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Reducing atmospheric carbon dioxide (CO2) concentrations to combat global warming is one of the greatest challenges of humanity.Marine alkalinity enhancement is a promising carbon dioxide removal measure with high potential to increase oceanic carbon uptake and storage. The natural processes of weathering on land sustains the alkalinity of the ocean and thereby removes CO2 from the atmosphere on geological time scales. The weathering can be enhanced by deploying fine-grained alkaline minerals to coastal areas, to directly supply more alkalinity to near-coastal waters. Nevertheless, the rate of CO2 consumption depends on the minerals used, grains size, temperature, pH, and salinity. During mineral dissolution, nutrients and trace elements are also released, which may affect marine biota. In order to evaluate the CO2 sequestration potential, ensuing biogeochemical and ecological impacts of alkalinity enhancement in intertidal environments, a novel in-situ experiment was installed in the Ria Formosa Coastal Lagoon, southern Portugal.The Ria Formosa is a highly dynamic lagoon system, with daily renewal of water and nutrients through multiple tidal inlets. A succession of salt marshes with varying zonation and faunal communities fringes the lagoon. The experiment was installed in an undisturbed zone colonized with Spartina maritima, in September 2022. The experimental set-up includes three replicate treatments with coarse olivine, fine olivine, coarse basalt, fine basalt, and an untreated control. Lagoonal, supernatant, and porewater waters are sampled from each treatment every month and analysed for temperature, salinity, oxygen concentration, pH, total alkalinity, nutrients, and trace metals. Preliminary data show an increase in total alkalinity in the supernatant and porewaters shortly after minerals deployment, by 0.36 and 2.05 mmol kg-1 on average, relative to the control. Lower values of total alkalinity were recorded in December 2022, followed by markedly lower salinities after heavy rainfall in the study area. The experiment will run over two years and monthly sampled for water properties. For monitoring potential biodiversity changes, sediment samples are analysed for faunal and floral composition. Results of this novel field experiment will provide strategic knowledge on the benefits and risks of alkalinity enhancement in intertidal environments.Acknowledgement. Research supported by the Portuguese Science Foundation, with the projects PTDC/CTA-CLI/1065/2021, UID/00350/2020CIMA and contracts DL57/2016/CP1361/CT0009, DL57/2016/CP1361/CT0002 and CEECINST/00146/2018/CP1493/CT0002.

Heavy metal pollution originating from anthropogenic sources like mining, industry and ship traffic is becoming an increasing threat for marine life. First evidences for human impacts on the heavy metal concentration in seawater were recorded in medieval times already. An emerging tool for palaeo-environmental applications is the heavy metal concentration in the tests of benthic foraminifera, which enable monitoring of the anthropogenic footprint on recent systems and the fossil record. The Helgoland mud area (HMA) in the German Bight is an important depositional area for fine sediments. Average sedimentation rates of up to 13 mm/yr offer high-resolution archives, which renders the site suitable for studying long-term variations of anthropogenic pollution on the marine system. The HMA also accumulates discharge from the Elbe and Weser rivers, which are affected by mining activities in their catchment areas since the Bronze Age. The concentration of chromium (Cr), manganese (Mn), nickel (Ni), copper (Cu), zinc (Zn), silver (Ag), tin (Sn) and lead (Pb) in tests of Ammonia batava was analysed along a sediment core from the HMA by laser ablation inductively coupled mass spectrometry (LA-ICP-MS). Radiocarbon datings and stable oxygen and carbon isotope stratigraphy revealed that the core is covering the Early Modern Period (~1550–1700 AD) and the post-World War II period (~1958–2009 AD). The basal unconformity of a tempestite depicts a hiatus and non-sequence between both periods ranging from 1700 to 1958 AD. The concentration of Cu, Zn, Ag and Pb was clearly elevated in the post-1950 AD part of the core indicating a markedly higher anthropogenic influence induced by industrialisation and rapid growth of cities since the 1950s. Variations in Cu, Zn, Ni, Ag and Pb concentrations were linked to mining activities and the production rates of Pb and Ag in the Harz Mountains. Redox-sensitive elements like Mn were correlated to storm surges resulting in the extensive reworking of sediment material. Therefore, the chemistry of the tests of A. batava reflects human activities and natural impacts at the same time, which allows its application for unravelling the environmental history of the North Sea and beyond.

Significance A substantial component of the global nitrogen cycle is the production of biologically inaccessible dinitrogen attributed to anaerobic denitrification by prokaryotes. Recent evidence identified a eukaryote, foraminifera, as new key players in this “loss” of bioavailable nitrogen. The evolution of denitrification in eukaryotes is a rare event, and the genetic mechanisms of the denitrification pathway in foraminifera are just starting to be elucidated. We present large-scale sequencing analyses of 10 denitrifying foraminiferal species, which reveals the high conservation of the foraminiferal denitrification pathway. We further find evidence for a complementation of denitrification by the foraminiferal microbiome. Together, these findings provide insights into the early evolution of a previously overlooked component in the marine nitrogen cycle. Abstract: Benthic foraminifera are unicellular eukaryotes that inhabit sediments of aquatic environments. Several foraminifera of the order Rotaliida are known to store and use nitrate for denitrification, a unique energy metabolism among eukaryotes. The rotaliid Globobulimina spp. has been shown to encode an incomplete denitrification pathway of bacterial origin. However, the prevalence of denitrification genes in foraminifera remains unknown, and the missing denitrification pathway components are elusive. Analyzing transcriptomes and metagenomes of 10 foraminiferal species from the Peruvian oxygen minimum zone, we show that denitrification genes are highly conserved in foraminifera. We infer the last common ancestor of denitrifying foraminifera, which enables us to predict the ability to denitrify for additional foraminiferal species. Additionally, an examination of the foraminiferal microbiota reveals evidence for a stable interaction with Desulfobacteraceae, which harbor genes that complement the foraminiferal denitrification pathway. Our results provide evidence that foraminiferal denitrification is complemented by the foraminifera-associated microbiome. The interaction of foraminifera with their resident bacteria is at the basis of foraminiferal adaptation to anaerobic environments that manifested in ecological success in oxygen depleted habitats.

Heavy metal pollution originating from anthropogenic sources, e.g. mining, industry and extensive land use, is increasing in many parts of the world and influences coastal marine environments even after the source has ceased to pollute. The elevated input of heavy metals into the marine system potentially affects the biota because of their toxicity, persistence and bioaccumulation. An emerging tool for environmental applications is the heavy metal incorporation into foraminiferal calcite tests, which facilitates monitoring of anthropogenic footprints on recent and past environmental systems. The aim of this study was to investigate whether the incorporation of heavy metals into foraminifera is a direct function of their concentration in seawater. Culturing experiments with a mixture of dissolved chromium (Cr), manganese (Mn), nickel (Ni), copper (Cu), zinc (Zn), silver (Ag), cadmium (Cd), tin (Sn), mercury (Hg) and lead (Pb) in artificial seawater were carried out over a wide concentration range to assess the uptake of heavy metals by the nearshore foraminiferal species Ammonia aomoriensis, Ammonia batava and Elphidium excavatum. Seawater analyses revealed increasing concentrations for most metals between culturing phases and high metal concentrations in the beginning of the culturing phases due to sudden metal addition. Furthermore, a loss of metals during the culturing process was discovered by an offset between the added and the actual concentrations of the metals in seawater. Laser ablation ICP-MS (inductively coupled plasma mass spectrometry) analysis of the newly formed calcite revealed species-specific differences in the incorporation of heavy metals. The foraminiferal calcite of all three species exhibited Pb and Ag concentrations strongly correlated with concentrations in the seawater culturing medium (partition coefficients and standard deviation for Ag – Ammonia aomoriensis, 0.50 ± 0.02; Ammonia batava, 0.17 ± 0.01; Elphidium excavatum, 0.47 ± 0.04; for Pb – Ammonia aomoriensis, 0.39 ± 0.01; Ammonia batava, 0.52 ± 0.01; Elphidium excavatum, 0.91 ± 0.01). Ammonia aomoriensis further showed a correlation with Mn and Cu, A. batava with Mn and Hg, and E. excavatum with Cr and Ni and partially also with Hg. However, Zn, Sn and Cd showed no clear trend for the species studied, which in the case of Sn was maybe caused by the lack of variation in the seawater Sn concentration. The calibrations and the calculated partition coefficients render A. aomoriensis, A. batava and E. excavatum as natural archives that enable the determination of variations in some heavy metal concentrations in seawater in polluted and pristine environments.

Benthic foraminifera are unicellular eukaryotes that inhabit sediments of aquatic environments. Several foraminifera of the order Rotaliida are known to store and use nitrate for denitrification, a unique energy metabolism among eukaryotes. The rotaliid Globobulimina spp. has been shown to encode an incomplete denitrification pathway of bacterial origin. However, the prevalence of denitrification genes in foraminifera remains unknown and the missing denitrification pathway components are elusive. Analysing transcriptomes and metagenomes of ten foraminiferal species from the Peruvian oxygen minimum zone, we show that denitrification genes are highly conserved in foraminifera. We infer of the last common ancestor of denitrifying foraminifera, which enables us to predict the ability to denitrify for additional foraminiferal species. Additionally, an examination of the foraminiferal microbiota reveals evidence for a stable interaction with Desulfobacteracea, which harbour genes that complement the foraminiferal denitrification pathway. Our results provide evidence that foraminiferal denitrification is complemented by the foraminifera-associated microbiome. The interaction of foraminifera with their resident bacteria is at the basis of foraminiferal adaptation to anaerobic environments that manifested in ecological success within oxygen depleted habitats.Significance StatementA substantial component of the global nitrogen cycle is the production of biologically inaccessible dinitrogen attributed to anaerobic denitrification by prokaryotes. Recent evidence identified a eukaryote – foraminifera – as new key players in this ‘loss’ of bioavailable nitrogen. The evolution of denitrification in eukaryotes constitutes a rare event and the genetic mechanisms of the denitrification pathway in foraminifera are just starting to be elucidated. We present large-scale sequencing analyses of ten denitrifying foraminiferal species, which reveals the high conservation of the foraminiferal denitrification pathway. We further find evidence for a complementation of denitrification by the foraminiferal microbiome. Together, these findings provide novel insights on the early evolution of a previously overlooked component in the marine nitrogen cycle.