Depth and intensity of the sulfate-methane transition zone control sedimentary molybdenum and uranium sequestration in a eutrophic low-salinity setting

Molybdenum (Mo) and uranium (U) contents in sedimentary archives are often used to reconstruct past changes in seafloor oxygenation. However, their sequestration processes are as yet poorly constrained in low-salinity coastal waters, which often suffer from anthropogenic eutrophication but only mild oxygen depletion. Due to the consequent lack of robust long-term paleo-redox reconstructions in such settings often characterized by a shallow front of dissolved sulfide accumulation within the sediment pore waters, inadequate understanding of the long-term drivers behind oxygen loss impedes cost-effective mitigation of this environmental problem. Here, we investigate the mechanisms of Mo and U sequestration in an oxic, low-salinity coastal setting in the northern Baltic Sea where anthropogenic eutrophication over the 20th century has resulted in formation of a shallow sulfate-methane transition zone (SMTZ) in the sediment column of this brackish-water basin. Our results demonstrate remarkably similar patterns for authigenic Mo and U sequestration, whereby the depth and intensity of the SMTZ exerts a first-order control on their solid-phase uptake. Sequential extraction analysis suggests that a large part of the authigenic Mo pool is hosted by refractory Fe-S phases such as pyrite and nanoscale FeMoS4, implying that the Fe-sulfide pathway is the dominating process of authigenic Mo scavenging. However, we also observe a pool of extremely labile Mo deep within the SMTZ, which might record an intermediate phase in authigenic Mo sequestration and/or partial switch to the organic matter (OM) pathway at low dissolved Fe levels. Authigenic U resides in acid-extractable and refractory phases, likely reflecting uptake into poorly crystalline monomeric U(IV) and crystalline uraninite, respectively. Similarly to Mo, authigenic U uptake is active at two fronts within the SMTZ, paralleled by increases in dissolved sulfide levels, suggesting coupling between sulfide production and U reduction. Our results imply that both Mo and U could provide viable proxies for mild bottom water deoxygenation in these settings, through the indirect link between seafloor oxygen conditions and the depth of SMTZ. Of these, Mo appears to more robustly capture variations in seafloor oxygen levels due to the significantly higher share of the authigenic pool. However, temporal resolution of these proxies is limited by the vertical offset between seafloor and the zone of authigenic uptake, and the superimposed character of the signal at a given depth due to vertical migrations of the SMTZ. These results have important implications for the use of Mo and U as paleo-redox proxies in other low-salinity coastal settings exposed to eutrophication.Peer reviewe

Terrestrial organic matter input drives sedimentary trace metal sequestration in a human-impacted boreal estuary

Coastal sediments play a fundamental role in processing anthropogenic trace metal inputs. Previous studies have shown that terrestrial organic matter (OM) is a significant vector for trace metal transport across the land-to-sea continuum, but little is known about the fate of land-derived metal-OM complexes in coastal sediments. Here, we use a comprehensive set of sediment pore water and solid-phase analyses to investigate how variations in terrestrial OM delivery since the 1950s have influenced trace metal accumulation and diagenesis in a human-impacted boreal estuary in the northern Baltic Sea. A key feature of our dataset is a strong correlation between terrestrial OM deposition and accumulation of metal-OM complexes in the sediments. Based on this strong coupling, we infer that the riverine input of terrestrial metal-OM complexes from the hinterland, followed by flocculation-induced settling in the estuary, effectively modulates sedimentary trace metal sequestration. While part of the trace metal pool associated with these complexes is efficiently recycled in the surface sediments during diagenesis, a substantial fraction is permanently buried as refractory metal-OM complexes or through incorporation into insoluble sulfides, thereby escaping further biological processing. These findings suggest that terrestrial OM input could play a more pivotal role in trace metal processing in coastal environments than hitherto acknowledged. (c) 2020 The Authors. Published by Elsevier B.V.Peer reviewe

16S and 18S rRNA Gene Metabarcoding Provide Congruent Information on the Responses of Sediment Communities to Eutrophication

Metabarcoding analyses of bacterial and eukaryotic communities have been proposed as efficient tools for environmental impact assessment. It has been unclear, however, to which extent these analyses can provide similar or differing information on the ecological status of the environment. Here, we used 16S and 18S rRNA gene metabarcoding to compare eutrophication-induced shifts in sediment bacterial and eukaryotic community structure in relation to a range of porewater, sediment and bottom-water geochemical variables, using data obtained from six stations near a former rainbow trout farm in the Archipelago Sea (Baltic Sea). Shifts in the structure of both community types were correlated with a shared set of variables, including porewater ammonium concentrations and the sediment depth-integrated oxygen consumption rate. Distance-based redundancy analyses showed that variables typically employed in impact assessments, such as bottom water nutrient concentrations, explained less of the variance in community structure than alternative variables (e.g., porewater NH4+ inventories and sediment depth-integrated O2 consumption rates) selected due to their low collinearity (up to 40 vs. 58% of the variance explained, respectively). In monitoring surveys where analyses of both bacterial and eukaryotic communities may be impossible, either 16S or 18S rRNA gene metabarcoding can serve as reliable indicators of wider ecological impacts of eutrophication.Peer reviewe

The influence of labile and refractory C-org on benthic foraminifera: a laboratory mesocosm study

Abstrac

16S rRNA Gene Metabarcoding Indicates Species-Characteristic Microbiomes in Deep-Sea Benthic Foraminifera

Foraminifera are unicellular eukaryotes that are an integral part of benthic fauna in many marine ecosystems, including the deep sea, with direct impacts on benthic biogeochemical cycles. In these systems, different foraminiferal species are known to have a distinct vertical distribution, i.e., microhabitat preference, which is tightly linked to the physico-chemical zonation of the sediment. Hence, foraminifera are well-adapted to thrive in various conditions, even under anoxia. However, despite the ecological and biogeochemical significance of foraminifera, their ecology remains poorly understood. This is especially true in terms of the composition and diversity of their microbiome, although foraminifera are known to harbor diverse endobionts, which may have a significant meaning to each species' survival strategy. In this study, we used 16S rRNA gene metabarcoding to investigate the microbiomes of five different deep-sea benthic foraminiferal species representing differing microhabitat preferences. The microbiomes of these species were compared intra- and inter-specifically, as well as with the surrounding sediment bacterial community. Our analysis indicated that each species was characterized with a distinct, statistically different microbiome that also differed from the surrounding sediment community in terms of diversity and dominant bacterial groups. We were also able to distinguish specific bacterial groups that seemed to be strongly associated with particular foraminiferal species, such as the family Marinilabiliaceae for Chilostomella ovoidea and the family Hyphomicrobiaceae for Bulimina subornata and Bulimina striata. The presence of bacterial groups that are tightly associated to a certain foraminiferal species implies that there may exist unique, potentially symbiotic relationships between foraminifera and bacteria that have been previously overlooked. Furthermore, the foraminifera contained chloroplast reads originating from different sources, likely reflecting trophic preferences and ecological characteristics of the different species. This study demonstrates the potential of 16S rRNA gene metabarcoding in resolving the microbiome composition and diversity of eukaryotic unicellular organisms, providing unique in situ insights into enigmatic deep-sea ecosystems.Peer reviewe

Depth and intensity of the sulfate-methane transition zone control sedimentary molybdenum and uranium sequestration in a eutrophic low-salinity setting

Molybdenum (Mo) and uranium (U) contents in sedimentary archives are often used to reconstruct past changes in seafloor oxygenation. However, their sequestration processes are as yet poorly constrained in low-salinity coastal waters, which often suffer from anthropogenic eutrophication but only mild oxygen depletion. Due to the consequent lack of robust long-term paleo-redox reconstructions in such settings often characterized by a shallow front of dissolved sulfide accumulation within the sediment pore waters, inadequate understanding of the long-term drivers behind oxygen loss impedes cost-effective mitigation of this environmental problem. Here, we investigate the mechanisms of Mo and U sequestration in an oxic, low-salinity coastal setting in the northern Baltic Sea where anthropogenic eutrophication over the 20th century has resulted in formation of a shallow sulfate-methane transition zone (SMTZ) in the sediment column of this brackish-water basin. Our results demonstrate remarkably similar patterns for authigenic Mo and U sequestration, whereby the depth and intensity of the SMTZ exerts a first-order control on their solid-phase uptake. Sequential extraction analysis suggests that a large part of the authigenic Mo pool is hosted by refractory Fe–S phases such as pyrite and nanoscale FeMoS4, implying that the Fe-sulfide pathway is the dominating process of authigenic Mo scavenging. However, we also observe a pool of extremely labile Mo deep within the SMTZ, which might record an intermediate phase in authigenic Mo sequestration and/or partial switch to the organic matter (OM) pathway at low dissolved Fe levels. Authigenic U resides in acid-extractable and refractory phases, likely reflecting uptake into poorly crystalline monomeric U(IV) and crystalline uraninite, respectively. Similarly to Mo, authigenic U uptake is active at two fronts within the SMTZ, paralleled by increases in dissolved sulfide levels, suggesting coupling between sulfide production and U reduction. Our results imply that both Mo and U could provide viable proxies for mild bottom water deoxygenation in these settings, through the indirect link between seafloor oxygen conditions and the depth of SMTZ. Of these, Mo appears to more robustly capture variations in seafloor oxygen levels due to the significantly higher share of the authigenic pool. However, temporal resolution of these proxies is limited by the vertical offset between seafloor and the zone of authigenic uptake, and the superimposed character of the signal at a given depth due to vertical migrations of the SMTZ. These results have important implications for the use of Mo and U as paleo-redox proxies in other low-salinity coastal settings exposed to eutrophication.</p

The ‘mica sandwich’; a remarkable new genus of Foraminifera (Protista, Rhizaria) from the Nazaré Canyon (Portuguese margin, NE Atlantic)

Based on morphological and molecular characteristics, we describe a new genus and species of monothalamous agglutinated foraminifera, Capsammina patelliformis, that occurs mainly at bathyal (1000-3400m) water depths in the Nazaré Canyon off Portugal. The test is strongly flattened, up to 500?m or more in maximum dimension, and 30-80?m thick. It lacks obvious apertures, and is typically composed of 2-3 large, plate-like grains of mica that form the upper and lower surface of the test; these are separated by fine-grained, white agglutinated material (‘mortar’) forming a ring around the cell body. The cytoplasm, visible through the mica plates, is whitish in colour with few obvious inclusions.Analysis of a fragment of the SSUrDNA gene indicates thatC. patelliformis belongs in a clade branching with Crithionina delacai, C. granum and an undetermined crithioninid species. However, the divergences between the new species and these Crithionina species range from 20% to 21%, and are therefore too high to classify it in the same genus. We also transfer a previously described species, Psammosphaera bowmanni Heron-Allen and Earland 1912, to Capsammina based on its use of mica flakes in test construction. Other monothalamous agglutinated foraminifera, including Psammosphera spp., are phylogenetically distant from Capsammina. The new species occupies a shallow infaunal microhabitat, living mainly in the top 0.5cm of sediment

The 'mica sandwich'; a remarkable new genus of Foraminifera (Protista, Rhizaria) from the Nazaré Canyon (Portuguese margin, NE Atlantic)

Based on morphological and molecular characteristics, we describe a new genus and species of monothalamous agglutinated foraminifera, Capsammina patelliforms, that occurs mainly at bathyal (1000-3400m) water depths in the Nazare Canyon off Portugal The test is strongly flattened, up to 500 mu m or more in maximum dimension, and 30-80 mu m thick It lacks obvious apertures. and is typically composed of 2-3 large, plate-like grains of mica that form the upper and lower surface of the test, these are separated by fine-grained, white agglutinated material ('mortar') forming a ring around the cell body The cytoplasm. visible through the mica plates, is whitish in colour with few obvious inclusions Analysis of a fragment of the SSUrDNA gene indicates that C patelliformis belongs in a chide branching with Crithionina delacai, C granum and an Undetermined crithioninid species However, the divergences between the new species and these Crithionina species range from 20% to 21%. and are therefore too high to classify it in the same genus We also transfer a previously described species. Psammosphaera bowmanni Heron-Allen and Earland 1912, to Capsammina based on its use of mica flakes in test construction Other monothalamous agglutinated foraminifera. including Psammosphera spp., are phylogenetically distant from Capsammina The new species occupies a shallow infaunal microhabitat, living mainly in the top 0 5cm of sediment

Metabarcoding insights into the trophic behavior and identity of intertidal benthic foraminifera

Foraminifera are ubiquitous marine protists with an important role in the benthic carbon cycle. However, morphological observations often fail to resolve their exact taxonomic placement and there is a lack of field studies on their particular trophic preferences. Here, we propose the application of metabarcoding as a tool for the elucidation of the in situ feeding behavior of benthic foraminifera, while also allowing the correct taxonomic assignment of the feeder, using the V9 region of the 18S (small subunit; SSU) rRNA gene. Living foraminiferal specimens were collected from two intertidal mudflats of the Wadden Sea and DNA was extracted from foraminiferal individuals and from the surrounding sediments. Molecular analysis allowed us to confirm that our foraminiferal specimens belong to three genetic types: Ammonia sp. T6, Elphidium sp. S5 and Haynesina sp. S16. Foraminiferal intracellular eukaryote communities reflected to an extent those of the surrounding sediments but at different relative abundances. Unlike sediment eukaryote communities, which were largely determined by the sampling site, foraminiferal intracellular eukaryote communities were driven by foraminiferal species, followed by sediment depth. Our data suggests that Ammonia sp. T6 can predate on metazoan classes, whereas Elphidium sp. S5 and Haynesina sp. S16 are more likely to ingest diatoms. These observations, alongside the use of metabarcoding in similar ecological studies, significantly contribute to our overall understanding of the ecological roles of these protists in intertidal benthic environments and their position and function in the benthic food webs