of pyrite‐forming sulfur compounds, may help to explain the unusual sulfur isotope composition of pyrite compared to its precursor compounds. This rapid process may leave an imprint in the geological pyrite record. Key Points: Two Chesapeake Bay sites feature net surficial pyrite precipitation in summer to early autumn and lose similar fractions of pyrite in winterHigher pyrite precipitation rates at burrowed site may be driven by contrasting effects of different sulfide oxidizing microbial communitiesPosition‐specific fractionation effects in polysulfides may generate δ34S depletions of 4.3–7.3 per mil in pyrites relative to coexisting S8 [ABSTRACT FROM AUTHOR]

Introduction: Coastal ecosystems worldwide are under the influence of local, regional and global stressors, such as pollution, eutrophication and climate change. Golfo Dulce is a relatively pristine and accessible deep tropical ecosystem that provides opportunities for comparative and collaborative research. Objective: To summarize published reports on past research conducted in this ecosystem, identify topics for further study, and suggest new research issues. Methods: A search was made on the web for reports based on research conducted in Golfo Dulce and published in scientific journals. Reports focusing on environmental parameters and on the biota were included. Results: A total of 123 studies that include data from Golfo Dulce are cited. The four topics more frequently addressed were reports based on the results of the R/V Victor Hensen expedition (1993-1994) and follow-up work on microbiology, studies on water parameters, research on vertebrates, and zooplankton studies. The reports focusing on vertical profiles of oxygen and temperature are discussed in detail, followed by those on the biota. Conclusions: Golfo Dulce has low oxygen concentrations below 50 m and is frequently anoxic at the 200 m deep basin with occasional formation of H2S. However, the ecosystem contains a relatively high diversity of identified organisms, from bacteria to whales. Of particular relevance for future studies are multidisciplinary surveys aiming at obtaining data on primary productivity, the diversity and biomass of the main groups of planktonic, demersal and benthic organisms, and the frequency and magnitude of the influx of deep offshore waters over the sill into the basin. These data, as well as the information gathered in the past, are essential for updating the trophic model developed more than 25 years ago and in support of new predictive models on the functioning of the ecosystem. [ABSTRACT FROM AUTHOR]

Precambrian filamentous microfossils are common and diverse. Nevertheless, their taxonomic assignment can be difficult owing to their overall simple shapes typically lacking in diagnostic features. Here, we report in situ communities of well-preserved, large filamentous impressions from the Ediacaran Itajaí Basin (ca 563 Ma) of Brazil. The filaments are uniserial (unbranched) and can reach up to 200 µm in width and up to 44 mm in length. They occur as both densely packed or sparsely populated surfaces, and typically show a consistent orientation. Although simple in shape, their preferred orientation suggests they were tethered to the seafloor, and their overall flexibility (e.g. bent, folded and twisted) supports a biological (rather than sedimentary) affinity. Biometric comparisons with modern filamentous groups further support their biological affinity, suggesting links with either large sulfide-oxidizing bacteria (SOB) or eukaryotes. Other morphological and palaeoecological characteristics further corroborates their similarities with modern large filamentous SOB. Their widespread occurrence and association with complex Ediacaran macrobiota (e.g. frondose organisms, Palaeopascichnus) suggest that they probably played an important role in the ecological dynamics of these early benthic communities by providing firm substrates for metazoans to inhabit. It is further hypothesized that the dynamic redox condition in the latest Ediacaran, with the noncontinuous rise in oxygen concentration and periods of hypoxia, may have created ideal conditions for SOB to thrive. [ABSTRACT FROM AUTHOR]

Cold seeps are widespread in the deep sea and, like other chemosynthesis-based ecosystems, often host high faunal biomass. Temporal changes at seeps have been inferred by comparing communities at different successional stages; nonetheless, temporal studies in seep ecosystems are rare. Using data collected as part of a benthic observatory, we characterized intra-annual microbial and faunal dynamics in a microbial mat habitat on the Håkon Mosby mud volcano (1256 m depth; Barents Sea), and evaluated the effects of a mud eruption on the biota. Video sequences recorded twice daily for 4.5 months with an autonomous imaging module were analyzed to quantify changes in microbial mat cover and megafaunal density and behavior. In addition, time series data for pressure, bottom currents, and seabed temperature were analyzed to characterize environmental changes. Of the five taxa observed on the video footage, the zoarcid Lycodes squamiventer, the pycnogonid Nymphon macronyx, and the skate Amblyraja hyperborea were the most common. Zoarcids made frequent incursions on microbial mats, but were more often documented on the adjacent sediments. The cyclic behavior of common trends identified between microbial mat cover, zoarcid densities, and abiotic variables using dynamic factor analysis suggested that seep ecosystem dynamics at the Håkon Mosby mud volcano may be modulated by tides and the activity of the volcano. Moreover, the eruption that occurred about 2 months after the start of the study had a significant impact on faunal densities, microbial activity, and environmental variables. The eruption was preceded by a steady increase in seabed temperatures, density of small holes (indicative of gas emissions), and microbial mat cover. It was followed by 3- and 5-fold increase in zoarcid and pycnogonid densities, respectively. To our knowledge, this study is the first to analyze intra-annual temporal data in a cold-seep ecosystem and to present data on the effects of a mud eruption on seep microbial and faunal dynamics. Overall, our results provide the first insight into ecosystem dynamics at a high-latitude cold seep and highlight the need to collect high-resolution temporal data to evaluate seep communities’ response to a warming Arctic and their potential role in mitigating methane emissions.

Mammal bones sustain rich chemoautotrophic microbial communities that are consumed by a range of marine invertebrates, with bacteria playing a fundamental role making the organic matter retained in the bones available to other organisms. Our major aim here is to characterize the phylogenetic diversity of bacteria associated with Mediterranean shallow-water bones (whale, pig, and cow) examined at two different times after colonization, and compare it to bacterial communities developing on whale bones in the Southern Ocean. We sequenced 16S amplicons for approximately 50 clones from each of the 5 bone samples studied here, resulting in 215 OTUs. Our preliminary microbial community analysis resulted into two groups. Cluster 1 with cow and whale bones from the Mediterranean (3 months) and whale bones from the Southern Ocean (1 year), characterized by abundant Epsilon-proteobacteria, usually the first colonizers in anaerobic environments. Cluster 2 with Mediterranean cow (9 months) and pig bones (12 months), characterized by abundant Thiotrichales. Among the Thiotrichales, two different species of Beggiatoa spp. appeared in two different bones in the Mediterranean samples, indicating that Beggiatoa spp. can occur in the same area. Our work confirms that microbial mats associated with mammal bones in the shallow waters of the Mediterranean and the Southern Ocean are highly diverse with a predominance of Gamma- and Epsilon-proteobacteria. [ABSTRACT FROM AUTHOR]

Dark carbon fixation (DCF) by chemoautotrophic microorganisms can sustain food webs in the seafloor by local production of organic matter independent of photosynthesis. The process has received considerable attention in deep sea systems, such as hydrothermal vents, but the regulation, depth distribution, and global importance of coastal sedimentary DCF have not been systematically investigated. Here we surveyed eight coastal sediments by means of stable isotope probing (13C‐DIC) combined with bacterial biomarkers (phospholipid‐derived fatty acids) and compiled additional rates from literature into a global database. DCF rates in coastal sediments range from 0.07 to 36.30 mmol C m−2 day−1, and there is a linear relation between DCF and water depth. The CO2 fixation ratio (DCF/CO2 respired) also shows a trend with water depth, decreasing from 0.09 in nearshore environments to 0.04 in continental shelf sediments. Five types of depth distributions of chemoautotrophic activity are identified based on the mode of pore water transport (advective, bioturbated, and diffusive) and the dominant pathway of microbial sulfur oxidation. Extrapolated to the global coastal ocean, we estimate a DCF rate of 0.04 to 0.06 Pg C year−1, which is less than previous estimates based on indirect measurements (0.15 Pg C year−1), but remains substantially higher than the global DCF rate at deep sea hydrothermal vents (0.001–0.002 Pg C year−1). Key Points: Coastal dark carbon fixation rates are highest in diffusion‐driven intertidal areas and lowest in permeable continental shelf sedimentsPore water transport and microbial sulfur oxidation mechanisms determine five depth distribution models of dark carbon fixation in sedimentsThe renewal of labile organic matter by dark carbon fixation is important in coastal carbon budgets [ABSTRACT FROM AUTHOR]

ABSTRACT Here we report on a new nanoscale secondary ion mass spectrometry (nanoSIMS) approach based on enzyme-mediated oxygen isotope exchange, which combines the visualization of general metabolic activity in the cytoplasm with insights into the activity of enzymes related to polyphosphate (polyP) inclusions. The polyP-accumulating strain of the large sulfur bacterium Beggiatoa was used as a model organism. Beggiatoa cultures were grown under oxic and anoxic conditions when exposed to either low- or high-sulfide conditions, which are known to influence polyP metabolism in this strain. Subsequent incubation with 18O-labeled water led to high 18O enrichments above the natural background in the cytoplasm and polyP granules derived from enzymatically mediated oxygen isotope exchange. The relative importance of polyP under the different sulfide regimes became evident by an apparent continued metabolic activity at polyP inclusions under stressfully high sulfide concentrations, in contrast to a decreased general metabolic activity in the cytoplasm. This finding confirms the role of polyP as a critical component in bacterial stress response and maintenance of a survival metabolism. IMPORTANCE Microbial organisms exert a large influence on the environment as they directly affect the turnover of essential elements. This is particularly true for polyphosphate-accumulating large sulfur bacteria, which can either accumulate phosphate as polyphosphate or degrade it and release phosphate into the environment, depending on environmental conditions. This study presents a new approach to simultaneously visualize general metabolic activity and enzymatic activity at polyphosphate granules by incubation with 18O-labeled water as the only stable isotope tracer. For this purpose, the well-studied Beggiatoa sp. strain 35Flor was used as a model organism and was exposed to different stress regimes. General metabolic activity was strongly impaired during high-stress regimes. In contrast, intense intracellular polyP cycling was not restricted to favorable or stressful conditions, highlighting the importance of polyP for general cell physiology, especially during hostile conditions. The nanoSIMS approach adds a new tool to study microorganisms involved in phosphorus cycling in the environment together with the identification of general metabolic activity.

Microbial eukaryotes have important roles in marine food webs, but their diversity and activities in hydrothermal vent ecosystems are poorly characterized. In this study, we analyzed microbial eukaryotic communities associated with bacterial (Beggiatoa) mats in the 2,000 m deep‐sea Guaymas Basin hydrothermal vent system using 18S rRNA gene high‐throughput sequencing of the V4 region. We detected 6,954 distinct Operational Taxonomic Units (OTUs) across various mat systems. Of the sequences that aligned with known protistan phylotypes, most were affiliated with alveolates (especially dinoflagellates and ciliates) and cercozoans. OTU richness and community structure differed among sediment habitats (e.g. different mat types and cold sediments away from mats). Additionally, full‐length 18S rRNA genes amplified and cloned from single cells revealed the identities of some of the most commonly encountered, active ciliates in this hydrothermal vent ecosystem. Observations and experiments were also conducted to demonstrate that ciliates were trophically active and ingesting fluorescent bacteria or Beggiatoa trichomes. Our work suggests that the active and diverse protistan community at the Guaymas Basin hydrothermal vent ecosystem likely consumes substantial amounts of bacterial biomass, and that the different habitats, often defined by distances of just a few 10s of cm, select for particular assemblages and levels of diversity. [ABSTRACT FROM AUTHOR]

The metabolic pathways of one-carbon compounds utilized by colorless sulfur bacterium Beggiatoa leptomitoformis D-402 were revealed based on comprehensive analysis of its genomic organization, together with physiological, biochemical and molecular biological approaches. Strain D-402 was capable of aerobic methylotrophic growth with methanol as a sole source of carbon and energy and was not capable of methanotrophic growth because of the absence of genes of methane monooxygenases. It was established that methanol can be oxidized to CO2 in three consecutive stages. On the first stage methanol was oxidized to formaldehyde by the two PQQ (pyrroloquinolinequinone)-dependent methanol dehydrogenases (MDH): XoxF and Mdh2. Formaldehyde was further oxidized to formate via the tetrahydromethanopterin (H4MPT) pathway. And on the third stage formate was converted to CO2 by NAD+-dependent formate dehydrogenase Fdh2. Finally, it was established that endogenous CO2, formed as a result of methanol oxidation, was subsequently assimilated for anabolism through the Calvin-Benson-Bassham cycle. The similar way of one-carbon compounds utilization also exists in representatives of another freshwater Beggiatoa species—B. alba. [ABSTRACT FROM AUTHOR]

ABSTRACT The largest known bacteria, Thiomargarita spp., have yet to be isolated in pure culture, but their large size allows for individual cells to be monitored in time course experiments or to be individually sorted for omics-based investigations. Here we investigated the metabolism of individual cells of Thiomargarita spp. by using a novel application of a tetrazolium-based dye that measures oxidoreductase activity. When coupled with microscopy, staining of the cells with a tetrazolium-formazan dye allows metabolic responses in Thiomargarita spp. to be to be tracked in the absence of observable cell division. Additionally, the metabolic activity of Thiomargarita sp. cells can be differentiated from the metabolism of other microbes in specimens that contain adherent bacteria. The results of our redox dye-based assay suggest that Thiomargarita is the most metabolically versatile under anoxic conditions, where it appears to express cellular oxidoreductase activity in response to the electron donors succinate, acetate, citrate, formate, thiosulfate, H2, and H2S. Under hypoxic conditions, formazan staining results suggest the metabolism of succinate and likely acetate, citrate, and H2S. Cells incubated under oxic conditions showed the weakest formazan staining response, and then only to H2S, citrate, and perhaps succinate. These results provide experimental validation of recent genomic studies of Candidatus Thiomargarita nelsonii that suggest metabolic plasticity and mixotrophic metabolism. The cellular oxidoreductase response of bacteria attached to the exterior of Thiomargarita also supports the possibility of trophic interactions between these largest of known bacteria and attached epibionts. IMPORTANCE The metabolic potential of many microorganisms that cannot be grown in the laboratory is known only from genomic data. Genomes of Thiomargarita spp. suggest that these largest of known bacteria are mixotrophs, combining lithotrophic metabolism with organic carbon degradation. Our use of a redox-sensitive tetrazolium dye to query the metabolism of these bacteria provides an independent line of evidence that corroborates the apparent metabolic plasticity of Thiomargarita observed in recently produced genomes. Finding new cultivation-independent means of testing genomic results is critical to testing genome-derived hypotheses on the metabolic potentials of uncultivated microorganisms.

We investigated the benthic foraminiferal faunal and stable carbon and oxygen isotopic composition of a 15-cm push core (NA075-092b) obtained on a Telepresence-Enabled cruise to the Southeast Seep on Kimki Ridge offshore southern California. The seep core was taken at a depth of 973 m in the vicinity of a Beggiatoa bacterial mat and vesicomyid clams ( Calyptogena ) and compared to previously published data of living assemblages from ~ 714 m, four reference cores obtained at ~ 1030 m, and another one at 739 m. All of the reference sites are also from the Inner Continental Borderland but with no evidence of methane seepage. No endemic species were found at the seep site and most of the taxa recovered there have been reported previously from other seep or low oxygen environments. Q- and R-mode cluster analyses clearly illustrated differences in the faunal assemblages of the seep and non-seep sites. The living assemblage at Southeast Seep was characterized by abundant Takayanagia delicata, Cassidulina translucens, and Spiroplectammina biformis , whereas the non-seep San Pedro Basin reference assemblage was comprised primarily of Chilostomella oolina and Globobulimina pacifica . Density and species richness were lower at the seep site compared to the non-seep site, reflecting the harsher living conditions there. The dead assemblage at the seep site was dominated by Gyroidina turgida compared to Cassidulina translucens at the ~ 1030 m non-seep site and Cassidulina translucens, Pseudoparrella pacifica, and Takayanagia delicata at the 739 m non-seep site. Density was three times lower at Southeast Seep than at the non-seep sites of comparable water depth but species richness was ~ 30% higher. Stable carbon isotopic values were considerably depleted in the seep samples compared to the non-seep samples, with a progression from lightest to heaviest average δ 13 C values evident at the seep site reflecting microhabitat preference and vital effect: the deep infaunal species of Globobulimina , the shallow infaunal species Uvigerina peregrina , the epifaunal species Cibicidoides wuellerstorfi , and the shallow infaunal but aragonite-shelled species Hoeglundina elegans . The δ 13 C values downcore among each benthic species indicates ongoing fluid seepage through at least the last 3800 cal yr B.P. at Southeast Seep. Besides the continual local seepage, evidence from δ 13 C values of planktic foraminifera in the seep core suggest two pulses of methane (at 3000 and 3700 cal yr B.P.) were released that were large enough to influence much of the water column. Paired benthic and planktic foraminiferal stable oxygen isotope records provide evidence that there were no paleoenvironmental changes such as increased bottom-water temperature or changes in oxygen isotopic composition of bottom and pore waters during this 3800-year record to induce the methane releases. Instead, Southeast Seep appears to be the result of local faulting providing pathways for fluid to flow to the seafloor at a fault stepover or transpressional bend in the regional strike-slip system. [ABSTRACT FROM AUTHOR]

Little is known about large sulfur bacteria (LSB) that inhabit sulfidic groundwater seeps in large lakes. To examine how geochemically relevant microbial metabolisms are partitioned among community members, we conducted metagenomic analysis of a chemosynthetic microbial mat in the Isolated Sinkhole, which is in a deep, aphotic environment of Lake Huron. For comparison, we also analyzed a white mat in an artesian fountain that is fed by groundwater similar to Isolated Sinkhole, but that sits in shallow water and is exposed to sunlight. De novo assembly and binning of metagenomic data from these two communities yielded near complete genomes and revealed representatives of two families of LSB. The Isolated Sinkhole community was dominated by novel members of the Beggiatoaceae that are phylogenetically intermediate between known freshwater and marine groups. Several of these Beggiatoaceae had 16S rRNA genes that contained introns previously observed only in marine taxa. The Alpena fountain was dominated by populations closely related to Thiothrix lacustris and an SM1 euryarchaeon known to live symbiotically with Thiothrix spp. The SM1 genomic bin contained evidence of H2-based lithoautotrophy. Genomic bins of both the Thiothrix and Beggiatoaceae contained genes for sulfur oxidation via the rDsr pathway, H2 oxidation via Ni-Fe hydrogenases, and the use of O2 and nitrate as electron acceptors. Mats at both sites also contained Deltaproteobacteria with genes for dissimilatory sulfate reduction (sat, apr, and dsr) and hydrogen oxidation (Ni-Fe hydrogenases). Overall, the microbial mats at the two sites held low-diversity microbial communities, displayed evidence of coupled sulfur cycling, and did not differ largely in their metabolic potentials, despite the environmental differences. These results show that groundwater-fed communities in an artesian fountain and in submerged sinkholes of Lake Huron are a rich source of novel LSB, associated heterotrophic and sulfate-reducing bacteria, and archaea. [ABSTRACT FROM AUTHOR]

Light-regulated enzymes enable organisms to quickly respond to changing light conditions. We characterize a photoactivatable adenylyl cyclase (AC) from Beggiatoa sp. (bPAC) that translates a blue light signal into the production of the second messenger cyclic AMP. bPAC contains a BLUF photoreceptor domain that senses blue light using a flavin chromophore, linked to an AC domain. We present a dark state crystal structure of bPAC that closely resembles the recently published structure of the homologous OaPAC from Oscillatoria acuminata . To elucidate the structural mechanism of light-dependent AC activation by the BLUF domain, we determined the crystal structures of illuminated bPAC and of a pseudo-lit state variant. We use hydrogen–deuterium exchange measurements of secondary structure dynamics and hypothesis-driven point mutations to trace the activation pathway from the chromophore in the BLUF domain to the active site of the cyclase. The structural changes are relayed from the residues interacting with the excited chromophore through a conserved kink of the BLUF β-sheet to a tongue-like extrusion of the AC domain that regulates active site opening and repositions catalytic residues. Our findings not only show the specific molecular pathway of photoactivation in BLUF-regulated ACs but also have implications for the general understanding of signaling in BLUF domains and of the activation of ACs. [ABSTRACT FROM AUTHOR]

The sediment-water interface is an important site for material exchange in marine systems and harbor unique microbial habitats. The flux of nutrients, metals, and greenhouse gases at this interface may be severely dampened by the activity of microorganisms and abiotic redox processes, leading to the "benthic filter" concept. In this study, we investigate the spatial variability, mechanisms and quantitative importance of a microbially-dominated benthic filter for dissolved sulfide in the Eastern Gotland Basin (Baltic Sea) that is located along a dynamic redox gradient between 65 and 173m water depth. In August-September 2013, high resolution (0.25 mm minimum) vertical microprofiles of redox-sensitive species were measured in surface sediments with solid-state gold-amalgam voltammetric microelectrodes. The highest sulfide consumption (2.73-3.38 mmol m-2 day-1) occurred within the top 5 mm in sediments beneath a pelagic hypoxic transition zone (HTZ, 80-120m water depth) covered by conspicuous white bacterial mats of genus Beggiatoa. A distinct voltammetric signal for polysulfides, a transient sulfur oxidation intermediate, was consistently observed within the mats. In sediments under anoxic waters (>140m depth), signals for Fe(II) and aqueous FeS appeared below a subsurface maximum in dissolved sulfide, indicating a Fe(II) flux originating from older sediments presumably deposited during the freshwater Ancylus Lake that preceded the modern Baltic Sea. Our results point to a dynamic benthic sulfur cycling in Gotland Basin where benthic sulfide accumulation is moderated by microbial sulfide oxidation at the sediment surface and FeS precipitation in deeper sediment layers. Upscaling our fluxes to the Baltic Proper; we find that up to 70% of the sulfide flux (2281 kton yr-1) toward the sediment-seawater interface in the entire basin can be consumed at the microbial mats under the HTZ (80-120m water depth) while only about 30% the sulfide flux effuses to the bottom waters (>120m depth). This newly described benthic filter for the Gotland Basin must play a major role in limiting the accumulation of sulfide in and around the deep basins of the Baltic Sea. [ABSTRACT FROM AUTHOR]

Finfish aquaculture can be installed over hard and patchy substrates where grab sampling is challenging and use of video can be an appropriate tool to document benthic changes. Video monitoring can show visual indicators of enrichment, namely flocculent matter, Beggiatoa -like mats, and opportunistic polychaete complexes (OPC). We examined factors influencing presence of indicators using 52 video monitoring reports collected in Newfoundland, Canada. The main driving factor was distance to cage, with indicators showing a higher probability of occurrence within 10 m from cages due to low current velocities. Indicators were less prevalent on sites dominated by hard substrates while OPC in particular were restricted to depths > 35 m. Beggiatoa- like bacteria covered a larger surface than the two other indicators; however, our results suggest the necessity of amalgamating information related to all the indicators (including bare stations that could indicate anoxia) to establish a more accurate evaluation of aquaculture impact. [ABSTRACT FROM AUTHOR]

A chemolithoautotrophic strain of the family Beggiatoaceae, Beggiatoa sp. strain 35Flor, was found to oxidize molecular hydrogen when grown in a medium with diffusional gradients of oxygen, sulfide, and hydrogen. Microsensor profiles and rate measurements suggested that the strain oxidized hydrogen aerobically when oxygen was available, while hydrogen consumption under anoxic conditions was presumably driven by sulfur respiration. Beggiatoa sp. 35Flor reached significantly higher biomass in hydrogen-supplemented oxygen-sulfide gradient media, but hydrogen did not support growth of the strain in the absence of reduced sulfur compounds. Nevertheless, hydrogen oxidation can provide Beggiatoa sp. 35Flor with energy for maintenance and assimilatory purposes and may support the disposal of internally stored sulfur to prevent physical damage resulting from excessive sulfur accumulation. Our knowledge about the exposure of natural populations of Beggiatoaceae to hydrogen is very limited, but significant amounts of hydrogen could be provided by nitrogen fixation, fermentation, and geochemical processes in several of their typical habitats such as photosynthetic microbial mats and submarine sites of hydrothermal fluid flow. [ABSTRACT FROM AUTHOR]

Little is known about the role of microbial biofilms in nutrient cycling and ecosystem processes within desert springs. However, biofilms produce microscale physicochemical variation important to ecosystem function. We used microelectrodes to measure microscale physicochemical (temperature, pH, O2, and H2S) heterogeneity in biofilms at Devils Hole, Nevada. Additionally, we measured water column and pore water nutrient concentrations in 2 autotrophic (Spirogyra, cyanobacteria) and 1 heterotrophic (Beggiatoa) biofilm types. Spirogyra and cyanobacteria followed similar physicochemical trends; however, Spirogyra had more pronounced diurnal and seasonal variation. Oxygen concentrations within the biofilms varied with sample month, light availability, and biofilm type. We also assessed the nutrient limitation of desert spring microbial biofilms using a nutrient-diffusing substrata experiment. Autotrophic and heterotrophic biofilms responded differently to nutrient treatments. The differences between cyanobacteria and Spirogyra have implications on the ecosystem function of desert springs, as well as other ecosystems with diverse biofilm communities, and suggest that a change in microbial biofilms could alter ecosystem biogeochemical dynamics. [ABSTRACT FROM AUTHOR]

The colourless mat covering organically enriched sediments underlying an intensive salmon farm in Estero Pichicolo, southern Chile, was surveyed by combined 454 PyroTag and conventional Sanger sequencing of 16S/18S ribosomal RNA genes for Bacteria and Eukarya . The mat was dominated by the sulphide-oxidizing bacteria (SOB) Candidatus Isobeggiatoa, Candidatus Parabeggiatoa and Arcobacter . By order of their abundances, sulphate-reducing bacteria (SRB) were represented by diverse deltaproteobacterial Desulfobacteraceae , but also within Desulfobulbaceae , Desulfuromonadaceae and Desulfovibrionaceae . The eukaryotic PyroTags were dominated by polychaetes, copepods and nematodes, however, ciliated protozoans were highly abundant in microscopy observations, and were represented by the genera Condylostoma , Loxophyllum and Peritromus . Finally, the abundant Sulfurimonas / Sulfurovum also suggest the occurrence of zero-valence sulphur oxidation, probably derived from Beggiatoaceae as a result of bacteriovorus infaunal activity or generated as free S 0 by the Arcobacter bacteria. The survey suggests an intense and complex sulphur cycle within the surface of salmon-farm impacted sediments. [ABSTRACT FROM AUTHOR]

Crusts and chimneys composed of authigenic barite are found at methane seeps and hydrothermal vents that expel fluids rich in barium. Microbial processes have not previously been associated with barite precipitation in marine cold seep settings. Here, we report on the precipitation of barite on filaments of sulfide-oxidizing bacteria at a brine seep in the Gulf of Mexico. Barite-mineralized bacterial filaments in the interiors of authigenic barite crusts resemble filamentous sulfide-oxidizing bacteria of the genus Beggiatoa. Clone library and i Tag amplicon sequencing of the 16 S r RNA gene show that the barite crusts that host these filaments also preserve DNA of Candidatus Maribeggiatoa, as well as sulfate-reducing bacteria. Isotopic analyses show that the sulfur and oxygen isotope compositions of barite have lower δ34S and δ18O values than many other marine barite crusts, which is consistent with barite precipitation in an environment in which sulfide oxidation was occurring. Laboratory experiments employing isolates of sulfide-oxidizing bacteria from Gulf of Mexico seep sediments showed that under low sulfate conditions, such as those encountered in brine fluids, sulfate generated by sulfide-oxidizing bacteria fosters rapid barite precipitation localized on cell biomass, leading to the encrustation of bacteria in a manner reminiscent of our observations of barite-mineralized Beggiatoa in the Gulf of Mexico. The precipitation of barite directly on filaments of sulfide-oxidizing bacteria, and not on other benthic substrates, suggests that sulfide oxidation plays a role in barite formation at certain marine brine seeps where sulfide is oxidized to sulfate in contact with barium-rich fluids, either prior to, or during, the mixing of those fluids with sulfate-containing seawater in the vicinity of the sediment/water interface. As with many other geochemical interfaces that foster mineral precipitation, both biological and abiological processes likely contribute to the precipitation of barite at marine brine seeps such as the one studied here. [ABSTRACT FROM AUTHOR]

A new species of dorvilleid polychaete, Ophryotrocha cyclops, has been observed on the rocky seafloor underneath deep salmonid aquaculture sites on the south coast of Newfoundland, Canada. The distribution of these opportunistic worms is likely related to organic matter accumulation on the seafloor, and this species may have a role in remediation processes. To better understand the functional role of O. cyclops at aquaculture sites, it is important to know what they feed upon. Here, stable isotope analyses (δ13C, δ15N and δ34S) and trace element analyses were performed on dorvilleids and their potential food sources at three aquaculture sites. Stable isotope analyses revealed spatial and temporal variation in the isotopic carbon signature of O. cyclops, highlighting possible differences in the food sources of individual dorvilleids within and between sites. The isotopic composition of dorvilleids was closest to that of fish pellets; the presence of abundant lipid droplets in gut epithelial cells of O. cyclops suggests the assimilation of fish pellet-derived lipids. Trace element analysis indicated that O. cyclops does not concentrate the aquaculture tracers Zn or Cu to a large extent. However, concentrations of sulphur were high in O. cyclops compared with other sources. Taken together, results show that O. cyclops most likely consume both fish pellets and flocculent matter-associated bacteria. As such, they are involved in sulphur cycling and fish pellet degradation at aquaculture sites. [ABSTRACT FROM PUBLISHER]

Large sulfur-oxidizing bacteria in the family Beggiatoaceae are important players in the global sulfur cycle. This group contains members of the well-known genera Beggiatoa, Thioploca, and Thiomargarita but also recently identified and relatively unknown candidate taxa, including "Candidatus Thiopilula" spp. and "Ca. Thiophysa" spp. We discovered a population of "Ca. Thiopilula" spp. colonizing cold seeps near Barbados at a∼4.7-km water depth. The Barbados population consists of spherical cells that are morphologically similar to Thiomargarita spp., with elemental sulfur inclusions and a central vacuole, but have much smaller cell diameters (5 to 40 ∼m). Metatranscriptomic analysis revealed that when exposed to anoxic sulfidic conditions, Barbados "Ca. Thiopilula" organisms expressed genes for the oxidation of elemental sulfur and the reduction of nitrogenous compounds, consistent with their vacuolated morphology and intracellular sulfur storage capability. Metatranscriptomic analysis further revealed that anaerobic methane-oxidizing and sulfate-reducing organisms were active in the sediment, which likely provided reduced sulfur substrates for "Ca. Thiopilula" and other sulfur-oxidizing microorganisms in the community. The novel observations of "Ca. Thiopilula" and associated organisms reported here expand our knowledge of the globally distributed and ecologically successful Beggiatoaceae group and thus offer insight into the composition and ecology of deep cold seep microbial communities. [ABSTRACT FROM AUTHOR]

Beggiatoaceae, giant sulphur-oxidizing bacteria, are well known to occur in cold and temperate waters, as well as hydrothermal vents, where they form dense mats on the floor. However, they have never been described in tropical marine mangroves. Here, we describe two new species of benthic Beggiatoaceae colonizing a marine mangrove adjacent to mangrove roots. We combined phylogenetic and lipid analysis with electron microscopy in order to describe these organisms. Furthermore, oxygen and sulphide measurements in and ex situ were performed in a mesocosm to characterize their environment. Based on this, two new species, Candidatus Maribeggiatoa sp. and Candidatus Isobeggiatoa sp. inhabiting tropical marine mangroves in Guadeloupe were identified. The species identified as Candidatus Maribeggiatoa group suggests that this genus could harbour a third cluster with organisms ranging from 60 to 120 μm in diameter. This is also the first description of an Isobeggiatoa species outside of Arctic and temperate waters. The multiphasic approach also gives information about the environment and indications for the metabolism of these bacteria. Our study shows the widespread occurrence of members of Beggiatoaceae family and provides new insight in their potential role in shallow-water marine sulphide-rich environments such as mangroves. [ABSTRACT FROM AUTHOR]

The photoactivated cyclase bPAC of the microbial mats bacterium Beggiatoa sp. consists of a BLUF domain and an adenylyl cyclase domain. It has strong activity of photo-induced cyclic adenylyl monophosphate (cAMP) formation and is therefore an important optogenetic tool in neuroscience applications. The SUMO-bPAC-Y7F mutant where Tyr-7 is replaced by Phe-7 in the BLUF domain has lost the typical BLUF domain photo-cycle dynamics. Instead, the investigated SUMO-bPAC-Y7F mutant consisted of three protein conformations with different triplet based photo-dynamics: (i) reversible flavin quinone (Fl) cofactor reduction to flavin semiquinone (FlH ), (ii) reversible violet/near ultraviolet absorbing flavin photoproduct (FlA) formation, and (iii) irreversible red absorbing flavin photoproduct (FlC) formation. Absorption and emission spectroscopic measurements on SUMO-bPAC-Y7F were carried out before, during and after light exposure. Flavin photo-dynamics schemes are developed for the SUMO-bPAC-Y7F fractions performing photo-induced FlH , FlA, and FlC formation. Quantitative parameters of the flavin cofactor excitation, relaxation and recovery dynamics in SUMO-bPAC-Y7F are determined. [ABSTRACT FROM AUTHOR]

Sulfide oxidizing bacterial mats are common in regions of the continental shelves characterized by high primary production and the resultant oxygen minimum zone. These mats are made up of several species of Beggiatoa and/or Thioploca, which oxidize sulfide that is generated in the sediment. Thioploca spp. inhabit a large polysaccharide sheath that encompasses bundles of 1-20 filaments (trichomes), each ranging from 3 to 60 μm in diameter. This sheath has been shown to be a critical component of the autecology of Thioploca. Analysis of Thioploca from cold seeps in Monterey Bay using light and transmission electron microscopy identified new and diverse microbial assemblages associated with interstitial spaces between trichomes, inside the sheath. Small diameter, non-vacuolate, filamentous prokaryotes were numerous. Amoebae, euglenozoa, ciliates and other protists of unknown affiliation were observed in sheaths. Most of the protists possessed food vacuoles and some protists showed ultrastructural evidence of endosymbionts. These observations suggest that Thioploca sheaths may serve as oases on the sea floor, providing nutritional and detoxification services to previously unrecognized microbial partners. [ABSTRACT FROM AUTHOR]

This study is a description of sediment sulphide levels and redox potential associated with different values of bacterial mats (Beggiatoa) coverage at stations in deep Subarctic finfish sites. Beggiatoa was not observed at reference stations and significant differences were found between geochemical parameters before aquaculture and at-harvest. Beggiatoa coverage should be considered in the light of other evidence gathered through video imaging such as benthic diversity and the presence of other visual indicators and not used as strict regulatory thresholds. [ABSTRACT FROM PUBLISHER]

Wrinkle structures are sedimentary features that are produced primarily through the trapping and binding of siliciclastic sediments by mat-forming micro-organisms. Wrinkle structures and related sedimentary structures in the rock record are commonly interpreted to represent the stabilizing influence of cyanobacteria on sediments because cyanobacteria are known to produce similar textures and structures in modern tidal flat settings. However, other extant bacteria such as filamentous representatives of the family Beggiatoaceae can also interact with sediments to produce sedimentary features that morphologically resemble many of those associated with cyanobacteria-dominated mats. While Beggiatoa spp. and cyanobacteria are metabolically and phylogenetically distant, genomic analyses show that the two groups share hundreds of homologous genes, likely as the result of horizontal gene transfer. The comparative genomics results described here suggest that some horizontally transferred genes may code for phenotypic traits such as filament formation, chemotaxis, and the production of extracellular polymeric substances that potentially underlie the similar biostabilizing influences of these organisms on sediments. We suggest that the ecological utility of certain basic life modes such as the construction of mats and biofilms, coupled with the lateral mobility of genes in the microbial world, introduces an element of uncertainty into the inference of specific phylogenetic origins from gross morphological features preserved in the ancient rock record. [ABSTRACT FROM AUTHOR]

Sulfur and iron concentrations in wood from three 17th century shipwrecks in the Baltic Sea, the Ghost wreck, the Crown and the Sword, were obtained by X-ray fluorescence (XRF) scanning. In near anaerobic environments symbiotic microorganisms degrade waterlogged wood, reduce sulfate and promote accumulation of low-valent sulfur compounds, as previously found for the famous wrecks of the Vasa and Mary Rose. Sulfur K-edge X-ray absorption near-edge structure (XANES) analyses of Ghost wreck wood show that organic thiols and disulfides dominate, together with elemental sulfur probably generated by sulfur-oxidizing Beggiatoa bacteria. Iron sulfides were not detected, consistent with the relatively low iron concentration in the wood. In a museum climate with high atmospheric humidity oxidation processes, especially of iron sulfides formed in the presence of corroding iron, may induce post-conservation wood degradation. Subject to more general confirmation by further analyses no severe conservation concerns are expected for the Ghost wreck wood. [ABSTRACT FROM AUTHOR]

Zero-valent sulfur is a key intermediate in the microbial oxidation of sulfide to sulfate. Many sulfide-oxidizing bacteria produce and store large amounts of sulfur intra- or extracellularly. It is still not understood how the stored sulfur is metabolized, as the most stable form of S0 under standard biological conditions, orthorhombic α-sulfur, is most likely inaccessible to bacterial enzymes. Here we analyzed the speciation of sulfur in single cells of living sulfide-oxidizing bacteria via Raman spectroscopy. Our results showed that under various ecological and physiological conditions, all three investigated Beggiatoa strains stored sulfur as a combination of cyclooctasulfur (S8) and inorganic polysulfides (Sn 2-). Linear sulfur chains were detected during both the oxidation and reduction of stored sulfur, suggesting that Sn 2- species represent a universal pool of bioavailable sulfur. Formation of polysulfides due to the cleavage of sulfur rings could occur biologically by thiol-containing enzymes or chemically by the strong nucleophile HS- as Beggiatoa migrates vertically between oxic and sulfidic zones in the environment. Most Beggiatoa spp. thus far studied can oxidize sulfur further to sulfate. Our results suggest that the ratio of produced sulfur and sulfate varies depending on the sulfide flux. Almost all of the sulfide was oxidized directly to sulfate under low-sulfide-flux conditions, whereas only 50% was oxidized to sulfate under high-sulfide-flux conditions leading to S0 deposition. With Raman spectroscopy we could show that sulfate accumulated in Beggiatoa filaments, reaching intracellular concentrations of 0.72 to 1.73 M. [ABSTRACT FROM AUTHOR]

The formation of marine phosphatic mineral deposits remains incompletely understood, despite decades of research. The involvement of bacteria in this process has long been suspected, and both modern and ancient associations between bacteria and phosphorites have been recorded. Only recently has a specific bacterial metabolic process associated with the formation of phosphorites been discovered. Recent studies demonstrate that polyphosphate utilization by sulfide-oxidizing bacteria results in the rapid precipitation of apatite – providing at least a partial mechanism to explain the close spatial correlation between accumulations of sulfide-oxidizing bacteria and modern phosphorites. Possible fossilized bacteria are known from ancient phosphatic mineral deposits. Potentially, the fossilized cells represent the remains of bacteria that induced the formation of those phosphorites. However, robust criteria for the recognition of these bacteria have yet to be identified.

The chemolithoautotrophic strain Beggiatoa sp. 35Flor shows an unusual migration behavior when cultivated in a gradient medium under high sulfide fluxes. As common for Beggiatoa spp., the filaments form a mat at the oxygen-sulfide interface. However, upon prolonged incubation, a subpopulation migrates actively downwards into the anoxic and sulfidic section of the medium, where the filaments become gradually depleted in their sulfur and polyhydroxyalkanoates (PHA) inclusions. This depletion is correlated with the production of hydrogen sulfide. The sulfur- and PHA-depleted filaments return to the oxygen-sulfide interface, where they switch back to depositing sulfur and PHA by aerobic sulfide oxidation. Based on these observations we conclude that internally stored elemental sulfur is respired at the expense of stored PHA under anoxic conditions. Until now, nitrate has always been assumed to be the alternative electron acceptor in chemolithoautotrophic Beggiatoa spp. under anoxic conditions. As the medium and the filaments were free of oxidized nitrogen compounds we can exclude this metabolism. Furthermore, sulfur respiration with PHA under anoxic conditions has so far only been described for heterotrophic Beggiatoa spp., but our medium did not contain accessible organic carbon. Hence the PHA inclusions must originate from atmospheric CO2 fixed by the filaments while at the oxygen-sulfide interface. We propose that the directed migration of filaments into the anoxic section of an oxygen-sulfide gradient system is used as a last resort to preserve cell integrity, which would otherwise be compromised by excessive sulfur deposition occurring in the presence of oxygen and high sulfide fluxes. The regulating mechanism of this migration is still unknown.

Little is known about protists at deep-sea hydrothermal vents. The vent sites at Guaymas Basin in the Gulf of California are characterized by dense mats of filamentous pigmented or nonpigmented Beggiatoa that serve as markers of subsurface thermochemical gradients. We constructed 18S rRNA libraries to investigate ciliate assemblages in Beggiatoa mats and from bare sediments at the Guaymas vent site. Results indicated a high diversity of ciliates, with 156 operational taxonomic units identified in 548 sequences. Comparison between mat environments demonstrated that ciliate and bacterial assemblages from pigmented mats, nonpigmented mats, and bare sediments were significantly different and highly correlated with bacterial assemblages. Neither bacterial nor ciliate assemblages were correlated with environmental factors. The most abundant ciliates at Guaymas were more likely to be represented in clone libraries from other hydrothermal, deep-sea, and/or anoxic or microaerophilic environments, supporting the hypothesis that these ciliate species are broadly distributed. The orange mat environment included a higher proportion of ciliate sequences that were more similar to those from other environmental studies than to cultured ciliate species, whereas clone libraries from bare sediments included sequences that were the most highly divergent from all other sequences and may represent species that are endemic to Guaymas. [ABSTRACT FROM AUTHOR]

The draft genome sequence of a single orange Beggiatoa ("Candidatus Maribeggiatoa") filament collected from a microbial mat at a hydrothermal site in Guaymas Basin (Gulf of California, Mexico) shows evidence of extensive genetic exchange with cyanobacteria, in particular for sensory and signal transduction genes. A putative homing endonuclease gene and group I intron within the 23S rRNA gene; several group II catalytic introns; GyrB and DnaE inteins, also encoding homing endonucleases; multiple copies of sequences similar to the fdxN excision elements XisH and XisI (required for heterocyst differentiation in some cyanobacteria); and multiple sequences related to an open reading frame (ORF) (00024_0693) of unknown function all have close non-Beggiatoaceae matches with cyanobacterial sequences. Sequences similar to the uncharacterized ORF and Xis elements are found in other Beggiatoaceae genomes, a variety of cyanobacteria, and a few phylogenetically dispersed pleiomorphic or filamentous bacteria. We speculate that elements shared among filamentous bacterial species may have been exchanged in microbial mats and that some of them may be involved in cell differentiation. [ABSTRACT FROM AUTHOR]

Orange, white, and yellow vacuolated Beggiatoaceae filaments are visually dominant members of microbial mats found near sea floor hydrothermal vents and cold seeps, with orange filaments typically concentrated toward the mat centers. No marine vacuolate Beggiatoaceae are yet in pure culture, but evidence to date suggests they are nitrate-reducing, sulfide-oxidizing bacteria. The nearly complete genome sequence of a single orange Beggiatoa ("Candidatus Maribeggiatoa") filament from a microbial mat sample collected in 2008 at a hydrothermal site in Guaymas Basin (Gulf of California, Mexico) was recently obtained. From this sequence, the gene encoding an abundant soluble orange-pigmented protein in Guaymas Basin mat samples (collected in 2009) was identified by microcapillary reverse-phase high-performance liquid chromatography (HPLC) nano-electrospray tandem mass spectrometry (pLC-MS-MS) of a pigmented band excised from a denaturing polyacrylamide gel. The predicted protein sequence is related to a large group of octaheme cytochromes whose few characterized representatives are hydroxylamine or hydrazine oxidases. The protein was partially purified and shown by in vitro assays to have hydroxylamine oxidase, hydrazine oxidase, and nitrite reductase activities. From what is known of Beggiatoaceae physiology, nitrite reduction is the most likely in vivo role of the octaheme protein, but future experiments are required to confirm this tentative conclusion. Thus, while present-day genomic and proteomic techniques have allowed precise identification of an abundant mat protein, and its potential activities could be assayed, proof of its physiological role remains elusive in the absence of a pure culture that can be genetically manipulated. [ABSTRACT FROM AUTHOR]

This study presents benthic data from 12 samplings from February to December 2010 in a 28m deep channel in the southwest Baltic Sea. In winter, the distribution of solutes in the porewater was strongly modulated by bioirrigation which efficiently flushed the upper 10 cm of sediment, leading to concentrations which varied little from bottom water values. Solute pumping by bioirrigation fell sharply in the summer as the bottom waters became severely hypoxic (<2 µMO2). At this point the giant sulfide-oxidizing bacteria Beggiatoa was visible on surface sediments. Despite an increase in O2 following mixing of the water column in November, macrofauna remained absent until the end of the sampling. Contrary to expectations, metabolites such as dissolved inorganic carbon, ammonium and hydrogen sulfide did not accumulate in the upper 10 cm during the hypoxic period when bioirrigation was absent, but instead tended toward bottom water values. This was taken as evidence for episodic bubbling of methane gas out of the sediment acting as an abiogenic irrigation process. Porewater--seawater mixing by escaping bubbles provides a pathway for enhanced nutrient release to the bottom water and may exacerbate the feed- back with hypoxia. Subsurface dissolved phosphate (TPO4) peaks in excess of 400 µM developed in autumn, resulting in a very large diffusive TPO4 flux to the water column of 0.7±0.2 mmol m-2 d-1. The model was not able to simulate this TPO4 source as release of iron-bound P (Fe--P) or organic P. As an alternative hypothesis, the TPO4 peak was reproduced using new kinetic expressions that allow Beggiatoa to take up porewater TPO4 and accumulate an intracellular P pool during periods with oxic bottom waters. TPO4 is then released during hypoxia, as previous published results with sulfide-oxidizing bacteria indicate. The TPO4 added to the porewater over the year by organic P and Fe--P is recycled through Beggiatoa, meaning that no additional source of TPO4 is needed to explain the TPO4 peak. Further experimental studies are needed to strengthen this conclusion and rule out Fe--P and organic P as candidate sources of ephemeral TPO4 release. A measured C/P ratio of <20 for the diffusive flux to the water column during hypoxia directly demonstrates preferential release of P relative to C under oxygen-deficient bottom waters. This coincides with a strong decrease in dissolved inorganic N/P ratios in the water column to ~1. Our results suggest that sulfide oxidizing bacteria could act as phosphorus capacitors in systems with oscillating redox conditions, releasing massive amounts of TPO4 in a short space of time and dramatically increasing the internal loading of TPO4 to the overlying water. [ABSTRACT FROM AUTHOR]