AbstractTwo ∼6 m long sediment cores were collected along the ∼300 m isobath on the Alaskan Beaufort Sea continental margin. Both cores showed distinct sulfate-methane transition zones (SMTZ) at 105 and 120 cm below seafloor (cmbsf). Sulfate was not completely depleted below the SMTZ but remained between 30 and 500 μM. Sulfate reduction and anaerobic oxidation of methane (AOM) determined by radiotracer incubations were active throughout the methanogenic zone. Although a mass balance could not explain the source of sulfate below the SMTZ, geochemical profiles and correlation network analyses of biotic and abiotic data suggest a cryptic sulfur cycle involving iron, manganese and barite. Inhibition experiments with molybdate and 2-bromoethanesulfonate (BES) indicated decoupling of sulfate reduction and AOM and competition between sulfate reducers and methanogens for substrates. While correlation network analyses predicted coupling of AOM to iron reduction, the addition of manganese or iron did not stimulate AOM. Since none of the classical archaeal anaerobic methanotrophs (ANME) were abundant, the involvement of unknown or unconventional phylotypes in AOM is conceivable. The resistance of AOM activity to inhibitors implies deviation from conventional enzymatic pathways. This work suggests that the classical redox cascade of electron acceptor utilization based on Gibbs energy yields does not always hold in diffusion-dominated systems, and instead biotic processes may be more strongly coupled to mineralogy.
AbstractOrganic material is degraded anaerobically by a complex community of microbes making up the anaerobic food chain. This community was examined in methane‐rich sediments of the Beaufort Sea, Alaska, using metagenomic sequencing, along with tag pyrosequencing and quantitative polymerase chain reaction analyses of 16S ribosomal ribonucleic acid genes. The goal of the study was to examine the relative abundance and taxonomic composition of organisms making up different parts of the anaerobic food chain. The metagenomic data suggested that genes for producing acetate via fermentation (“acetate fermentation”) were more common than genes producing other fermentation by‐products, but acetate fermentation genes made up only 32% of all genes for fermentation pathways while genes for fermentation to ethanol accounted for 27%. The genes for the production of other compounds, including propionate (15%), butyrate (11%), lactate (4%), and hydrogen gas (11%), were also often abundant. Similar results were observed when the same approach was used to analyze metagenomic data previously collected from two low‐latitude systems with methane‐rich sediments. In all of these sediments, genes for pathways producing organic acids, ethanol, and hydrogen gas were about 30‐fold more abundant than the genes for sulfate reduction and methanogenesis, processes which consume those compounds. Our results suggest that the type and abundance of fermentative microbes potentially affect the taxonomic composition of sulfate‐reducing bacteria and methanogenic archaea and rates of organic carbon mineralization by the anaerobic food chain.
AbstractProkaryote communities were investigated on the seasonally stratified Alaska Beaufort Shelf (ABS). Water and sediment directly underlying water with origin in the Arctic, Pacific or Atlantic oceans were analyzed by pyrosequencing and length heterogeneity-PCR in conjunction with physicochemical and geographic distance data to determine what features structure ABS microbiomes. Distinct bacterial communities were evident in all water masses. Alphaproteobacteria explained similarity in Arctic surface water and Pacific derived water. Deltaproteobacteria were abundant in Atlantic origin water and drove similarity among samples. Most archaeal sequences in water were related to unclassified marine Euryarchaeota. Sediment communities influenced by Pacific and Atlantic water were distinct from each other and pelagic communities. Firmicutes and Chloroflexi were abundant in sediment, although their distribution varied in Atlantic and Pacific influenced sites. Thermoprotei dominated archaea in Pacific influenced sediments and Methanomicrobia dominated in methane-containing Atlantic influenced sediments. Length heterogeneity-PCR data from this study were analyzed with data from methane-containing sediments in other regions. Pacific influenced ABS sediments clustered with Pacific sites from New Zealand and Chilean coastal margins. Atlantic influenced ABS sediments formed another distinct cluster. Density and salinity were significant structuring features on pelagic communities. Porosity co-varied with benthic community structure across sites and methane did not. This study indicates that the origin of water overlying sediments shapes benthic communities locally and globally and that hydrography exerts greater influence on microbial community structure than the availability of methane.
This study was carried out to address how hydrodynamic dispersal and local geochemical features shape marine sediment microbiomes across geographic distance. The work follows a study that investigated how regional water masses, their stratification, and physical properties shape surface sediment microbiomes on continental shelves. This project focused on two groups of sediment samples from the Alaska Beaufort Shelf, and the Hikurangi Margin of New Zealand. The cores were selected based on geochemical profiles of pore-water sulfate, chloride, and dissolved inorganic carbon concentration, and sediment methane concentration. Cores were pair matched with a geochemically analogous core. The study tested the hypothesis that in geochemically analogous environments separated by geographic distance, but within the same ocean mass, the same communities would be present in similar relative abundance at different geochemical horizons down core: consequently, the same biogeochemical functions would be performed by like taxonomic groups. The study used a computational approach to extract functional data from tag-sequence data. The analysis demonstrated in samples near the sediment-water interface there was covariance in terms of taxonomic composition and metabolic function across sites. However, in general, down core, in geochemically analogous settings from different locations, metabolic functions were carried out by different taxonomic groups. The work identified limitations with current databases to extract functional information from sequence data, but provides a potential framework for extracting information from archived datasets, where real-time functional analyses are not possible.