Marine microbial communities in sediments play a critical role in ecosystems functioning and are the main drivers of biogeochemical cycling of carbon, nitrogen and sulfur. They constitute a huge biomass portion of the Earth, are highly taxonomic diverse, and responsible for the majority of metabolic activity in the ocean including the polar regions (Parkes et al., 2000; Azam and Malfatti, 2007; Fuhrman, 2012). Various physical and chemical parameters can affect the marine sediment communities, in particular the organic matter. Studies conducted in the sediments of the Ross Sea (Carr et al., 2013) and the Drake Passage at the Antarctic Polar Front (Ruff et al., 2014) showed that the organic matter can increase the microbial abundance based on phospholipids and DNA sequencing, respectively. The organic matter can also influence communities composed by lithotrophic and heterotrophic microbes, associated with degradation of organics from sinking particles and fecal pellets in the Western Antarctica (Learman et al., 2016).

As consequences of climate change and rapid warming registered in the Western Antarctic Peninsula, the sea ice declines and glacial inputs increase. These conditions can favor phytoplankton and seaweed blooms (Meredith and King, 2005; Clarke et al., 2007; Montes-Hugo et al., 2009; Pearson et al., 2015), which then increase the quantity of organic matter transported from surface layers to the ocean floor (ex. Ducklow et al., 2006; Gillies et al., 2012). The quantity and type of carbon could also be altered due to the introduction of more terrestrial carbon into marine and benthic environments in polar systems (Learman et al., 2016).

Admiralty Bay is the largest bay on King George Island (South Shetlands Archipelago), located within the maritime Antarctic region. It was designated an Antarctic Specially Managed Area no. 1 (ASMA 1) and contains the Antarctic Specially Protected Area 128 (ASPA 128, former SSSI N° 8). Its hydrology is complex, as it receives different contributions from water masses originating in the Bransfield Strait, and also from ice melt within the bay (Szafrański and Lipski, 1982). Depending on the bathymetry, regional water circulation, winds and seasonal regime, waters from the Bransfield Strait that penetrate the bay originate from the adjacent warm and low saline Bellingshausen Sea (normally in the summer) or the cold and saline waters of Weddell Sea (in the winter) (Gordon and Nowlin, 1978; Tokarczyk, 1987; Huneke et al., 2016). The fjord-like shape of Admiralty Bay reflects freshwater input due to a strong glacial influence and high water column turbidity caused by suspension of soft sediments (Pichlmaier et al., 2004). All this organic matter falls to the seafloor, providing organic carbon to the reservoir, which is partially consumed by the cold-adapted microorganisms from the sediments, leading to low oxygen conditions (Orcutt et al., 2011).

To understand how the contribution of the organic matter from different sources can shape the microbial community in the Admiralty Bay and surrounding areas, in this study the bacterial diversity and community structure of the sediment samples from Admiralty Bay (AB) and adjacent areas of North Bransfield Basin (NBB) were compared. The results contribute to a better understanding of the sequenced microbial community structure in this polar area, and show differences in richness and community structure between the two sites across physical-chemical characteristics of the sediments.

The present study compared the bacterial diversity and community structure found in marine sediments of AB and NBB. Although similar taxonomic groups were identified, mostly related to heterotrophic metabolism, their relative abundances, as well as alpha-diversity values and bacterial community structure have differed between the two sampling regions, possibly reflecting the distinct physical-chemical characteristics of the sediments (e.g., grain size, organic matter, chlorophyll, carbonate percentages, carbon and nitrogen concentrations). Additionally, hydro-oceanographic conditions such as bathymetry, regional water circulation and winds regime, continental inputs from glaciers melting, contribution of primary producers, seasonal entrance of water masses, and turbidity, might have favored the organic matter supply in the area. Indeed, a combination of physical and chemical parameters can influence the marine sediment community structure (e.g., Austen et al., 2002; Schauer et al., 2010; Zinger et al., 2011; Bienhold et al., 2012; Nguyen and Landfald, 2015; Learman et al., 2016).

The most abundant phylum (Proteobacteria) was represented by the classes Gamma-, Alpha- and Deltaproteobacteria in different proportions between the samples. This supports the results from previous studies showing that this phylum has a wide phylogenetic and phenotypic diversity in several marine and benthic environments (Sogin et al., 2006; Danovaro et al., 2010; Williams et al., 2010; Zinger et al., 2011; Lyra et al., 2013; Sun et al., 2013). Similarly, Proteobacteria has been reported from sediments of Antarctic Polar Front (Ruff et al., 2014), Ross Sea (Baldi et al., 2010) and Western Antarctic Peninsula (Learman et al., 2016).

The report of Psychrobacter as the dominant genus of Gammaproteobacteria in Antarctic sediments has not been previously reported. These heterotrophic versatile microorganisms include piezophilic and halophilic species (Kawasaki et al., 2002; Nogi et al., 2002), found in shallow and deep-sea sediments, in the water column, as part of the fish and krill microbiomes, and in association with brown macroalgae (Bozal et al., 2003; Lee et al., 2006; Dang et al., 2008; Teske et al., 2011; Tropeano et al., 2012). Most species of this genus can adapt to cold conditions, such as polar permafrost and ice, and are capable of reproducing at temperatures ranging from -10°C to 40°C (Rodrigues et al., 2009). They often produce low temperature-adaptive lipases (Bozal et al., 2003; Yumoto et al., 2003) and play essential roles in fat decomposition reactions (Li et al., 2013). The polar environment therefore constitutes an ecological niche for Psychrobacter strains (Bozal et al., 2003).

Alphaproteobacterial sequences affiliated with Loktanella, Litorimicrobium and Hoefla, which play a common role in the nitrogen cycle, were present in all samples, although in higher relative abundances outside the bay. They are able to reduce nitrate to nitrite, degrade aromatic compounds, and oxidize sulfur, ammonia, carbon monoxide, iron and manganese (Van Trappen et al., 2004a; Ivanova et al., 2005; Jin et al., 2011; Jung et al., 2013).

Flavobacteria, generally a major clade of Bacteroidetes in marine environments, was the third most abundant class. The clade was mainly represented by the genus Gillisia, specifically in samples 15 (4.1%) and 13 (3.4%), both located in NBB. Several strains have previously been isolated from marine and polar environments, such as Antarctic lakes (Van Trappen et al., 2004b). These heterotrophs are especially important, as they break down complex organic matter using exoenzymes to degrade algal cells and algal-detrital particles (Kirchman, 2002; Gómez-Pereira et al., 2012; Teeling et al., 2012). In fact, the presence of such distinct taxa in particular at stations placed outside the bay, indicated by alpha-diversity and community composition, helps to explain the differences in bacterial community between the two study areas.

The microbial community composition found in AB and NBB showed the prevalence of microorganisms related to heterotrophic metabolism. High relative abundance of heterotrophs can be explained by the high input of organic matter from different sources. The phytodetritus in deep sediments are allochthonous and can be derived from microphytobenthos, abundant in shallow areas of AB, or deposition of phytoplankton and macroalgae fragments (Gheller, 2014). Macroalgae can cover about 30% of the seabed surface of AB, and produce ca. 74,000 tons of wet biomass (Zielinski, 1990; Nedzarek and Rakusa-Suszczewski, 2004), resulting in substrates that favor the presence of these heterotrophs.

Phytoplankton blooms can also contribute to the transfer of organic matter to the seafloor. They normally occur in the study area during the austral summer due to the advection of continental input of ice melts (Nedzarek, 2008) and coastal upwelling (Brandini and Rebello, 1994; Schloss et al., 2002). This rapidly transported phytoplankton-derived organic carbon will be recycled by heterotrophic microbes fueling a diverse microbial community in deep sediments, as previously shown by Ruff et al. (2014). Moreover, the system of currents in the study area, influenced by the warmer and less saline waters from Bellingshausen Sea and the cold and saline waters from Weddell, may also indirectly influence microbial composition. The currents within the bay are more intense when compared to those in the inlets (Gordon and Nowlin, 1978; Huneke et al., 2016), thus enabling the transport, deposition and homogenization of the organic matter that drives the heterotrophic bacterial community.

The next generation DNA sequencing data of 15 samples of seafloor sediments provides the first results characterizing the sediment microbial communities of the Northwestern Antarctic Peninsula, in a transect from AB to the NBB. Our study revealed high prevalence of heterotrophic gammaproteobacterial phylotypes, and differences in bacterial diversity and community structure between the two sites. A combination of conditions that favor the organic matter input, like regional water circulation and winds regime, bathymetry, continental influence from glaciers melting, inputs from primary producers of the euphotic zone or continental areas, water masses carrying nutrients, besides the grain size and sediment characteristics (organic matter, carbon and nitrogen concentrations), may contribute to shape the marine sediment communities in Antarctica.

DF, RD, and CS analyzed data and prepared figures and tables. All authors contributed to writing the paper.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.