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The aim of this study was to describe the microzooplankton composition under the sea ice in the Ross Sea (Antarctica) and its possible role as seed for the open-water community. We compared the under-sea-ice data to the communities present in the ice-free waters and at the retreating ice edge in order to test the hypothesis that the microzooplankton composition and abundance depend on the temporal development of the ecosystem. In December 2011, microzooplankton were sampled through two boreholes in sea ice, and 1 month later, in nearby stations that had been free from ice for a short time. The data from these samples were compared with a previous campaign (January 2011) when all of the area had been free of ice for a longer period. Our data indicate that under the sea ice the communities were very different from those of the ice-free waters, both in terms of abundance and diversity, with the prevalence of heterotrophic dinoflagellates and aloricate ciliates. Tintinnids were almost absent below the sea ice, and their abundance increased during the sampling at the intermediate depths. The present study provides evidence for the effect of the environment on the microzooplankton distributions, and indicates that the microzooplankton presence mainly depends on the sampling period. The community under the sea ice appears not to seed the open water community once the ice has retreated. The presence of the ice appears to influence the whole water-column production, which might thus not feed the deeper communities. The most important constraint appears to be the time that has lapsed from the presence of ice cover to the ice-free period. [ABSTRACT FROM AUTHOR]

Abstract: The identification of bacterial community structure has led, since the beginning of the 1990s, to the idea that bacterioplankton populations are stratified in the water column and that diverse lineages with mostly unknown phenotypes dominate marine microbial communities. The diversity of depth-related assemblages is also reflected in their patterns of activities, as bacteria affiliated to different groups can express different activities in a given ecosystem. We analysed bacterial assemblages (DGGE fingerprinting) and their activities (prokaryotic carbon production, protease, phosphatase, chitinase, beta-glucosidase and lipase activities) in two areas in the Ross Sea, differing mainly in their productivity regime: two stations are located in the Terra Nova Bay polynya area (highly productive during summer) and two close to Cape Adare (low phytoplankton biomass and activity). At every station a pronounced stratification of bacterial assemblages was identified, highlighting epipelagic communities differing substantially from the mesopelagic and the bathypelagic communities. Multivariate analysis suggested that pressure and indirectly light-affected variables (i.e. oxygen and fluorescence) had a great effect on the bacterial communities outcompeting the possible influences of temperature and dissolved organic carbon concentration. Generally activities decreased with depth even though a signal of the Circumpolar Deep Water (CDW) at one of the northern stations corresponded to an increase in some of the degradative activities, generating some ‘hot spots’ in the profile. We also found that similar assemblages express similar metabolic requirements reflected in analogous patterns of activity (similar degradative potential and leucine uptake rate). Furthermore, the presence of eukaryotic chloroplasts’ 16S rDNA in deep samples highlighted how in some cases the dense surface-water formation (in this case High Salinity Shelf Water—HSSW) and downwelling can affect, at least for some phylotypes, the bacterial (16S rDNA based) community structure of the dark ocean. [Copyright &y& Elsevier]

Deep-sea environments host the largest pool of microbes and represent the last largely unexplored and poorly known ecosystems on Earth. The Ross Sea is characterized by unique oceanographic dynamics and harbors several water masses deeply involved in cooling and ventilation of deep oceans. In this study the V9 region of the 18S rDNA was targeted and sequenced with the Ion Torrent high-throughput sequencing technology to unveil differences in protist communities (>2 μm) correlated with biogeochemical properties of the water masses. The analyzed samples were significantly different in terms of environmental parameters and community composition outlining significant structuring effects of temperature and salinity. Overall, Alveolata (especially Dinophyta), Stramenopiles and Excavata groups dominated mesopelagic and bathypelagic layers, and protist communities were shaped according to the biogeochemistry of the water masses (advection effect and mixing events). Newly-formed High Salinity Shelf Water (HSSW) was characterized by high relative abundance of phototrophic organisms that bloom at the surface during the austral summer. Oxygen-depleted Circumpolar Deep Water (CDW) showed higher abundance of Excavata, common bacterivores in deep water masses. At the shelf-break, Antarctic Bottom Water (AABW), formed by the entrainment of shelf waters in CDW, maintained the eukaryotic genetic signature typical of both parental water masses. [ABSTRACT FROM AUTHOR]