CO2 leakage alters biogeochemical and ecological functions of submarine sands

Physiological and behavioral responses of the Baltic clam Macoma balthica to a laboratory simulated CO2-leakage from a subseabed carbon storage site

Carbon capture and storage in sub-seabed geological reservoirs is now officially included in the atmospheric CO2 emissions reduction policy and meets the agenda of Sustainable Development Goals (SDGs). Over the last few years biological risk assessment studies have delivered substantial empirical data on possible consequences of CO2 leakages from underwater storage sites on benthic systems. Current knowledge on Carbon Capture and Storage CCS associated risks is limited to marine systems. Yet there are multiple areas identified as suitable for carbon storage, but their hydrogeochemical features are so distinct that they should be studied as separate cases. Baltic Sea is one example of an area but is host to a unique - in a world scale - ecosystem with low salinity in combination with reduced oxygen availability in the benthic zone. Geological surveys have designated a potential storage site in the Southern Baltic Sea, namely the B3 oil field. Thus, this study focuses on biological effects of seawater acidification caused by a simulated CO2 leakage scenarios under laboratory conditions on a model macrobenthic in-faunal species. Baltic clams Macoma balthica were exposed to different environmental pH scenarios: pH 7.7 (no leakage), pH 7.0 (moderate hypercapnia) and pH 6.3 (severe hypercapnia) in three independent experiments conducted with the use of a hyperbaric tank (Karl Eric Titank) mimicking hydrostatic pressure of 900 kPa, relevant to conditions at the B3 field. Selected physiological aspects of the Baltic clam, such as survival, shell growth rate, morphometric condition and biochemical composition were investigated along with their behavioral responses, i.e. sediment burrowing activity. The results showed modest effects of hypercapnia on physiological performance of the clams that did not lead to greater mortality in neither of the tested leakage scenarios. Apart from high survival of the clams even in the lowest seawater pH (6.3) there were only little changes observed in the burrowing depth of the clams and biochemical composition of their soft tissues related to seawater acidification. The most evident physiological responses of the clams to prolonged hypercapnia (40 days at pH 6.3) were manifested in decreased shell growth.

Contrasting effects of increasing dissolved iron on photosynthesis and O2 availability in the gastric cavity of two Mediterranean corals

Iron (Fe) plays a fundamental role in coral symbiosis, supporting photosynthesis, respiration, and many important enzymatic reactions. However, the extent to which corals are limited by Fe and their metabolic responses to inorganic Fe enrichment remains to be understood. We used respirometry, variable chlorophyll fluorescence, and O2 microsensors to investigate the impact of increasing Fe(III) concentrations (20, 50, and 100 nM) on the photosynthetic capacity of two Mediterranean coral species, Cladocora caespitosa and Oculina patagonica. While the bioavailability of inorganic Fe can rapidly decrease, we nevertheless observed significant physiological effects at all Fe concentrations. In C. caespitosa, exposure to 50 nM Fe(III) increased rates of respiration and photosynthesis, while the relative electron transport rate (rETR(II)) decreased at higher Fe(III) exposure (100 nM). In contrast, O. patagonica reduced respiration, photosynthesis rates, and maximum PSII quantum yield (Fv/Fm) across all iron enrichments. Both corals exhibited increased hypoxia (<50 µmol O2 L-1) within their gastric cavity at night when exposed to 50 and 100 nM Fe(III), leading to increased polyp contraction time and reduced O2 exchange with the surrounding water. Our results indicate that C. caespitosa, but not O. patagonica, might be limited in Fe for achieving maximal photosynthetic efficiency. Understanding the multifaceted role of iron in corals' health and their response to environmental change is crucial for effective coral conservation.

Effects of acidification on the biogeochemistry of unvegetated and seagrass marine sediments

Many studies addressed ocean acidification (OA) effects on marine life, whereas its effects on sedimentary organic matter (OM) have received less attention. We investigated differences in OM features in sediments from unvegetated and seagrass (Posidonia oceanica) beds in a shallow hydrothermal area (Aeolian Archipelago, Mediterranean Sea), under natural (8.1-8.0) and acidified (7.8-7.9) conditions. We show that a pH difference of -0.3 units have minor effects on OM features in unvegetated sediments, but relevant consequences within acidified seagrass meadows, where OM quantity and nutritional quality are lower than those under natural pH conditions. Effects of acidified conditions on OM biogeochemistry vary between unvegetated and seagrass sediments, with lower C degradation rates and longer C turnover time in the former than in the latter. We conclude that OA, although with effects not consistent between unvegetated and vegetated sediments, can affect OM quantity, composition, and degradation, thus having possible far-reaching consequences for benthic trophic webs.

Ecological and Biotechnological Relevance of Mediterranean Hydrothermal Vent Systems

Marine hydrothermal systems are a special kind of extreme environments associated with submarine volcanic activity and characterized by harsh chemo-physical conditions, in terms of hot temperature, high concentrations of CO2 and H2S, and low pH. Such conditions strongly impact the living organisms, which have to develop adaptation strategies to survive. Hydrothermal systems have attracted the interest of researchers due to their enormous ecological and biotechnological relevance. From ecological perspective, these acidified habitats are useful natural laboratories to predict the effects of global environmental changes, such as ocean acidification at ecosystem level, through the observation of the marine organism responses to environmental extremes. In addition, hydrothermal vents are known as optimal sources for isolation of thermophilic and hyperthermophilic microbes, with biotechnological potential. This double aspect is the focus of this review, which aims at providing a picture of the ecological features of the main Mediterranean hydrothermal vents. The physiological responses, abundance, and distribution of biotic components are elucidated, by focusing on the necto-benthic fauna and prokaryotic communities recognized to possess pivotal role in the marine ecosystem dynamics and as indicator species. The scientific interest in hydrothermal vents will be also reviewed by pointing out their relevance as source of bioactive molecules.

Ocean Acidification Induces Changes in Virus-Host Relationships in Mediterranean Benthic Ecosystems

Acidified marine systems represent "natural laboratories", which provide opportunities to investigate the impacts of ocean acidification on different living components, including microbes. Here, we compared the benthic microbial response in four naturally acidified sites within the Southern Tyrrhenian Sea characterized by different acidification sources (i.e., CO₂ emissions at Ischia, mixed gases at Panarea and Basiluzzo and acidified freshwater from karst rocks at Presidiana) and pH values. We investigated prokaryotic abundance, activity and biodiversity, viral abundance and prokaryotic infections, along with the biochemical composition of the sediment organic matter. We found that, despite differences in local environmental dynamics, viral life strategies change in acidified conditions from mainly lytic to temperate lifestyles (e.g., chronic infection), also resulting in a lowered impact on prokaryotic communities, which shift towards (chemo)autotrophic assemblages, with lower organic matter consumption. Taken together, these results suggest that ocean acidification exerts a deep control on microbial benthic assemblages, with important feedbacks on ecosystem functioning.

Climate change mitigation effects: How do potential CO2 leaks from a sub-seabed storage site in the Norwegian Sea affect Astarte sp. bivalves?

Carbon capture and storage (CCS) is one of the most promising mitigation strategies for reducing the emissions of carbon dioxide (CO₂) to the atmosphere and may substantially help to decelerate global warming. There is an increasing demand for CCS sites. Nevertheless, there is a lack of knowledge of the environmental risk associated with potential leakage of CO₂ from the storage sites; and even more, what happens when the seepage stops. Can the environment return to the initial equilibrium? Potential effects on native macrofauna were studied under a scenario of a 50-day CO₂ leakage, and the subsequent leak closure. To accomplish the objective, Trondheim Fjord sediments and clams were exposed to an acidified environment (pH 6.9) at 29 atm for 7 weeks followed by a 14-day recovery at normal seawater conditions (pH 8.0, 29 atm). Growth and survival of clams exposed to pressure (29 atm) and reduced pH (6.9) did not significantly differ from control clams kept at 1 atm in natural seawater. Furthermore, bioaccumulation of elements in the soft tissue of clams did not register significant variations for most of the analysed elements (Cd, Cr, Pb, and Ti), while other elements (As, Cu, Fe, Ni) had decreasing concentrations in tissues under acidified conditions in contrast to Na and Mg, which registered an uptake (Ku) of 111 and 9.92 μg g^-1dw d^-1, respectively. This Ku may be altered due to the stress induced by acidification; and the element concentration being released from sediments was not highly affected at that pH. Therefore, a 1 unit drop in pH at the seafloor for several weeks does not appear to pose a risk for the clams.

Quantitative assessment of deep-seated CO2 leakage around CO2-rich springs with low soil CO2 efflux using end-member mixing analysis and carbon isotopes

This study examined a mountainous area with two hydrochemically distinct CO₂-rich springs to understand the origin, flow, and leakage of CO₂, which may provide implications for precise monitoring of CO₂ leakage in geological carbon storage (GCS) sites. The carbon isotopic compositions of dissolved inorganic carbon (DIC) in CO₂-rich water (δ¹³C_DIC) and those of soil CO₂ (δ¹³C_CO2) indicated a deep-seated CO₂ supply to the near-surface environment in the study area. The hydrochemical difference (e.g. pH, total dissolved solids) for the two CO₂-rich springs separated by 7 m, despite similar δ¹³C_DIC and partial pressure of CO₂, was considered as the result of different evolution of shallow groundwater affected by deep-seated CO₂ preferentially rising along fracture zones. Electrical resistivity tomography also suggested flow through fracture zones beneath the CO₂-rich springs, showing low resistivity compared to other surveyed zones. However, soil CO₂ efflux was low compared to that in other natural CO₂ emission sites, and in particular it was noticeably low near the CO₂-rich springs, whereas δ¹³C_CO2 was high close the CO₂-rich springs. The dissolution of CO₂ in the near-surface water body seemed to decrease the deep-seated CO₂ leakage through the soil layer, while δ¹³C_CO2 imprinted the source. End-member mixing analysis was performed to assess the contribution of deep-seated CO₂ to the low soil CO₂ efflux by assuming that atmospheric CO₂ and soil CO₂ (by respiration) as well as deep-seated CO₂ contribute to the soil CO₂ efflux. For each end-member, characteristic δ¹³C_CO2 and CO₂ concentrations were defined, and then their apportionment to soil CO₂ efflux was estimated. The resultant proportion of deep-seated CO₂ was up to 8.8%. Unlike the spatial distribution of high soil CO₂ efflux, high proportions exceeding 3% were found around the CO₂-rich springs along the east-west valley. The study results indicate that soil CO₂ efflux measurement should be combined with carbon isotopic analysis in GCS sites for CO₂ leakage detection because CO₂ dissolution in the underground water body may blur leakage detection on the surface. The implication of this study is the need to quantitatively assess the contribution of deep-seated CO₂ using the soil CO₂ concentration, soil CO₂ efflux, and δ¹³C_CO2 at each measurement site.

Leakage of CO2 from geological storage and its impacts on fresh soil-water systems: a review

Leakage of CO₂ from the geological storage is a serious issue for the sustainability of the receiving fresh soil-water systems. Subsurface water quality issues are no longer related to one type of pollution in many regions around the globe. Thus, an effort has been made to review studies performed to investigate supercritical CO₂ (scCO₂) and CO₂ enrich brine migration and it's leakage from geological storage formations. Further, the study also reviewed it's impacts on fresh soil-water systems, soil microbes, and vegetation. The first part of the study discussed scCO₂/CO₂ enrich brine migration and its leakage from storage formations along with it's impact on pore dynamics of hydrological regimes. Later, a state-of-the-art literature survey has been performed to understand the role of CO₂-brine leakage on groundwater dynamics and its quality along with soil microbes and plants. It is observed in the literature survey that most of the studies on CO₂-brine migration in storage formations reported significant CO₂-brine leakage due to over-pressurization through wells (injections and abandoned), fracture, and faults during CO₂ injection. Thus, changes in the groundwater flow and water table dynamics can be the first impact of the CO₂-brine leakage. Subsequently, three major alterations may also occur-(i) drop in pH of subsurface water, (ii) enhancement of organic compounds, and (iii) mobilization of metals and metalloids. Geochemical alteration depends on the amount of CO₂ leaked and interactions with host rocks. Therefore, such alteration may significantly affect soil microbial dynamics and vegetation in and around CO₂ leakage sites_. In-depth analysis of the available literature fortifies that a proper subsurface characterization along with the bio-geochemical analysis is extremely important and should be mandatory to predict the more accurate risk of CO₂ capture and storage activities on soil-water systems.

Ocean acidification and hypoxia alter organic carbon fluxes in marine soft sediments

Anthropogenic stressors can alter the structure and functioning of infaunal communities, which are key drivers of the carbon cycle in marine soft sediments. Nonetheless, the compounded effects of anthropogenic stressors on carbon fluxes in soft benthic systems remain largely unknown. Here, we investigated the cumulative effects of ocean acidification (OA) and hypoxia on the organic carbon fate in marine sediments, through a mesocosm experiment. Isotopically labelled macroalgal detritus (¹³ C) was used as a tracer to assess carbon incorporation in faunal tissue and in sediments under different experimental conditions. In addition, labelled macroalgae (¹³ C), previously exposed to elevated CO₂ , were also used to assess the organic carbon uptake by fauna and sediments, when both sources and consumers were exposed to elevated CO₂ . At elevated CO₂ , infauna increased the uptake of carbon, likely as compensatory response to the higher energetic costs faced under adverse environmental conditions. By contrast, there was no increase in carbon uptake by fauna exposed to both stressors in combination, indicating that even a short-term hypoxic event may weaken the ability of marine invertebrates to withstand elevated CO₂ conditions. In addition, both hypoxia and elevated CO₂ increased organic carbon burial in the sediment, potentially affecting sediment biogeochemical processes. Since hypoxia and OA are predicted to increase in the face of climate change, our results suggest that local reduction of hypoxic events may mitigate the impacts of global climate change on marine soft-sediment systems.

Potential CO2 intrusion in near-surface environments: a review of current research approaches to geochemical processes

Carbon dioxide (CO₂) capture and storage (CCS) plays a crucial role in reducing carbon emissions to the atmosphere. However, gas leakage from deep storage reservoirs, which may flow back into near-surface and eventually to the atmosphere, is a major concern associated with this technology. Despite an increase in research focusing on potential CO₂ leakage into deep surface features and aquifers, a significant knowledge gap remains in the geochemical changes associated with near-surface. This study reviews the geochemical processes related to the intrusion of CO₂ into near-surface environments with an emphasis on metal mobilization and discusses about the geochemical research approaches, recent findings, and current knowledge gaps. It is found that the intrusion of CO₂(g) into near-surface likely induces changes in pH, dissolution of minerals, and potential degradation of surrounding environments. The development of adequate geochemical research approaches for assessing CO₂ leakage in near-surface environments, using field studies, laboratory experiments, and/or geochemical modeling combined with isotopic tracers, has promoted extensive surveys of CO₂-induced reactions. However, addressing knowledge gaps in geochemical changes in near-surface environments is fundamental to advance current knowledge on how CO₂ leaks from storage sites and the consequences of this process on soil and water chemistry. For reliable detection and risk management of the potential impact of CO₂ leakage from storage sites on the environmental chemistry, currently available geochemical research approaches should be either combined or used independently (albeit in a manner complementarily to one another), and the results should be jointly interpreted.

Simulating and Quantifying Multiple Natural Subsea CO2 Seeps at Panarea Island (Aeolian Islands, Italy) as a Proxy for Potential Leakage from Subseabed Carbon Storage Sites

Carbon dioxide (CO₂) capture and storage (CCS) has been discussed as a potentially significant mitigation option for the ongoing climate warming. Natural CO₂ release sites serve as natural laboratories to study subsea CO₂ leakage in order to identify suitable analytical methods and numerical models to develop best-practice procedures for the monitoring of subseabed storage sites. We present a new model of bubble (plume) dynamics, advection-dispersion of dissolved CO₂, and carbonate chemistry. The focus is on a medium-sized CO₂ release from 294 identified small point sources around Panarea Island (South-East Tyrrhenian Sea, Aeolian Islands, Italy) in water depths of about 40-50 m. This study evaluates how multiple CO₂ seep sites generate a temporally variable plume of dissolved CO₂. The model also allows the overall flow rate of CO₂ to be estimated based on field measurements of pH. Simulations indicate a release of ∼6900 t y^-1 of CO₂ for the investigated area and highlight an important role of seeps located at >20 m water depth in the carbon budget of the Panarea offshore gas release system. This new transport-reaction model provides a framework for understanding potential future leaks from CO₂ storage sites.

CO2 leakage can cause loss of benthic biodiversity in submarine sands

One of the options to mitigate atmospheric CO₂ increase is CO₂ Capture and Storage in sub-seabed geological formations. Since predicting long-term storage security is difficult, different CO₂ leakage scenarios and impacts on marine ecosystems require evaluation. Submarine CO₂ vents may serve as natural analogues and allow studying the effects of CO₂ leakage in a holistic approach. At the study site east of Basiluzzo Islet off Panarea Island (Italy), gas emissions (90-99% CO₂) occur at moderate flows (80-120 L m^-2 h^-1). We investigated the effects of acidified porewater conditions (pH_T range: 5.5-7.7) on the diversity of benthic bacteria and invertebrates by sampling natural sediments in three subsequent years and by performing a transplantation experiment with a duration of one year, respectively. Both multiple years and one year of exposure to acidified porewater conditions reduced the number of benthic bacterial operational taxonomic units and invertebrate species diversity by 30-80%. Reduced biodiversity at the vent sites increased the temporal variability in bacterial and nematode community biomass, abundance and composition. While the release from CO₂ exposure resulted in a full recovery of nematode species diversity within one year, bacterial diversity remained affected. Overall our findings showed that seawater acidification, induced by seafloor CO₂ emissions, was responsible for loss of diversity across different size-classes of benthic organisms, which reduced community stability with potential relapses on ecosystem resilience.

The Importance of Natural Acidified Systems in the Study of Ocean Acidification: What Have We Learned?

Human activity is generating an excess of atmospheric CO₂, resulting in what we know as ocean acidification, which produces changes in marine ecosystems. Until recently, most of the research in this area had been done under small-scale, laboratory conditions, using few variables, few species and few life cycle stages. These limitations raise questions about the reproducibility of the environment and about the importance of indirect effects and synergies in the final results of these experiments. One way to address these experimental problems is by conducting studies in situ, in natural areas where expected future pH conditions already occur, such as CO₂ vent systems. In the present work, we compile and discuss the latest research carried out in these natural laboratories, with the objective to summarize their advantages and disadvantages for research to improve these investigations so they can better help us understand how the oceans of the future will change.