Posts Tagged 'sediment'

Fish trait-based indicators of mercury bioaccumulation under natural ocean acidification: insights from a mediterranean CO₂ vent system

Published 22 July 2026 Science Closed
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Highlights

  • Low pH conditions at CO₂ vents increase THg bioavailability and fish exposure.
  • Functional traits explain THg patterns better than species identity.
  • Under low pH, less-mobile benthic fish accumulate more THg than highly-mobile fish.
  • Trophic position drives stronger THg biomagnification at low pH sites.

Abstract

Ocean acidification is not an isolated climate-related threat to marine organisms, as it may act in combination with other stressors such as trace element contamination. Here, we took advantage of naturally acidified conditions and mercury spill-out from the shallow CO2 vent of Vulcano Island (Italy) to test whether fish functional traits explain contaminant dynamics better than species identity under ocean acidification scenarios. Specifically, we investigated if trophic group and mobility influence total mercury (THg) bioaccumulation and trophic transfer in fish inhabiting Cymodocea nodosa seagrass meadows under low and ambient pH conditions. Low-mobility benthic fish, mainly represented by invertebrate feeders and small piscivores, exhibited higher THg concentration at the vent site than at reference sites, while highly mobile zooplanktivorous fish showed no significant differences, highlighting the importance of fish mobility and local trophic benthic pathways in shaping contaminant exposure and bioaccumulation. Moreover, the relationship between trophic position and THg concentration in fish was stronger in the low pH site, indicating enhanced trophic transfer and biomagnification when Hg availability is higher. In contrast, weak or absent biomagnification in the reference sites suggests that Hg transfer depends more on contaminant availability and food web structure than trophic position alone. Overall, these findings indicate that functional traits better predict Hg bioaccumulation patterns than species identity, supporting trait-based approach to assess contaminant dynamics in future ocean acidification scenarios.

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Unveiling carbonate dissolution in coastal sediments and its influence on seawater buffering capacity with δ13CDIC and 224Ra–228Th disequilibria

Published 21 July 2026 Science Closed
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Organic carbon mineralization is generally recognized as the primary source of dissolved inorganic carbon (DIC) released from sediments in coastal seas. The CO2 accumulation or the formation of corrosive microenvironment induced by organic carbon degradation can promote the dissolution of calcium carbonate (CaCO3) in sediments, complicating the efficiency of carbon burial and total alkalinity (TA) inputs to aquatic environments. However, quantitative assessments of sediment CaCO3 dissolution and its impacts on the seawater carbonate remain poorly constrained. In this study, we selected typical high-productivity regions, mariculture farms, and applied the 224Ra–228Th disequilibrium approach to quantify the effluxes of DIC and TA across the sediment-water interface. Stable carbon isotopes of DIC (δ13CDIC) were employed to trace DIC sources in porewater. The results showed that CaCO3 dissolution in sediments accounted for 27–56 % of the benthic DIC efflux. Notably, a high contribution of CaCO3 dissolution did not coincide with strong organic carbon degradation across sites, suggesting that dynamic disturbance on sediments, which weakened the metabolic CO2 accumulation in porewater, was also a crucial factor affecting carbonate dissolution. According to the evaluation of the influence that benthic DIC and TA efflux exerted on the seawater CO2 content, the TA supplied by the CaCO3 dissolution was identified to enhance the carbonate buffering capacity of seawater and counteracted the acidification driven by organic matter remineralization. This indicates that CaCO3 dissolution in sediments should be involved in coastal carbon cycling and assessments on coastal ecosystem resilience under the risk of CO2 elevation.

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Thermal regulation of benthic fluxes in temperate estuaries

Published 2 July 2026 Science Closed
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The effects of short-term heatwave extremes on biogeochemical cycling and fluxes in a temperate estuary of a semi-dry climate were studied using an experimental setup of temperature-controlled benthic incubations. The results demonstrated a strong thermal effect, notably under extreme warming events, for shifts in exchanges across the sediment-water interface. Extreme heatwave conditions (+5 °C of the seasonal mean) boosted acidification, hypoxia, and ammonification, due to accelerated remineralization rates, resulting in strong effluxes of NH4, Si(OH)4, and PO4 to the overlying water. These excessive nutrient loads may increase eutrophication risk via runoff or tidal action, specifically in adjacent oligotrophic coastal waters. CO2 production rates reached ~4000 µatm under extreme hypoxia and acidification, 2.3-fold higher than the ambient rate, with a maximal flux of ~27.0 mmol m-2 d-1. Hence, our experiments show that marine heatwaves amplify CO2 emissions while reducing the CO2 buffering capacity of temperate estuaries. It emphasizes temperate estuaries as highly sensitive ecosystems to climate change.

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Shifts of tentacles-associated prokaryotes of Anemonia viridis along a natural pH gradient

Published 18 June 2026 Science Closed
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Highlights

  • A. viridis tentacle microbiomes were studied under changing natural pH conditions.
  • Notable shifts in the abundance of specific taxa emerged in the acidified sites.
  • Differences in seawater emphasized the host’s unique microbial signature.
  • Rickettsiales predominance suggested a specialized ecological role in symbiosis.
  • Further research is needed to discern the role of microbes for host resilience.

Abstract

Marine hydrothermal vents are extreme environments that naturally select for organisms with strong resistance and the ability to cope with special conditions of acidification. Sea anemones are an interesting example that are able to buffer intracellular pH conditions. In this study, the influence of a natural pH gradient on microbial communities associated with Anemonia viridis (Cnidaria, Anthozoa) tentacles was investigated. We hypothesized that exposure to a natural pH gradient would be associated with changes in the structure and activity of A. viridis-associated microbial communities, potentially contributing to the host’s resilience in hydrothermal environments. Microbial enzymatic activities within anemones’ tentacles were investigated by incubation with fluorogenic compounds. The leucine amino peptidase activity was highest in the tentacles of specimens living in more acidified sites. A microbial biodiversity loss was observed in bacterial symbionts from less acidified to more acidified sites, with a reduction of relative abundance in certain groups (i.e., Planctomycetota, Firmicutes, and Desulfobacterota). Results obtained by a metabarcoding approach provided interesting insights into the taxonomic shifts of the A. viridis holobiont system in naturally acidified environments.

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Changes in salinity impact nitrogen removal and carbon preservation in coastal wetlands sediment

Published 18 May 2026 Science Closed
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Highlights

  • Coastal freshening suppressed N removal via denitrification while accelerating net organic carbon mineralization.
  • Salinity shaped N removal vs. N retention and carbonate vs. alkalinity balance through sulfate availability.
  • Salinity changes had concurrent implications for coastal eutrophication and ocean acidification.

Abstract

Coastal wetlands naturally remediate nitrogen (N) pollution through microbial pathways that either remove reactive N via denitrification and anammox, or retain it via dissimilatory nitrate reduction to ammonium (DNRA). The balance among three processes is closely linked to the carbon (C) cycle, as both heterotrophic denitrification and DNRA consume organic C and release alkalinity. While salinity fluctuations can disrupt these processes through direct ionic stress or sulfur (S) cycling, their net impact on N removal and C preservation services remains unclear. Here, we deployed microcosm experiments using mangrove sediments under a large salinity gradient (0-30 psu). We quantified N transformation rates using 15N isotope tracing technique, combined with geochemical analysis, and functional genes quantification. Freshening from ambient 30 psu to 10 psμ decreased N removal efficiency by ∼20%. This decline was caused by reduced denitrification, whereas anammox and DNRA were unaffected. Meanwhile, lower salinity appears to have stimulated C decomposition via reduced ionic stress. The reduced sulfate input diminished total alkalinity (TA) generation relative to dissolved inorganic carbon (DIC). The stoichiometric shift of TA:DIC ratio could further contribute to acidification in adjacent coastal waters. Additionally, the S-mediated regulation of N partitioning appears to be nitrate-dependent: under nitrate limitation, higher sulfate favored N retention; conversely, with enriched nitrate, it potentially favored N removal. Integrating the coupling effect of salinity on interaction between N, C and S cycles, our study demonstrates that coastal water freshening may weaken wetlands’ ability to remove N and preserve C.

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Ordovician sedimentary processes and related driving forces: Jordan, Arabian Plate

Published 7 May 2026 Science Closed
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The Ordovician-Lower Silurian siliciclastics deposited on the Jordanian Platform represent a transitional sedimentary system between their granitoid Gondwana source area and the Paleo-Tethys. While fluvial fining upward cycles (FUCs) of quartz arenite dominate braid plain deltas/upper shore face environments of the Lower Ordovician, arkosic tempestite and oxygen-deficient bituminous pelite/tuffite cycles cover upper/lower shore face environments of the Sandbian and Katian. The mineral deficit (feldspar, unstable heavy minerals) relates to acid sturz-rain events during volcanic degassing (SO2, HCl, HF, NOx) sourced in an Infracambrian/Cambrian Large Igneous Province (LIP) around S Sinai/Wadi Araba Rift-Zone. The change of sedimentary architectural elements/lithofacies types during the Upper Darriwilian took place after an L-chondrite of the Main Asteroid Belt (MAB) crossed the Earth’s orbit (~470 Ma), which resulted in some small meteorite craters (i.e., Lockne). Through the Sandbian and Katian, this insignificant impact series was accompanied by massive tephra production during worldwide explosive subduction-related volcanic arc magmatism. During the Upper Ordovician High Stand-System Tract (HST), the glass-bearing tephras were transformed under marine conditions into montmorillonite (K-bentonite), contributing to green tuffitic pelite interbedded with storm-generated arkosic clastics. Transtensional tectonics (pull-apart type) caused the main Ordovician-Silurian unconformity (“paleovalleys”) in SE Jordan and Saudi Arabia. Their sedimentary fills expose arkosic FUCs originated by shallow-water turbidites during the Hirnantian. The intensive explosive volcanism generated almost continuously negative climate forcing (“cosmic winter”) by tephra, aerosols, smog, and clouding that led to regional glaciation in the S Hemisphere. The abrupt 87Sr/86Sr-ratio decrease accompanies, at the Sandbian base, the onset of magmatism, while δ13C excursions follow a Transgressive System Tract (TST) and three T-maxima indicating increasing phytoplankton growth. The undulation—0% mirrors a cyclicity of volcanic events, climate forcing, Eh, and pH conditions. The δ18O rise shows a continuous CO2 assimilation until its stop (~1200 ppm CO2) and the following formation of black-shale facies.

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Combined effects of ammonium and pH on sea urchin embryogenesis: insights for sediment quality assessment

Published 30 April 2026 Science Closed
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Highlights

  • Reduced pH enhances ammonium toxicity on sea urchin embryos in filtered seawater.
  • In elutriates ammonium is a major driver of P. lividus embryotoxicity.
  • Data support setting ammonium thresholds in sediment quality frameworks.
  • Ocean acidification potentially increases ammonium toxicity for sea urchin larvae.

Abstract

Ammonium is a key component of coastal marine systems, originating from both natural and anthropogenic sources, with possible toxic effects on marine organisms depending on the concentration and pH. This study evaluates, for the first time, the combined effects of ammonium and seawater acidification on early development of the sea urchin Paracentrotus lividus under both laboratory conditions and exposure to environmental matrices derived by dredged sediments from harbor area. Embryos were incubated with increasing concentrations of ammonium in filtered seawater at pH 8.1 and 7.6, as well as in sediment elutriates from the Pescara harbor (Adriatic Sea, Italy), selected as a case study with relevant concentrations of ammonium (0.1–3.5 mg/L). A combined effect between ammonium and pH was observed, with increasing ammonium toxicity by ∼20% at pH 7. Moreover, in sediment elutriates, ammonium affect sea urchin embryo development, with EC50 ranging between 1.388 and 1.538 mg/L NH4+ at pH 8.1 and 7.6, respectively, without significant differences due to pH. Chemical analyses of sediments confirmed low levels of trace metals and organic pollutants, indicating that ammonium is the primary driver of embryotoxicity without a direct toxic effect of other contaminants. The results further underscore the need to integrate ammonium assessment into sediment quality frameworks and for management strategies, particularly in the context of future ocean acidification, to safeguard the early life stages of sensitive marine invertebrates.

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Light and tidal inundation and exposure regulate the sensitivity of estuarine benthic greenhouse gas fluxes to warming and ocean acidification

Published 28 April 2026 Science Closed
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Coastal sediments are globally significant sources and sinks of greenhouse gases (GHGs), yet their contributions to climate feedbacks of warming and ocean acidification remain uncertain, in part due to limited understanding of short-term variability. Here, we use a fully factorial laboratory experiment to disentangle how diel light–dark and tidal inundation and exposure interact with warming and elevated pCO2 to regulate benthic fluxes of CO2, CH4, and N2O in estuarine sediments, alongside concurrent changes in benthic oxygen exchange. While warming and pCO2 exerted strong independent effects, their influence was shaped by diel and tidal fluctuations in redox conditions and oxygen availability, reflecting shifts in metabolic balance between primary production and respiration. Light consistently limited CO2, CH4, and N2O emissions through enhanced autotrophic uptake and oxygenation, while dark promoted anaerobic production pathways. N2O showed the greatest sensitivity to the combined effects of climate forcing and redox dynamics. Despite warming-driven stimulation of benthic heterotrophy and the production of all GHGs, CO2 remained the dominant greenhouse gas, with minimal CH4 and N2O fluxes due to the limited organic matter availability within the sediment. This reflects the strong redox controls on CH4 and N2O production, which relies on both oxygen depletion and organic substrate supply. Our findings emphasize that fine-scale temporal variability can significantly shape both the magnitude and climate sensitivity of benthic GHG emissions. Capturing these fine-scale controls is essential for accurately modeling the contributions of estuarine sediments to global GHG budgets and their feedbacks.

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A biogeochemical perspective on acidification and buffering capacity in the Piscataqua Estuary

Published 18 February 2026 Science Closed
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Coastal acidification is influenced not only by rising atmospheric CO2 and river-ocean mixing, but also by metabolic processes that alter seawater carbonate chemistry and buffering capacity. This study examines how sedimentary biogeochemical processes contribute to carbonate system variability in the Piscataqua Estuary, a tidally dynamic channel connecting Great Bay to the Gulf of Maine. The biogeochemical processes considered include sedimentary aerobic respiration, denitrification, sulfate reduction, and carbonate dissolution or precipitation. Two incubation experiments were conducted in September and October of 2024 at the University of New Hampshire’s Coastal Marine Laboratory (CML) to quantify changes in pH, dissolved inorganic carbon (DIC), and total alkalinity (TA) in the overlying water arising from sediment-water biogeochemical exchange. Sediment cores were collected to be paired with overlying water from slack low and slack high tides during each month. Across both experiments, sediment cores consistently exhibited greater acidification and larger shifts in DIC and TA concentrations compared to water-only cores, indicating strong sedimentary biogeochemical influence. Among the processes considered, sulfate reduction is likely the most influential driver of carbonate system variability, contributing to increases in both DIC and TA. Linking experimental results to in-situ measurements at CML revealed that variability observed over individual ebb or flood tides primarily reflected processes associated with tidal advection (ie, river-ocean mixing and water-column biogeochemical activity). However, when evaluating net changes over both tidal transitions (ebb and flood), contributions from sedimentary biogeochemical processes were comparable in magnitude to those of the other processes during September and October. Sedimentary biogeochemical processes also appear to exert more consistent contributions to DIC and TA than water-column biogeochemical processes. Together, these findings demonstrate that sedimentary biogeochemical processes play a major role in regulating carbonate system variability in the Piscataqua Estuary. This study underscores the importance of examining carbonate system variability across multiple timescales to obtain a more comprehensive understanding of estuarine carbonate dynamics. Additional experimental work is needed to further resolve the influence of metabolic processes on coastal carbonate systems under changing environmental conditions.

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Co–occurring aquatic acidification and hypoxia promote methane emissions from estuarine ecosystems

Published 14 January 2026 Science Closed
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Highlights

  • Acidification, hypoxia, and the combined effect enhanced CH4 emission from estuary.
  • Acidification and hypoxia exerted contrasting regulatory mechanisms on CH4 emission.
  • Acidification raised CH4 release by suppressing methanotrophs more than methanogens.
  • Hypoxia preferentially enhanced methanogenic activity over CH4 oxidation.
  • Oxygen availability dominated CH4 dynamics under acidification–hypoxia interactions.

Abstract

Estuaries worldwide are experiencing intensifying acidification and hypoxia, driven synergistically by anthropogenic activities and global climate change. Nevertheless, their combined impact on the emissions of the potent greenhouse gas methane (CH4) and its underlying regulatory mechanisms remains poorly understood, undermining our ability to project climate feedbacks. Here, we integrated 13C stable isotope tracing, DNA/mRNA–based qPCR, and amplicon/metagenomic sequencing to unravel how acidification–hypoxia interactions regulate the complex balance between CH4 production and consumption in estuarine sediments. Results showed that aquatic acidification and hypoxia combined to significantly increase CH4 emissions from estuarine sediments (P < 0.05), in a non-additive (antagonistic) manner where oxygen availability was the dominant factor governing this response. Notably, acidification increased CH4 emissions by suppressing methanotrophy more strongly than methanogenesis, whereas hypoxia preferentially stimulated methanogenic activity over CH4 oxidation. These response patterns were further demonstrated by metagenomic sequencing and mRNA-based quantitative PCR analyses, which revealed coordinated shifts in both the relative abundance and transcriptional activity of key functional genes. These findings uncover a previously overlooked mechanism whereby the worldwide co-occurrence of acidification and hypoxia in estuarine ecosystems jointly promote CH4 emissions, providing a scientific basis for improving predictive models of the global CH4 cycle and its climate feedbacks under combined anthropogenic and climatic stressors.

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Sediment topography enhances the response of coral reef carbonate sediment dissolution to ocean acidification

Published 13 November 2025 Science Closed
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The interaction between water flow and sediment topography (e.g., surface ripples) in shallow, permeable coral reef carbonate sediments establishes pressure gradients that increase the rate of sediment–water solute exchange relative to water flow along a flat bottom. It is unknown how this effect from surface ripples may modify the rate at which the sediment porewater is exposed to future chemical changes in the overlying water column, such as elevated pCO2 that is causing ocean acidification (OA). To address this question, this study used a series of 22-h incubations in flume aquaria with live permeable calcium carbonate sediment communities and examined the interactive effect of pCO2 (400 and 1000 µatm) and surface topography (flat and rippled sediments) on invertebrate infaunal activity, carbonate sediment microbial metabolism, and inorganic carbonate dissolution. Results show that the introduction of oxygen into flat sediments was largely driven by infaunal activity, whereas introduction of oxygen into rippled sediments was largely driven by physical flow processes. Rippled sediments exhibited rates of respiration and gross primary production that were ~ 45% and ~ 50% higher, respectively, than flat sediments. An increase in pCO2 shifted the sediments in the flat flumes from net calcifying to net dissolving, an effect that was amplified an additional ~ 60% in rippled sediments. These results suggest that current estimates of coral reef carbonate sediment calcification may be underestimating the dissolution response to OA where the carbonate sediment environment exhibits ripples in the topography.

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Variations of coccolith morphology and their influencing factors in the northeastern South China Sea since the last glacial maximum

Published 15 April 2025 Science Closed
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Coccoliths are widely present in marine sediments of the South China Sea and closely associated with paleoenvironmental changes. This study investigates the morphological variations and driving factors of coccoliths since the Last Glacial Maximum(LGM) by analyzing the morphology and related indicators of Noelaerhabdaceae coccoliths in sediment samples from core MD18-3569 (0.01~12.41 m; 0.69~26.58 ka) in the northeastern South China Sea(22°09.30’N, 119°49.24’E at water depth 1320 m), and a total of 155 samples were collected, with a sampling resolution of approximately 167 years. Morphological attributes such as coccolith length, thickness, area, and mass were obtained through microscopic measurements and computational formulas. Coccolith morphological constants were used to evaluate preservation conditions. Based on these data, morphological divergence index and calcification index were calculated. Using the PyCO2SYS model, ocean carbonate system parameters were reconstructed to explore their impact on coccolith morphology.

The results show that coccolith length ranged from 2.69 μm to 3.86 μm(mean: 3.20 μm) since the LGM, with significant variability but no clear trend. Thickness ranged from 0.07 μm to 0.17 μm(mean: 0.13 μm) and exhibited a decreasing trend since the LGM. Coccolith mass varied between 2.31 pg and 9.02 pg(mean: 5.61 pg), also showing a decreasing trend since the LGM. Coccolith morphological constants indicate good preservation and high data reliability. Morphological divergence index results suggest a decline in coccolith size diversity, reflecting reduced seasonal variation and regional productivity. Calcification intensity metrics indicate a weakening of calcification since the LGM.

By comparing temperature, salinity, and ocean carbonate system parameters, the study identifies rising atmospheric CO2 concentrations and resultant ocean acidification as the primary factors contributing to reduced coccolith calcification. The impacts of temperature and salinity were relatively minor. These findings demonstrate the complex response of Noelaerhabdaceae coccoliths to environmental changes and highlight the significant roles of regional climate variations and carbonate system evolution in shaping coccolith morphology.

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The effect of carbonate mineral additions on biogeochemical conditions in surface sediments and benthic–pelagic exchange fluxes (update)

Published 11 February 2025 Science Closed
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Coastal sediments are hotspots of biogeochemical processes that are impacting subsurface and overlying water conditions. Fluid composition in sediments is altered through the mineralization of organic matter which, under oxic conditions, further lowers both pH and the carbonate saturation state. As a potential mitigation strategy for this sediment acidification, we explored the effects of mineral additions to coastal sediments. We experimentally quantified carbonate mineral dissolution kinetics of carbonate shells suitable for field application and then integrated these data into a reactive transport model that represents early diagenetic cycling of C, O, N, S, and Fe and traces total alkalinity, pH, and saturation state of CaCO3. Model simulations were carried out to delineate the impact of mineral type and amount added, porewater mixing, and organic matter mineralization rates on sediment alkalinity and its flux to the overlying water. Model results showed that the added minerals undergo initial rapid dissolution and generate saturated conditions demonstrating the potential of alkalinity enhancement in mitigating surface sediment acidification. Aragonite dissolution led to higher total alkalinity concentrations than calcite. Simulations of carbonate mineral additions to sediment environments with low rates of organic matter mineralization exhibited a substantial increase in mineral saturation state compared to sediments with high CO2 production rates, highlighting the environment-specific extent of the effect of mineral addition. Our work indicates that carbonate additions have the potential to effectively buffer surficial sediments over multiple years, yielding biogeochemical conditions that counteract the detrimental effect of low-pH sediment conditions on larval recruitment and potentially increase benthic alkalinity fluxes to support marine carbon dioxide removal (mCDR) in the overlying water.

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Effects of sediment acidification on germinability of Scrippsiella acuminata cysts in hypoxic zones

Published 16 January 2025 Science Closed
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We investigated the calcareous cysts of Scrippsiella acuminata, with a focus on morphological changes from spiny to naked types in the surface sediments of hypoxic zones. The cyst-type abundance and bottom environmental conditions at two stations, representing hypoxic and normoxic conditions, were compared. Germination tests simulating in situ pH conditions were conducted to elucidate differences in germinability between spiny and naked cysts. The pH values at the hypoxic station reached a minimum of 7.2 in September, coinciding with high bottom-water temperatures and low dissolved oxygen levels. Significant differences in cyst abundance were observed; naked and intermediate cysts dominated the hypoxic station, whereas spiny cysts were more abundant at the normoxic station. Both cyst types exhibited a similar negative effect of germinability decrease (62 to 25% for spiny cysts and 75 to 32% for naked cysts) in acidic conditions (7.2) compared to normal pH conditions (7.7). Morphological changes in ungerminated cysts, such as cytoplasmic degradation and wall thickening, occurred under acidified conditions.

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Toxicity of PAHs-enriched sediments on meiobenthic communities under ocean warming and CO2-driven acidification scenarios

Published 6 January 2025 Science Closed
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Highlights

  • Temperature rise reduced the densities of Copepoda and certain Nematoda groups.
  • CO2 acidification prevented some Nematoda groups from increasing at high temperatures.
  • CO2 acidification reduced Copepoda and nauplii densities, but increased Ostracoda.
  • Complex interactions increased certain meiobenthic groups exposed to sediment PAHs.
  • Global change and pollution showed interactive effects in meiobenthic communities.

Abstract

This study aimed to assess the interactive effects of CO2-driven acidification, temperature rise, and PAHs toxicity on meiobenthic communities. Laboratory microcosms were established in a full factorial experimental design, manipulating temperature (25 °C and 27 °C), pH (8.1 and 7.6), and PAH contamination (acenaphthene + benzo(a)pyrene spiked sediments and negative control). Temperature rise and CO2-driven acidification led to a decrease in the densities of Copepoda. The density of nematodes Pseudochromadora and Daptonema also decreased, while Sphaerotheristus and Sabatieria densities increased, particularly in the absence of CO2-driven acidification. Ostracoda densities increased in the acidified scenario. PAH contamination resulted in decreased Daptonema densities but increased Turbellaria and certain Nematoda genera (e.g. Pseudochromadora). Overall, the results indicate that the changes of meiobenthic communities caused by CO2 acidification, warming, and PAH contamination are shaped by the vulnerability and tolerance of each taxonomic group, alongside indirect effects observed in Nematoda assemblages.

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Extreme abiotics drive sediment biocomplexity along pH gradients in a shallow submarine volcanic vent

Published 3 January 2025 Science Closed
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Highlights

  • Shallow CO2 vents generate pH gradients that influence sediment biogeochemistry
  • Sedimentary organic matter (SOM) and prokaryotic community were analysed along a pH gradient
  • Environmental gradients drive distribution and abundance of benthic prokaryotic communities and origin of SOM
  • Vent-derived sources contributed largely to SOM up to 350 m from the vent
  • CO2-driven benthic community shifts affect spatial dynamics of SOM origin and composition with expected rebounds on biota

Abstract

Volcanic emissions in shallow vents influence the biogeochemistry of the sedimentary compartment, creating marked abiotic gradients. We assessed the spatial dynamics of the sediment compartment, as for the composition and origin of organic matter and associated prokaryotic community, in a volcanic shallow CO2 vent (Vulcano Island, Italy). Based on elemental (carbon, nitrogen content and their ratio) and isotopic composition (δ13C, δ15N and δ34S), the contribution of vent-derived organic matter (microbial mats) to sedimentary organic matter was high close to the vent, while the marine-derived end-members (seagrasses) contributed highly at increasing distance. Chemoautotrophic Campylobacterota and hyperthermophilic Achaea prevailed close to the vent, whilst phototrophic and chemoheterotrophic members dominated at increasing distance. Abiotic gradients generated by the volcanic CO2 vent drive relevant changes in the composition, origin and nutritional quality of sedimentary organic matter, and influence the structure and complexity of associated prokaryotic communities, with expected relevant impact on the entire food-web.

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Seawater warming rather than acidification profoundly affects coastal geochemical cycling mediated by marine microbiome

Published 28 November 2024 Science Closed
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Highlights

  • The structure and function of coastal microbial communities are influenced by ocean warming and acidification.
  • Elevated temperature more profoundly impacts microbial communities than does acidification.
  • Warming promotes denitrification that may increase nitrogen loss.
  • The nitrogen, sulfur cycles, and carbon-fixation pathways exhibit distinct variation patterns under warming.

Abstract

The most concerning consequences of climate change include ocean acidification and warming, which can affect microbial communities and thus the biogeochemical cycling they mediate. Therefore, it is urgent to study the impact of ocean acidification and warming on microbial communities. In the current study, metagenomics was utilized to reveal how the structure and function of marine microorganisms respond to ocean warming and acidification. In terms of community structure, Non-metric Multidimensional Scaling analysis visualized the similarity or difference between the control and the warming or acidification treatments, but the inter-group differences were not significant. In terms of gene functionality, warming treatments showed greater effects on microbial communities than acidification. After treatment with warming, the relative abundance of genes associated with denitrification increased, suggesting that ocean nitrogen loss can increase with increased temperature. Conversely, acidification treatments apparently inhibited denitrification. Warming treatment also greatly affected sulfur-related microorganisms, increasing the relative abundance of certain sulfate-reducing prokaryote, and enriched microbial carbon-fixation pathways. These results provide information on the response strategies of coastal microorganisms in the changing marine environments.

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Paleo-atmospheric CO2 reconstructions from deep-ocean sediments

Published 22 November 2024 Science Closed
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Biological remains in ocean sediments document the remarkable history of atmospheric CO2 and its fundamental control on Earth’s climate. Higher resolution studies are needed to better understand the short-term processes that inform imminent anthropogenic climate changes.

Human activities have increased the concentration of carbon dioxide in our atmosphere from 280 ppm before industrialization to 424 parts per millin (ppm) in 2024. Without reductions in emissions, CO2 is projected to rise to >800 ppm by the end of this century, driving warming well in excess of the 1°C already recorded (IPCC 2021). How warm it will get can be projected by complex numerical climate models whose skills are validated using the detailed relationship between atmospheric CO2 and global climate in Earth’s history. Instrumental measurements of CO2 have been collected since 1958 (Lan et al. 2024), and ancient air trapped in Antarctic ice documents Earth’s atmospheric composition over hundreds of thousands of years prior (Bereiter et al. 2015; Yan et al. 2019). However, CO2 during this geologically recent past was generally lower than today, and global temperatures colder. Much warmer intervals occurred in the distant past, but because the atmosphere of that time cannot be sampled directly, paleo-CO2 reconstructions rely on indirect proxies preserved in the sedimentary record.

Reconstructing CO2 from ocean sediments

Deep-sea sediments are key to paleoreconstructions; they are globally distributed and gradually accumulate biogenic and inorganic proxy materials over tens of millions of years, thereby providing excellent age stratigraphy. Uniquely useful in documenting past surface-ocean temperatures and the partial pressure of CO2 (PCO2) are the mineralized and organic remains left behind by organisms that once inhabited the ancient surface ocean. This is because gas exchange at the air-sea interface drives PCO2 in seawater towards equilibrium with PCO2 in the atmosphere. Once absorbed in seawater, CO2 reacts with water (H2O) and forms a suite of carbon species whose abundances are controlled by well-understood chemical equilibrium reactions that also determine seawater acidity (i.e. pH).

Not all oceanic regions are appropriate for paleo-CO2 studies because vigorous photosynthesis can diminish sea-surface CO2 while upwelling of deeper waters delivers respired CO2 to the surface, disturbing the air–sea equilibrium. Therefore, paleo-CO2 studies focus on off-shore regions such as subtropical gyres, where photosynthesis is weak and downwelling of surface waters allows air–sea equilibrium to be established.

There are two main frameworks for marine-based CO2 reconstructions: the stable carbon isotopic composition of organic phytoplankton (δ13Cphytoplankton) remains and the boron isotopic composition (δ11B) of fossilized CaCO3 shells. Briefly, the δ13Cphytoplankton proxy assumes CO2 passively diffuses into an algae cell, and the CO2-fixing enzyme RuBisCo preferentially takes up 12C over 13C during oxygenic photosynthesis. When CO2 is abundant, 12C is preferentially incorporated into organic matter (resulting in relatively lower δ13Cphytoplankton). The opposite occurs at low CO2 (Fig. 1). Although first applied to bulk organic matter (Popp et al. 1989), selective preservation and mixed organic sources imposed problems. These challenges have been resolved by using: (1) specific compounds produced by select algae (e.g. alkenones from Haptophytes); (2) specific compounds produced by the broader phytoplankton community (e.g. chlorophyll), enabling greater spatial and temporal diversity of reconstructions; and (3) organic carbon bound to mineral or organic exteriors of e.g. coccolithophores, diatoms or dinoflagellates. The detailed systematics of these approaches are reviewed in Hollis et al. (2019).

Figure 1: Basic systematics of the two marine CO2 proxies. Fossil organic compounds and CaCO3 shells are preserved in layered ocean sediments that can be extracted by deep-ocean drilling

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Coupled decline in ocean pH and carbonate saturation during the Palaeocene–Eocene Thermal Maximum

Published 20 November 2024 Science Closed
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The Palaeocene–Eocene Thermal Maximum, a climate event 56 million years ago, was characterized by rapid carbon release and extensive ocean acidification. However, our understanding of acidification and the evolution of ocean saturation states continues to be hindered by considerable uncertainties, primarily stemming from the limited availability of proxy data. Under such conditions, data assimilation allows for an internally consistent assessment of atmospheric CO2 changes, ocean acidification and carbonate saturation state during this period. Here, we present a reconstruction of the Palaeocene–Eocene Thermal Maximum carbon cycle perturbation by assimilating seafloor sediment CaCO3 and sea surface temperature proxy data with simulations from an Earth system model, which includes a comprehensive carbonate system. Our reconstructions indicate a substantial increase in atmospheric CO2 from 890 ppm (95% credible interval: 680–1,170 ppm) to 1,980 ppm (1,680–2,280 ppm), coupled with a notable decline in pH (0.46 units, ranging from 0.31 to 0.63 units) and surface-water calcite saturation state, decreasing from 10.2 (7.5–12.8) in the pre-event period to 3.8 (2.8–5.1) during the thermal maximum. Carbonate undersaturation intensified substantially in high-latitude surface waters during the Palaeocene–Eocene Thermal Maximum, paralleling the current decline in Arctic aragonite saturation driven by anthropogenic CO2 emissions.

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Competitive dissolution of mixed carbonate solids under simulated ocean acidification

Published 12 November 2024 Science Closed
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It is estimated that at least 25 % of the anthropogenic carbon dioxide (CO2) emitted to the atmosphere since the start of the industrial revolution has been absorbed and dissolved by the oceans. The uptake of CO2 by the oceans leads to an increase in the seawater proton concentration ([H+]), and decreases in seawater pH, carbonate ion concentration ([CO32–]), and saturation state (Ω) with respect to calcium carbonate (CaCO3) minerals; a process commonly referred to as “ocean acidification”. Shallow-water (<200 m), high-magnesium, biogenic calcites are expected to be amongst the first to respond to ocean acidification, and it has been proposed that they will dissolve selectively and sequentially according to their solubility in seawater. In this study, we test this competitive dissolution hypothesis by reacting a mixture of biogenic and synthetic carbonates of varying Mg content with acidified, natural seawater to simulate the progressive acidification of surface-ocean waters by anthropogenic CO2. The results of this study confirm the hypothesis that carbonates will dissolve sequentially according to their respective solubility. They also reveal that the dissolution of high Mg-calcites will proceed incongruently. The originality of this contribution rests with the demonstration that the presence of a single high Mg-calcite will generate, like in a sediment of mixed mineralogy, a continuum of transient states as lower Mg-calcites of greater stability are precipitated and dissolved. Consequently, in a semi-closed or closed system, the pH buffering of the acidified seawater solution will be progressive rather than occur in steps according to changes in the solubility of the individual carbonate phases that compose a sediment. Hence, we expect that, as the oceans take up more anthropogenic CO2 and further acidify, the average mineralogy and composition (Mg content) of shallow-water carbonate sediments and reef structures will change over the next few centuries as the most soluble carbonate phases (high-Mg calcites) are dissolved and no longer precipitated.

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