Metagenomic analysis of carbohydrate-active enzymes and their contribution to marine sediment biodiversity.

Marine sediments constitute the world's most substantial long-term carbon repository. The microorganisms dwelling in these sediments mediate the transformation of fixed oceanic carbon, but their contribution to the carbon cycle is not fully understood. Previous culture-independent investigations into sedimentary microorganisms have underscored the significance of carbohydrates in the carbon cycle. In this study, we employ a metagenomic methodology to investigate the distribution and abundance of carbohydrate-active enzymes (CAZymes) in 37 marine sediments sites. These sediments exhibit varying oxygen availability and were isolated in diverse regions worldwide. Our comparative analysis is based on the metabolic potential for oxygen utilisation, derived from genes present in both oxic and anoxic environments. We found that extracellular CAZyme modules targeting the degradation of plant and algal detritus, necromass, and host glycans were abundant across all metagenomic samples. The analysis of these results indicates that the oxic/anoxic conditions not only influence the taxonomic composition of the microbial communities, but also affect the occurrence of CAZyme modules involved in the transformation of necromass, algae and plant detritus. To gain insight into the sediment microbial taxa, we reconstructed metagenome assembled genomes (MAG) and examined the presence of primary extracellular carbohydrate active enzyme (CAZyme) modules. Our findings reveal that the primary CAZyme modules and the CAZyme gene clusters discovered in our metagenomes were prevalent in the Bacteroidia, Gammaproteobacteria, and Alphaproteobacteria classes. We compared those MAGs to organisms from the same taxonomic classes found in soil, and we found that they were similar in its CAZyme repertoire, but the soil MAG contained a more abundant and diverse CAZyme content. Furthermore, the data indicate that abundant classes in our metagenomic samples, namely Alphaproteobacteria, Bacteroidia and Gammaproteobacteria, play a pivotal role in carbohydrate transformation within the initial few metres of the sediments.

Human-induced metals accumulation in sediments of an industrialized bay of northern Chile. An enrichment and ecological risk assessment based on preindustrial values.

Environmental assessment in marine systems is difficult because most of the time their preindustrial characteristics are not available. Four sediment cores taken in Mejillones bay (northern Chile) were used to establish preindustrial metals concentrations, and to assess the environmental status of this industrialized zone. According to historical documents, the beginning of the industrial era start in 1850 CE. Considering this, the preindustrial concentration of some metals was established, by mean of a statistical approach. Most of the metals increase their concentration from the preindustrial to the industrial period. Environmental assessment showed an enrichment of Zr and Cr, a moderately polluted condition, and a low probability of toxic effect on the biological communities. The preindustrial values derived from sediment cores are a good tool to evaluate the environmental condition of Mejillones bay. However new information (backgrounds with a higher spatial representativity, toxicological threshold, and others) it is necessary to improve the environmental assessment of this environment.

Deep-Sea Sediments from the Southern Gulf of Mexico Harbor a Wide Diversity of PKS I Genes.

The excessive use of antibiotics has triggered the appearance of new resistant strains, which is why great interest has been taken in the search for new bioactive compounds capable of overcoming this emergency in recent years. Massive sequencing tools have enabled the detection of new microorganisms that cannot be cultured in a laboratory, thus opening the door to the search for new biosynthetic genes. The great variety in oceanic environments in terms of pressure, salinity, temperature, and nutrients enables marine microorganisms to develop unique biochemical and physiological properties for their survival, enhancing the production of secondary metabolites that can vary from those produced by terrestrial microorganisms. We performed a search for type I PKS genes in metagenomes obtained from the marine sediments of the deep waters of the Gulf of Mexico using Hidden Markov Models. More than 2000 candidate genes were detected in the metagenomes that code for type I PKS domains, while biosynthetic pathways that may code for other secondary metabolites were also detected. Our research demonstrates the great potential use of the marine sediments of the Gulf of Mexico for identifying genes that code for new secondary metabolites.

Seasonal tendencies of microplastics around coral reefs in selected Marine Protected National Parks of Gulf of California, Mexico.

This study focuses on the presence of MPs in the sediment beds around coral reefs of MPNPs in Baja California Sur, México. Based on seasonal sampling results, comparison of MPs from Cabo Pulmo (avg. 680.25 items/100 g-1 d.w) recorded higher values than Espiritu Santo Island (avg. 321.75 items/100 g-1 d.w) from backshore/foreshore regions. Fibrous MPs are the dominant morphotypes followed by fragments and spheres. SEM/EDS analysis revealed that the MPs are altered texturally in surface and is bioavailable to marine organisms independent of size/shape. FTIR analysis indicate different polymers (in %) in the form of PP (70), PET (65), HDPE (59), LDPE (50), PS (30), PC (18), PU (10) and RYN (10). Most of the MPs are secondary in origin resulting from man-made and tourist's activities controlled by wave transportation and tidal currents. Existence of MPs in sediment beds around the coral reefs signals the ways for future investigations.

Physical and anthropogenic drivers shaping the spatial distribution of microplastics in the marine sediments of Chilean fjords.

Several studies have focused on the presence and distribution of microplastics within the water column of coastal waters, but the dynamics of these particles in sediments have received little attention. Here we examine the concentrations and characteristics of microplastics in sediment samples collected from 35 stations within the Inner Sea of Chiloé, Chilean Patagonia. Current velocity, grain size, intensity of salmon farming activities, and human population density were all evaluated as factors potentially explaining concentrations and distribution of microplastic particles within these sediments. Microplastics were detected in all samples, with the highest abundance represented by fibers (88%), fragments (10%) and films (2%). Across the sampled sites, microplastic concentrations averaged 72.2 ± 32.4 (SD) items per kg dw (dry weight) sediment, with the principal polymers identified as polyethylene terephthalate (PET), acrylic, polypropylene (PP) and polyurethane (PUR). Approximately 40% of the variability in distribution and abundance of microplastics was explained by current velocity combined with proximity and intensity of local salmon production activities. SYNOPSIS: Marine currents and aquaculture intensity explain abundance and dynamics of microplastics in marine sediments.

Microscale extraction combined with gas chromatography/mass spectrometry for the simultaneous determination of polycyclic aromatic hydrocarbons and polycyclic aromatic sulfur heterocycles in marine sediments.

This paper describes a novel method based on an ultrasound-assisted extraction microscale device (UAE-MSD) for the rapid and simultaneous determination of polycyclic aromatic hydrocarbons (PAH) and polycyclic aromatic sulfur heterocycles (PASH) in marine sediments. Solvent extraction conditions were optimized by applying a simplex-centroid mixture design. Optimum conditions were used to validate and determine the concentrations of 17 PAH and 7 PASH. The best conditions were obtained by extracting sediments with 500 µL of DCM:MeOH (65:35, v:v) over 23 min of sonication. Analytes were determined by gas chromatography/mass spectrometry in selective ion monitoring (GC-MS/SIM). Matrix effects were evaluated, and matrix-matched calibration was used for quantitation. Analytical method validation was carried out using the certified reference material NIST SRM 1941b, as well as sediment spiked with PASH at three concentration levels. Recoveries ranged between 70.0 ± 3.5% and 119 ± 9.1% for PAH and 80.6 ± 10.4% and 120 ± 10% for PASH. Linearity (R2) was ≥0.99 for all compounds. Method detection limits ranged from 8.8 to 30.2 ng g-1, while limits of quantification ranged from 29.4 to 1011 ng g-1. UAE-MSD was applied to marine sediments exposed to different anthropogenic impacts collected in Todos os Santos Bay, Brazil. PAH concentrations ranged from

Diversity of microbial community and its metabolic potential for nitrogen and sulfur cycling in sediments of Phu Quoc island, Gulf of Thailand.

Although Phu Quoc island, Gulf of Thailand possesses diverse marine and coastal ecosystems, biodiversity and metabolic capability of microbial communities remain poorly investigated. The aim of our study was to evaluate the biodiversity and metabolic potential of sediment microbial communities in Phu Quoc island. The marine sediments were collected from three different areas and analyzed by using 16S rRNA gene-based amplicon approach. A total of 1,143,939 reads were clustered at a 97% sequence similarity into 8,331 unique operational taxonomic units, representing 52 phyla. Bacteria and archaea occupied averagely around 86% and 14%, respectively, of the total prokaryotic community. Proteobacteria, Planctomycetes, Chloroflexi, and Thaumarchaeota were the dominant phyla in all sediments, which were involved in nitrogen and sulfur metabolism. Sediments harboring of higher nitrogen sources were found to coincide with increased abundance of archaeal phylum Thaumarchaeota. Predictive functional analysis showed high abundance prokaryotic genes associated with nitrogen cycling including nifA-Z, amoABC, nirA, narBIJ, napA, nxrAB, nrfA-K, nirBD, nirS, nirK, norB-Z, nlnA, ald, and ureA-J, based on taxonomic groups detected by 16S rRNA sequencing. Although the key genes involved in sulfur cycling were found to be at low to undetectable levels, the other genes encoding for sulfur-related biological processes were present, suggesting that alternative pathways may be involved in sulfur cycling at our study site. In conclusion, our study for the first time shed light on diversity of microbial communities in Phu Quoc island.

Characterization of sediment microbial communities at two sites with low hydrocarbon pollution in the southeast Gulf of Mexico.

BACKGROUND: Coastal ecosystems are prone to hydrocarbon pollution due to human activities, and this issue has a tremendous impact on the environment, socioeconomic consequences, and represents a hazard to humans. Bioremediation relies on the ability of bacteria to metabolize hydrocarbons with the aim of cleaning up polluted sites. METHODS: The potential of naturally occurring microbial communities as oil degraders was investigated in Sisal and Progreso, two port locations in the southeast Gulf of Mexico, both with a low level of hydrocarbon pollution. To do so, we determined the diversity and composition of bacterial communities in the marine sediment during the dry and rainy seasons using 16S rRNA sequencing. Functional profile analysis (PICRUTSt2) was used to predict metabolic functions associated with hydrocarbon degradation. RESULTS: We found a large bacterial taxonomic diversity, including some genera reported as hydrocarbon-degraders. Analyses of the alpha and beta diversity did not detect significant differences between sites or seasons, suggesting that location, season, and the contamination level detected here do not represent determining factors in the structure of the microbial communities. PICRUTSt2 predicted 10 metabolic functions associated with hydrocarbon degradation. Most bacterial genera with potential hydrocarbon bioremediation activity were generalists likely capable of degrading different hydrocarbon compounds. The bacterial composition and diversity reported here represent an initial attempt to characterize sites with low levels of contamination. This information is crucial for understanding the impact of eventual rises in hydrocarbon pollution.

Oil leakage induces changes in microbiomes of deep-sea sediments of Campos Basin (Brazil).

The Campos Basin (100,000 km2) is located on the continental shelf of southeastern Brazil. Despite the significant oil and gas industrial activities underway in the Campos Basin, scarce information is available regarding the hydrocarbon contents and microbial communities in the deep-sea sediments. To gain new insights on these aspects, we first obtained deep-sea sediment samples with different degrees of oil exposure. We obtained samples from a seabed fissure (N = 28), surroundings (250 m to 500 m from the fissure; N = 24), and a control area (N = 4). We used shotgun metagenomics to characterize the taxonomic and metabolic diversity and analyzed biogeochemical parameters (metal and oil concentration) of all samples. The high levels of unresolved complex mixture of hydrocarbons in the fissure indicate a potentially recent petrogenic contribution in these sediments. The fissure area was found to have a higher abundance of hydrocarbonoclastic bacterial genera and hydrocarbon degradation genes. These bacteria may be used as biosensors of sediment contamination. The effects of oil contamination, mainly around the fissure, are less clear at 250 m and 500 m, suggesting that the surroundings may not have been heavily affected by the oil leakage. Our study demonstrates that metagenomics can disclose biosensors for environmental monitoring.

Anthropogenic and natural impacts in the marine area of influence of the Grijalva - Usumacinta River (Southern Gulf of Mexico) during the last 45 years.

The development of the Grijalva-Usumacinta river basin exerts modifications on its discharge area. A sediment core was studied to reconstruct environmental changes and trace element contamination status during the past 45 years. 210Pb-derived mass accumulation rates indicate higher sediment input to the area since 1995, related to increased precipitation and floodings in the catchment area. Sediments show finer particles from the late 1970s on, likely related to dams construction upriver and/or land use changes. Heavy metal enrichment factors (EF < 2) suggest minimum contamination. Benthic foraminifera and redox-sensitive - elements (As, Ba, Co, Cr, Cu, Ni, Pb, V and Zn) indicate the sediments before 2000 were deposited under oxygenated conditions. Afterwards, environmental conditions changed and benthic foraminifera and dinocysts assemblages changed suggesting eutrophication and lower oxygen conditions during the last 20 years. Monitoring should be continued to assess eutrophication/hypoxic/pollution trends that could become deleterious to the marine biota.

Biochemical and Genomic Characterization of the Cypermethrin-Degrading and Biosurfactant-Producing Bacterial Strains Isolated from Marine Sediments of the Chilean Northern Patagonia.

Pesticides cause severe environmental damage to marine ecosystems. In the last ten years, cypermethrin has been extensively used as an antiparasitic pesticide in the salmon farming industry located in Northern Patagonia. The objective of this study was the biochemical and genomic characterization of cypermethrin-degrading and biosurfactant-producing bacterial strains isolated from cypermethrin-contaminated marine sediment samples collected in southern Chile (MS). Eleven strains were isolated by cypermethrin enrichment culture techniques and were identified by 16S rDNA gene sequencing analyses. The highest growth rate on cypermethrin was observed in four isolates (MS13, MS15a, MS16, and MS19) that also exhibited high levels of biosurfactant production. Genome sequence analyses of these isolates revealed the presence of genes encoding components of bacterial secondary metabolism, and the enzymes esterase, pyrethroid hydrolase, and laccase, which have been associated with different biodegradation pathways of cypermethrin. These novel cypermethrin-degrading and biosurfactant-producing bacterial isolates have a biotechnological potential for biodegradation of cypermethrin-contaminated marine sediments, and their genomes contribute to the understanding of microbial lifestyles in these extreme environments.

Metabolomic Diversity and Identification of Antibacterial Activities of Bacteria Isolated From Marine Sediments in Hawai'i and Puerto Rico.

Antibiotic resistance is a growing concern worldwide and consequently metabolomic tools are being applied increasingly in efforts aimed at identifying new antimicrobial compounds. Marine bacteria-derived compounds have shown great promise in this area. A metabolomics-based study was undertaken to study the diversity of secondary metabolites from marine sediment bacteria isolated from different locations of Hawai'i and Puerto Rico. This effort included characterizing the biodiversity in the sediment samples and searching for antibacterial activity and associated compounds. Bacterial strains were isolated using several different nutrient agars and culture conditions. DNA sequencing (16s rDNA) was used for phylogenetic characterization. Antibacterial activity was assessed against antibiotic-resistant strains of Escherichia coli, Salmonella enterica, Acinetobacter baumannii, Staphylococcus aureus, and Enterococcus faecalis. Ethyl acetate extracted bacterial secondary metabolites were measured by ultra-performance liquid chromatography-mass spectrometry, processed in Progenesis QI and further analyzed by partial least squares-discriminant analysis using MetaboAnalyst 3. Among the strains (n = 143) that were isolated from these two geographical areas and tested for antibiotic activity, 19 exhibited antibacterial activity against at least one antibiotic-resistant human pathogen. One strain from Hawai'i possessed broad-spectrum activity against all five pathogens. Metabolite profiles were diverse and separated the strains into two clusters in PCA analysis that mirrored geographical origin of the isolated strains. A diversity of bacteria and potential antibacterial compounds were observed in this study. Marine environments represent an opportunity to discover a rich diversity of antibacterial compounds for which resistance mechanisms may be uncommon in human pathogens.

Cultivable fungi present in deep-sea sediments of Antarctica: taxonomy, diversity, and bioprospecting of bioactive compounds.

We accessed the culturable mycobiota present in marine sediments at different depths in Antarctica Ocean. Acremonium fusidioides, Penicillium allii-sativi, Penicillium chrysogenum, Penicillium palitans, Penicillium solitum, and Pseudogymnoascus verrucosus were identified. Penicillium allii-sativi was the dominant species. At least one isolate of each species was capable to present antifungal, trypanocidal, leishmanicidal, antimalarial, nematocidal, or herbicidal activities. Penicillium produced extracts with strong trypanocidal and antimalarial activities, and the extracts of P. solitum and P. chrysogenum demonstrated strong antimalarial activities. Acremonium fusidioides and P. verrucosus displayed strong selective herbicidal properties. The 1H NMR signals for extracts of A. fusidioides, P. chrysogenum, and P. solitum indicated the presence of highly functionalized secondary metabolites, which may be responsible for the biological activities detected. In the deep marine Antarctic sediments, we detected fungal assemblages in which the Penicillium species were found to be dominant and demonstrated capabilities to survive and/or colonise that poly-extreme habitat. Penicillium being a polyextremophile Antarctic species, exhibited strong biological activities and the presence of aromatic compounds in its extracts may indicate that they are wild ancient strains with high genetic and biochemical potentials that enable them to produce bioactive compounds which can be researched in further studies and used in the chemotherapy of neglected tropical diseases as well as in agriculture.

Spatial monitoring of metals and As in coastal sediments of northern Chile: An evaluation of background values for the analysis of local environmental conditions.

To assess environmental conditions along the coast of northern Chile, 457 sediment samples were collected between 22° S and 28° S in order to evaluate the Cu, Zn, Pb, Ni, V, Cd, As and Al content as well as its corresponding relationship to nearby anthropogenic activities. In general, the results of most metals showed that bays without industrial activity accumulate lower concentrations of contaminant elements in the sediments and thus mostly represent natural conditions. The highest Cu, Pb and V levels were measured in coastal zones were different industrial mining activities have been developed during the last decades. Results of different indices of environmental evaluation and ecological impact suggest that, metal content of sediments from pollution-free zones characterized by similar oceanographic and geological conditions to those under study, would be the most appropriate background level for assessing the anthropogenic impact on coastal areas of northern Chile.

Natural and anthropogenic oil impacts on benthic foraminifera in the southern Gulf of Mexico.

The Campeche Sound is the major offshore oil producing area in the Southern Gulf of Mexico (SGoM). To evaluate the impact of oil related activities in the ocean floor sediments, we analyzed the geochemical (major and trace element, organic carbon and hydrocarbon concentrations) and biological (benthic foraminifera) composition of 62 superficial sediment samples, from 13 to 1336 m water depth. Cluster and Factor analysis of all the variables indicate that their distribution patterns are mainly controlled by differences between the terrigenous and carbonate platforms in the SGoM. Benthic foraminiferal assemblages were abundant and diverse, and their distribution patterns are mainly determined by water depth and sedimentary environment. However, most of the abundant species are opportunistic and/or low-oxygen tolerant, and many of their tests show oil stains and infillings, characteristic of oil polluted locations, suggesting the environment has been modified by natural seepage or oil-related activities. To determine if these conditions are natural or anthropogenic in origin, pre - industrial settings should be studied. Organic carbon (Corg) content (0.6-2.9%) and total hydrocarbon concentrations (PAHs 1.0-29.5 µg kg-1) were usually higher around the oil platforms area, the natural hydrocarbon seeps ("chapopoteras") area and offshore rivers, but there is no accumulation of oil related trace elements in these areas. However, the comparison with international sediment quality benchmarks indicates that Cd, Cr and Ni concentrations are above the threshold effect level, and also As, Ba and Cu are above the probable effect level benchmarks, which indicate that these element concentrations might be of potential ecological concern. Comprehensive studies involving different proxies, and assessing pre-industrial conditions, must be undertaken before assessing environmental health of marine benthic ecosystems.

Ecological Strategies Behind the Selection of Cultivable Actinomycete Strains from the Yucatan Peninsula for the Discovery of Secondary Metabolites with Antibiotic Activity.

The quest for novel natural products has recently focused on the marine environment as a source for novel microorganisms. Although isolation of marine-derived actinomycete strains is now common, understanding their distribution in the oceans and their adaptation to this environment can be helpful in the selection of isolates for further novel secondary metabolite discovery. This study explores the taxonomic diversity of marine-derived actinomycetes from distinct environments in the coastal areas of the Yucatan Peninsula and their adaptation to the marine environment as a first step towards novel natural product discovery. The use of simple ecological principles, for example, phylogenetic relatedness to previously characterized actinomycetes or seawater requirements for growth, to recognize isolates with adaptations to the ocean in an effort to select for marine-derived actinomycete to be used for further chemical studies. Marine microbial environments are an important source of novel bioactive natural products and, together with methods such as genome mining for detection of strains with biotechnological potential, ecological strategies can bring useful insights in the selection and identification of marine-derived actinomycetes for novel natural product discovery.

Drastic difference in cadmium concentration in mussels (Mytilus chilensis) observed between seasons in natural bed and aquaculture systems in Chile.

Globally, Chile is the second largest producer of mussels, with 99% of production concentrated in the inland sea of the Los Lagos Region, Southern Chile. This study reveals that seasons produce a drastic difference in the cadmium concentration (Cd) in marine mussels in bay, channel, and fjord ecosystems in this area. As the global mussel industry continues its rapid expansion, a complete understanding of cadmium pathways is critical in order to minimize the cadmium content in harvests. In this study, biweekly sampling was conducted in Chiloé (Southern Chile), during five consecutive seasons from June 2014 to November 2015. Cadmium in the soft tissues (ST) and in the content of the digestive gland (CDG) of Mytilus chilensis were investigated, in addition to resuspensions and seston to determine the effect of the seasons on metal bioassimilation capacity. In spring, the (Cd) between CDG and ST varied by approximately 2 mg Cd kg-1 dry mass (DM). In summer and autumn, the (Cd) in CDG increased from 3 to 6 mg Cd kg-1 while the (Cd) in ST decreased from 2.5 to 1.5 mg Cd kg-1 DM. The three ecosystems showed the same cadmium bioconcentration trends in all seasons, revealing coherent global trends. These findings should caution the industry and coastal populations about the seasonal variability and intensity of cadmium metal transfer to biofilters, especially because of the adverse effects of cadmium consumption on human health. Additionally, this study found that mussels in natural beds concentrate more Cd (> 1 mg Cd kg-1 DM) than in industrial facilities. Multiregression analysis showed and explained the cadmium in CDG for three ecosystems: channel (R2 0.9537), bay (R2 0.5962), and fjord (R2 0.4009). The independent variable nocturnal seston was able to explain the increase in cadmium.

An environmental magnetism approach to assess impacts of land-derived sediment disturbances on coral reef ecosystems (Cartagena, Colombia).

We used environmental magnetism methods to study recently deposited marine sediments from the estuarine ecosystems on the Caribbean coast of Colombia. Cartagena region has undergone an increasing sediment load during the last decades via sediment plumes from Magdalena River and its distributary man-made channel. Concentration dependent magnetic parameters show an increasing abundance of ferrimagnetic minerals on the uppermost sediments on sites located close to the continent (remanent magnetization SIRM = 5.4-9.5 × 10-3Am2 kg-1) as well as faraway sites (SIRM = 0.5-1.7 × 10-3Am2 kg-1 near Rosario Islands coral reef complex). The magnetic grain size and mineralogy along the cores are variable, showing the dominance of the magnetite-like minerals (remanent coercivity Hcr = 34.3-45.3 mT), with a minor contribution of high-coercivity minerals (Hcr = 472-588 mT). In addition, there is a moderate enrichment of elements Cu, Mo, and Zn (enrichment factor EF = 1.5-3.8) that indicates the additional land-derived contribution on sediments. The environmental magnetism approach, which shows significant signals of magnetic minerals and trace elements, is a reliable tool to prove the presence of continental sediment supply in coral reef ecosystems.

Up-flow anaerobic sediment trapped (UAST) reactor as a new configuration for the enrichment of anammox bacteria from marine sediments.

A novel reactor configuration for the enrichment of anammox bacteria from marine sediments was developed. Marine sediments were successfully kept inside the bioreactors during the enrichment process by strategically installing traps at different depths to prevent the wash-out of sediments. Three up-flow anaerobic sediment trapped (UAST) reactors were set up (α, ß and ω supplied with 50, 150 and 300mgCa2+/L, respectively). Nitrogen removal rates (NRR) of up to 3.5gN/L-d and removal efficiencies of >95% were reached. Calcium enhanced biomass production as evidenced by increased volatile suspended solids and extracellular polymeric substances. After the long-term operation, dominant families detected were Rhodobacteracea, Flavobacteracea, and Alteromonadacea, while the main anammox genera detected in the three reactors were Candidatus Kuenenia and Candidatus Anammoximicrobium. The UAST reactor is proposed as suitable technology for the enrichment of anammox bacteria applicable for the treatment of saline industrial wastewaters with high nitrogen content.

High Prevalence of Gammaproteobacteria in the Sediments of Admiralty Bay and North Bransfield Basin, Northwestern Antarctic Peninsula.

Microorganisms dominate most Antarctic marine ecosystems, in terms of biomass and taxonomic diversity, and play crucial role in ecosystem functioning due to their high metabolic plasticity. Admiralty Bay is the largest bay on King George Island (South Shetland Islands, Antarctic Peninsula) and a combination of hydro-oceanographic characteristics (bathymetry, sea ice and glacier melting, seasonal entrance of water masses, turbidity, vertical fluxes) create conditions favoring organic carbon deposition on the seafloor and microbial activities. We sampled surface sediments from 15 sites across Admiralty Bay (100-502 m total depth) and the adjacent North Bransfield Basin (693-1147 m), and used the amplicon 454-sequencing of 16S rRNA gene tags to compare the bacterial composition, diversity, and microbial community structure across environmental parameters (sediment grain size, pigments and organic nutrients) between the two areas. Marine sediments had a high abundance of heterotrophic Gammaproteobacteria (92.4% and 83.8% inside and outside the bay, respectively), followed by Alphaproteobacteria (2.5 and 5.5%), Firmicutes (1.5 and 1.6%), Bacteroidetes (1.1 and 1.7%), Deltaproteobacteria (0.8 and 2.5%) and Actinobacteria (0.7 and 1.3%). Differences in alpha-diversity and bacterial community structure were found between the two areas, reflecting the physical and chemical differences in the sediments, and the organic matter input.