Description: Colonization of newly ice-free areas by marine benthic organisms intensifies burial of macroalgae detritus in Potter Cove coastal surface sediments (Western Antarctic Peninsula). Thus, fresh and labile macroalgal detritus serves as primary organic matter (OM) source for microbial degradation. Here, we investigated the effects on post-depositional microbial iron reduction in Potter Cove using sediment incubations amended with pulverized macroalgal detritus as OM source, acetate as primary product of OM degradation and lepidocrocite as reactive iron oxide to mimic in situ conditions. Humic substances analogue anthraquinone-2,6-disulfonic acid (AQDS) was also added to some treatments to simulate potential for electron shuttling. Microbial iron reduction was promoted by macroalgae and further enhanced by up to 30-folds with AQDS. Notably, while acetate amendment alone did not stimulate iron reduction, adding macroalgae alone did. Acetate, formate, lactate, butyrate and propionate were detected as fermentation products from macroalgae degradation. By combining 16S rRNA gene sequencing and RNA stable isotope probing, we reconstructed the potential microbial food chain from macroalgae degraders to iron reducers. Psychromonas, Marinifilum, Moritella, and Colwellia were detected as potential fermenters of macroalgae and fermentation products such as lactate. Members of class deltaproteobacteria including Sva1033, Desulfuromonas, and Desulfuromusa together with Arcobacter (former phylum Epsilonbacteraeota, now Campylobacterota) acted as dissimilatory iron reducers. Our findings demonstrate that increasing burial of macroalgal detritus in an Antarctic fjord affected by glacier retreat intensifies early diagenetic processes such as iron reduction. Under scenarios of global warming, the active microbial populations identified above will expand their environmental function, facilitate OM remineralisation, and contribute to an increased release of iron and CO2 from sediments. Such indirect consequences of glacial retreat are often overlooked but might, on a regional scale, be relevant for the assessment of future nutrient and carbon fluxes.

Description: Iron (Fe) fluxes from reducing sediments and subglacial environments are potential sources of bioavailable Fe into the Southern Ocean. Stable Fe isotopes (δ56Fe ) are considered a proxy for Fe sources and reaction pathways, but respective data are scarce and Fe cycling in complex natural environments is not understood sufficiently to constrain respective δ56Fe “endmembers” for different types of sediments, environmental conditions, and biogeochemical processes. We present δ56Fe data from pore waters and sequentially extracted sedimentary Fe phases of two contrasting sites in Potter Cove (King George Island, Antarctic Peninsula), a bay that is affected by fast glacier retreat. Sediments close to the glacier front contain more easily reducible Fe oxides and pyrite and show a broader ferruginous zone, compared to sediments close to the icefree coast, where surficial oxic meltwater streams discharge into the bay. Pyrite in sediments close to the glacier front predominantly derives from eroded bedrock. For the high amount of easily reducible Fe oxides proximal to the glacier we suggest mainly subglacial sources, where Fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering processes (likely pyrite oxidation or dissimilatory iron reduction, DIR). Our strongest argument for a subglacial source of the highly reactive Fe pool in sediments close to the glacier front is its predominantly negative δ56Fe signature that remains constant over the whole ferruginous zone. This implies in situ DIR does not significantly alter the stable Fe isotope composition of the accumulated Fe oxides. The nonetheless overall light δ56Fe signature of easily reducible Fe oxides suggests pre-depositional microbial cycling as it occurs in potentially anoxic subglacial environments. The strongest 56Fe-depletion in pore water and most reactive Fe oxides was observed in sediments influenced by oxic meltwater discharge. The respective site showed a condensed redox zonation and a pore water δ56Fe profile typical for in-situ Fe cycling. We demonstrate that the potential of pore water δ56Fe as a proxy for benthic Fe fluxes is not straight-forward due to its large variability in marine shelf sediments at small spatial scales (- 2.4‰ at the site proximal to oxic meltwater discharge vs. -0.9‰ at the site proximal to the marine glacier terminus, both at 2 cm sediment depth). The controlling factors are multifold and include the amount and reactivity of reducible Fe oxides and organic matter, the isotopic composition of the primary and secondary ferric substrates, sedimentation rates, and physical reworking (bioturbation, ice scraping). The application of δ56Fe geochemistry may prove valuable in investigating biogeochemical weathering and Fe cycling in subglacial environments. This requires, however (similarly to the use of δ56Fe for the quantification of benthic fluxes), that the spatial and temporal variability of the isotopic endmember is known and accounted for. Since geochemical data from subglacial environments are very limited, further studies are needed in order to sufficiently assess Fe cycling and fractionation at glacier beds and the composition of discharges from those areas.

Description: Iron stable isotope signatures (δ56Fe) in hemolymph (bivalve blood) of the Antarctic bivalve Laternula elliptica were analyzed by Multiple Collector - Inductively Coupled Plasma - Mass Spectrometry (MC-ICP-MS) to test whether the isotopic fingerprint can be tracked back to the predominant sources of the assimilated Fe. An earlier investigation of Fe concentrations in L. elliptica hemolymph suggested that an assimilation of reactive and bioavailable Fe (oxyhydr)oxide particles (i.e. ferrihydrite), precipitated from pore water Fe around the benthic boundary, is responsible for the high Fe concentration in L. elliptica (Poigner et al., 2013b). At two stations in Potter Cove (King George Island, Antarctica) bivalve hemolymph showed mean δ56Fe values of −1.19 ± 0.34‰ and -1.04 ± 0.39‰, respectively, which is between 0.5‰ and 0.85‰ lighter than the pool of easily reducible Fe (oxyhydr)oxides of the surface sediments (−0.3‰ to −0.6‰). This is in agreement with the enrichment of lighter Fe isotopes at higher trophic levels, resulting from the preferential assimilation of light isotopes from nutrition. Nevertheless, δ56Fe hemolymph values from both stations showed a high variability, ranging between −0.21‰ (value close to unaltered/primary Fe(oxyhydr)oxide minerals) and −1.91‰ (typical for pore water Fe or diagenetic Fe precipitates), which we interpret as a “mixed” δ56Fe signature caused by Fe assimilation from different sources with varying Fe contents and δ56Fe values. Furthermore, mass dependent Fe fractionation related to physiological processes within the bivalve cannot be ruled out. This is the first study addressing the potential of Fe isotopes for tracing back food sources of bivalves.

Description: Antarctic shelf regions are potential carbon and nutrient cycling hotspots where rapid climatic changes are projected to affect seasonal sea ice cover, water column stratification, and thus surface primary production and associated fluxes of organic carbon to the seafloor. Here, we report on surface sediment oxygen profiles and respective fluxes in combination with pore water profiles of dissolved iron (DFe) and phosphate (PO43-) from 7 stations along a 400 mile transect with variable sea ice cover and water column stratification from the East Antarctic Peninsula to the west of South Orkney Islands. Our results show that sea ice concentrations and stratification of the upper water column decreased across the transect. We defined a marginal sea ice index of 5-35% sea ice cover which was positively correlated with the benthic carbon mineralization rate. C-mineralization rates increased gradually between the heavy ice-covered station and the marginal sea ice stations from 1.1 to 7.3 mmol C m-2 d-1, respectively. The rates decreased again to 1.8 mmol C m-2 d-1 at the ice-free station, likely attributed to a deeper water column mixed layer depth, which decreases primary production and thus organic carbon export to the sediment. Iron cycling in the sediment was elevated at the marginal sea ice stations where Fe-reduction led to DFe fluxes in the pore water of up to 0.379 mmol DFe m-2 d-1, while moderate (0.068 mmol DFe m-2 d-1) and negligible fluxes were observed at ice-free and ice-covered stations, respectively. In pore waters, concentrations of DFe and PO43- were significantly correlated with almost identical flux ratios of 0.33 mol PO43- per mol DFe for most of the stations, indicating a strong control of the iron cycling on the phosphate release to the water column. The high benthic DFe and PO43- fluxes highlight the importance of sediments underlying the marginal ice zone as source for limiting nutrients to the shelf waters.