Combining voltammetric and mass spectrometric data to evaluate iron organic speciation in subsurface coastal seawater samples of the Ross sea (Antarctica).

Iron (Fe) is the most important trace element in the ocean, as it is required by phytoplankton for photosynthesis and nitrate assimilation. Iron speciation is important to better understand the biogeochemical cycle and availability of this micronutrient, in particular in the Southern Ocean. Dissolved Fe (dFe) concentration and speciation were determined in 24 coastal subsurface seawater samples collected in the western Ross sea (Antarctica) during the austral summer 2017 as part of the CELEBeR (CDW Effects on glacial mElting and on Bulk of Fe in the Western Ross sea) project. ICP-DRC-MS was used for dFe determination, whereas CLE-AdSV was used to obtain the concentration of complexed and free dFe, of the ligands, and the values of the stability constants of the complexes. Dissolved Fe values ranged from 0.4 to 2.5 nM and conditional stability constant (logK'Fe'L) from 13.0 to 15.0, highlighting the presence of Fe-binding organic complexes of different stabilities. Principal component analysis (PCA) allowed us to point out that Terra Nova Bay and the neighboring area of Aviator and Mariner Glaciers were different in terms of chemical, physical, and biological parameters. A qualitative investigation on the nature of the organic ligands was carried out by HPLC-ESI-MS/MS. Results showed that siderophores represented a heterogeneous class of organic ligands pool.

Bacterioplankton Diversity and Distribution in Relation to Phytoplankton Community Structure in the Ross Sea Surface Waters.

Primary productivity in the Ross Sea region is characterized by intense phytoplankton blooms whose temporal and spatial distribution are driven by changes in environmental conditions as well as interactions with the bacterioplankton community. However, the number of studies reporting the simultaneous diversity of the phytoplankton and bacterioplankton in Antarctic waters are limited. Here, we report data on the bacterial diversity in relation to phytoplankton community structure in the surface waters of the Ross Sea during the Austral summer 2017. Our results show partially overlapping bacterioplankton communities between the stations located in the Terra Nova Bay (TNB) coastal waters and the Ross Sea Open Waters (RSOWs), with a dominance of members belonging to the bacterial phyla Bacteroidetes and Proteobacteria. In the TNB coastal area, microbial communities were characterized by a higher abundance of sequences related to heterotrophic bacterial genera such as Polaribacter spp., together with higher phytoplankton biomass and higher relative abundance of diatoms. On the contrary, the phytoplankton biomass in the RSOW were lower, with relatively higher contribution of haptophytes and a higher abundance of sequences related to oligotrophic and mixothrophic bacterial groups like the Oligotrophic Marine Gammaproteobacteria (OMG) group and SAR11. We show that the rate of diversity change between the two locations is influenced by both abiotic (salinity and the nitrogen to phosphorus ratio) and biotic (phytoplankton community structure) factors. Our data provide new insight into the coexistence of the bacterioplankton and phytoplankton in Antarctic waters, suggesting that specific rather than random interaction contribute to the organic matter cycling in the Southern Ocean.

Phytoplankton blooms during austral summer in the Ross Sea, Antarctica: Driving factors and trophic implications.

During the austral summer of 2014, an oceanographic cruise was conducted in the Ross Sea in the framework of the RoME (Ross Sea Mesoscale Experiment) Project. Forty-three hydrological stations were sampled within three different areas: the northern Ross Sea (RoME 1), Terra Nova Bay (RoME 2), and the southern Ross Sea (RoME 3). The ecological and photophysiological characteristics of the phytoplankton were investigated (i.e., size structure, functional groups, PSII maximum quantum efficiency, photoprotective pigments), as related to hydrographic and chemical features. The aim was to identify the mechanisms that modulate phytoplankton blooms, and consequently, the fate of organic materials produced by the blooms. The observed biomass standing stocks were very high (e.g., integrated chlorophyll-a up to 371 mg m-2 in the top 100 m). Large differences in phytoplankton community composition, relative contribution of functional groups and photosynthetic parameters were observed among the three subsystems. The diatoms (in different physiological status) were the dominant taxa in RoME 1 and RoME 3; in RoME 1, a post-bloom phase was identified, whereas in RoME 3, an active phytoplankton bloom occurred. In RoME 2, diatoms co-occurred with Phaeocystis antarctica, but were vertically segregated by the upper mixed layer, with senescent diatoms dominating in the upper layer, and P. antarctica blooming in the deeper layer. The dominance of the phytoplankton micro-fraction over the whole area and the high Chl-a suggested the prevalence of non-grazed large cells, independent of the distribution of the two functional groups. These data emphasise the occurrence of significant temporal changes in the phytoplankton biomass in the Ross Sea during austral summer. The mechanisms that drive such changes and the fate of the carbon production are probably related to the variations in the limiting factors induced by the concurrent hydrological modifications to the Ross Sea, and they remain to be fully clarified. The comparison of conditions observed during summer 2014 and those reported for previous years reveal considerably different ecological assets that might be the result of current climate change. This suggests that further changes can be expected in the future, even at larger oceanic scales.

Using proteolytic enzymes to assess metal bioaccessibility in marine sediments.

In this work we tested two proteolytic enzymes (trypsin and pepsin) to evaluate metal bioaccessibility in marine sediments. Some working conditions were studied, in particular incubation time, enzyme concentration and solution/solid ratio, and metals release mechanisms were investigated using both untreated and denatured enzymes. The results obtained with trypsin and pepsin were compared with each other and with results from proteinase K extraction, carried out on the same sediment samples, following a procedure reported in the literature. Moreover, a comparison with the first step of BCR sequential procedure was made. Metal orders of mobility determined with the different enzymes and with acetic acid were compared with the one defined by the enrichment factors in the gut fluids of deposit-feeder organisms. The overall picture suggests that proteinase K has a better capability of reproducing natural phenomena and is therefore more suitable to determine metal bioaccessibility in marine sediments, when the target of the investigation is deposit-feeder organisms.

Simultaneous determination of CFC-11, CFC-12 and CFC-113 in seawater samples using a purge and trap gas-chromatographic system.

We have optimized the analytical parameters of a homemade instrument for the simultaneous measurement of the chlorofluorocarbons CCl(2)F(2) (CFC-12), CCl(3)F (CFC-11) and C(2)Cl(3)F(3) (CFC-113) in seawater. Seawater samples are flame sealed into 60 ml glass ampoules avoiding any contact with the atmosphere and stored in cold, dark condition until analysis. In the laboratory, after cracking the ampoule in an enclosed chamber filled with ultra-pure nitrogen, the seawater sample is transferred to a stripping chamber, where ultra-pure nitrogen is used to purge the dissolved CFCs from the seawater. The extracted gases are then cryogenically trapped, subsequently the trap is isolated and heated and the CFCs are transferred by a carrier gas stream into a precolumn and then are separated on a gaschromatographic packed column. To separate adequately CFC-12 from N(2)O, during the early part of the chromatographic run, the gas stream passes through a molecular sieve, which is then isolated and backflushed. The CFCs are detected on an electron capture detector ((63)Ni ECD). After a careful choice of the experimental conditions, the performances of the system were evaluated. The detection limits for seawater samples are: 0.0081 pmol kg(-1) for CFC-12, 0.0073 pmol kg(-1) for CFC-11 and 0.0043 pmol kg(-1) for CFC-113. The reproducibility of replicate samples lies within 5% for the three CFCs. The system has been successfully employed for CFC measurements in seawater samples collected in the Ross Sea (Antarctica) in the framework of the Italian Antarctic research project.