Mercury loads from Submarine Groundwater Discharge (SGD) may represent an overlooked source of methylmercury (MeHg) to the ocean, affecting human and ecosystem health. The SGD process involves the flow of fresh, saline, or mixed groundwater from coastal aquifers into the oceans classified in different components. Existing studies rarely report the fluxes supplied by the different SGD components; therefore, the relevance of SGD as a source of mercury remains unclear. We aimed to quantify SGD-driven mercury/methylmercury fluxes to the coast, focusing on the largest coastal lagoon in the western Mediterranean. We measured total dissolved mercury and MeHg in surficial and porewaters during the summer and autumn. Porewaters were enriched in total dissolved Hg and MeHg compared to the lagoon. Lagoon shore waters and porewaters with a high concentration of labile dissolved organic matter were prone to MeHg formation and thus had higher MeHg concentrations. The mercury input through SGD to the Mar Menor lagoon (4300 mmol year–1) was similar to atmospheric deposition and 1 order of magnitude greater than the stream input. Among different components of the SGD, long-scale lagoon water recirculation dominated. These findings have substantial implications for the regional Hg budget and raise awareness of the importance of considering the different SGD components for an accurate estimation of SGD-based Hg input to the coastal ocean., This research was funded by the Marie Curie Postdoctoral Fellowship HORIZON-MSCA-2022-PF-01 project grant number 101106387. The research was also supported by MerTerMar (PID2019-lll722RJ-I00), the OPAL (PID2019-110311RB-C21), MINIOM (PID2022-142480NB-I00), WINDERS (TED2021-130710B-I00) funded by the Spanish Ministry of Science Innovation and Universities (MICINN) and by European Union NextGenerationEU/PRTR. This study also formed part of the THINKINAZUL program and was supported by MICIU with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Fundación Séneca with funding from Comunidad Autónoma Región de Murcia (CARM) with additional funds also provided by the D.G. Mar Menor (CARM). C. Ruiz-González. was further supported by a RyC contract (RYC2019-026758-I). A. G. Bravo was supported by RyC contract (RYC2019-028400-I). J. Rodriguez-Puig acknowledges financial support from FPU grant FPU20/01369, and the institutional support of the María de Maeztu Programme (CEX2019-000940-M) for Units of Excellence of the Spanish Ministry of Science and Innovation. M. Montero-Curiel acknowledges the funding from the FPI grant (PRE2020-096147), with the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S-20-5). D. Romano-Gude acknowledges the funding from the FPI grant (PRE2020-095468), with the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S-20-2). I. Alorda-Montiel acknowledges financial support from FPI grant PRE2020-092343. M. Diego-Feliu acknowledges financial support from grant JDC2022-050316-I funded by MCIN/AEI/10.13039/501100011033 and by the European Union - NextGenerationEU/PRTR, and support from the Grup d’Hidrologia Subterrània - GHS (Universitat Politècnica de Catalunya) funded by grant 2021-SGR 00609 from Generalitat de Catalunya. L-E. Heimbürger-Boavida acknowledges financial support from the French National Research Agency (ANR) through the project HydrOThermal Mercury (ANR-21-CE34-0026), and from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie GMOSTrain, grant agreement no. 860497., Peer Reviewed, Article signat per xx autors/es: Céline Lavergne,* Júlia Rodriguez-Puig, Clara Ruiz-González, María Montero-Curiel, Gemma Casas,Daniel Romano-Gude, Irene Alorda-Montiel, Júlia Dordal-Soriano, Aaron Alorda-Kleinglass,Marc Diego-Feliu, Javier Gilabert, Alex Campillo-de La Maza, Cristina Romera-Castillo, Natalia Torres-Rodriguez, Lars-Eric Heimbürger-Boavida, Jordi García-Orellana, Valentí Rodellas,and Andrea G. Bravo*, Postprint (published version)

13 pages, 6 figures, 2 tables, supplementary material www.frontiersin.org/articles/10.3389/feart.2025.1560106/full#supplementary-material.-- Data availability statement: The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author., The Putaendo watershed (Central Andes, Chile), notable for its pre-colonial history and as the first independent Chilean town (1817), also exhibits special hydrological features. It is one of the few areas in the Andes Cordillera where the inhabitants rely almost entirely on periglacial water sources. Since 2010, the region has also experienced a severe megadrought, drastically altering the water supply and straining the livelihoods of pastoralists and peasants to the limit. However, despite the significant decrease in precipitation recorded during the megadrought, water continued to flow from the headwaters to the Putaendo River. To elucidate the mechanisms behind this water persistence, we quantified the individual contributions of different water sources (snow, rock glaciers, and groundwater) within this basin through the analysis of stable water isotopes and major ions. The region’s simple winter precipitation regime, another crucial hydrological characteristic, further facilitates the clear discrimination and quantification of meltwater inputs distinct from precipitation using physicochemical tracers. Additionally, to comprehensively understand public knowledge regarding water origin, the impact of the megadrought and climate change, and the potential development of mega-mining projects (as this is the last basin without this activity in the Chilean Central Andes), we conducted a social perception analysis using a cross-sectional descriptive survey with non-probabilistic causal sampling. Upstream of the Chacrillas dam’s, where there is minimal human intervention, the river in this arid water cycle exhibited a marked predominance of water contributions from rock glaciers (56.1%), followed by groundwater (32.7%) and snow (11.2%). The inhabitants of the territory domonstrates a remarkable awareness and prior knowledge, with 45.5% of respondents identifying rock glaciers as the main source of water during dry years. There was also a clear negative public opinion regarding the developing of mega-mining projects in the valley (84.1%). Integrating these perceptions of water scarcity’s social complexities with an advanced understanding of water source contribution provides crucial information for regional water security management, This work also acknowledges to ANID/FONDAP No. 1511009; to the Severo Ochoa Centre of Excellence accreditation (CEX2024-001494-S), funded by AEI 10.13039/501100011033, and to the project Highlands.3 from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 872328, Projects: UCV2095, Centro de Acción Climática-PUCV, DI emergent 039.340/2021 PUCV and IAEA project CRPF33031. Celine L. was funded by the projects ANID InES I + D 2021 grant number INID210013 and Marie Curie Postdoctoral Fellowship HORIZON-MSCA-2022-PF-01 project grant number 101106387, Peer reviewed

12 pages, 5 figures, 1 table, supporting information https://doi.org/10.1021/acs.est.5c07191.-- The complete dataset can be downloaded at https://doi.org/10.5281/zenodo.17045205, Mercury loads from Submarine Groundwater Discharge (SGD) may represent an overlooked source of methylmercury (MeHg) to the ocean, affecting human and ecosystem health. The SGD process involves the flow of fresh, saline, or mixed groundwater from coastal aquifers into the oceans classified in different components. Existing studies rarely report the fluxes supplied by the different SGD components; therefore, the relevance of SGD as a source of mercury remains unclear. We aimed to quantify SGD-driven mercury/methylmercury fluxes to the coast, focusing on the largest coastal lagoon in the western Mediterranean. We measured total dissolved mercury and MeHg in surficial and porewaters during the summer and autumn. Porewaters were enriched in total dissolved Hg and MeHg compared to the lagoon. Lagoon shore waters and porewaters with a high concentration of labile dissolved organic matter were prone to MeHg formation and thus had higher MeHg concentrations. The mercury input through SGD to the Mar Menor lagoon (4300 mmol year–1) was similar to atmospheric deposition and 1 order of magnitude greater than the stream input. Among different components of the SGD, long-scale lagoon water recirculation dominated. These findings have substantial implications for the regional Hg budget and raise awareness of the importance of considering the different SGD components for an accurate estimation of SGD-based Hg input to the coastal ocean, This research was funded by the Marie Curie Postdoctoral Fellowship HORIZON-MSCA-2022-PF-01 project grant number 101106387. The research was also supported by MerTerMar (PID2019-111722RJ-I00), the OPAL (PID2019-110311RB-C21), MINIOM (PID2022-142480NB-I00), WINDERS (TED2021-130710B-I00) funded by the Spanish Ministry of Science Innovation and Universities (MICINN) and by European Union NextGenerationEU/PRTR. This study also formed part of the THINKINAZUL program and was supported by MICIU with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Fundación Séneca with funding from Comunidad Autónoma Región de Murcia (CARM) with additional funds also provided by the D.G. Mar Menor (CARM). C. Ruiz-González. was further supported by a RyC contract (RYC2019-026758-I). A. G. Bravo was supported by RyC contract (RYC2019-028400-I). J. Rodriguez-Puig acknowledges financial support from FPU grant FPU20/01369, and the institutional support of the María de Maeztu Programme (CEX2019-000940-M) for Units of Excellence of the Spanish Ministry of Science and Innovation. M. Montero-Curiel acknowledges the funding from the FPI grant (PRE2020-096147), with the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S-20-5). D. Romano-Gude acknowledges the funding from the FPI grant (PRE2020-095468), with the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S-20-2). I. Alorda-Montiel acknowledges financial support from FPI grant PRE2020-092343. M. Diego-Feliu acknowledges financial support from grant JDC2022-050316-I funded by MCIN/AEI/10.13039/501100011033 and by the European Union - NextGenerationEU/PRTR, and support from the Grup d’Hidrologia Subterrània - GHS (Universitat Politècnica de Catalunya) funded by grant 2021-SGR 00609 from Generalitat de Catalunya. L-E. Heimbürger-Boavida acknowledges financial support from the French National Research Agency (ANR) through the project HydrOThermal Mercury (ANR-21-CE34-0026), and from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie GMOSTrain, grant agreement no. 860497, This work acknowledges the Severo Ochoa Centre of Excellence accreditation (CEX2024-001494-S) funded by AEI 10.13039/501100011033. Support from the Direcció General de Recerca from Generalitat de Catalunya (2021-SGR-00640) through the Marine and Environmental Biogeosciences Research Group-MERS is also acknowledged, Peer reviewed

15 pages, 4 figures, 4 tables, supplementary data https://doi.org/10.1016/j.ecolind.2025.113679.-- Data availability: Data will be made available on request, Coastal aquatic ecosystems are highly dynamic and influenced by multiple natural and anthropogenic factors that can contribute to their degradation. In this study, we present an innovative and interdisciplinary approach that integrates the analysis of the physicochemical properties, metal concentrations, and microbial communities in the San Antonio River and Petrel Lagoon, located in Central Chile, to evaluate the ecological health of this coastal ecosystem. Nutrient concentrations, in particular NO3− (values ranged from 26.6 ± 0.0 and 37.6 ± 9.4 mg/L) and PO42− (values > 8.0 mg/L), were elevated upstream, reflecting agricultural inputs, while microbial indicators (E. coli) surpassed regulatory thresholds (>1,000MPN/100 ml) in several sites. Metal analyses revealed high Na and Mg, concentrations (>2,000 mg/L and > 280 mg/L, respectively) in waters and in Al, Fe and Li in sediments (values ranged from 20,000 to 1.0x106 mg/kg). Microbial community analyses in waters identified that most of the sites were dominated by Idiomarinaceae (16 % of total community) and Marinicella (5 % of total community), and with enrichment in Owenweeksia (12 % of total community) in downstream sites. Potential pathogenic genera, including Cutibacterium acnes, were detected, suggesting risks to water quality and public health. Multivariate analyses revealed spatial structuring driven by salinity and nutrient inputs, emphasizing the influence of hydrodynamics and land use. These findings highlight significant environmental gradients and contamination hotspots, stressing the need for a comprehensive approach that integrates a similar approach to better understand microbial dynamics, metal behavior, and their combined impact on water quality. Future studies should aim to overcome the limitations of this work in order to identify relevant indicators and develop effective monitoring strategies that support the sustainability of ecosystem services, This work was supported by the Ministry of Education of Chile through the project Project RED21992 “Sistema articulado de investigación en Cambio Climático y Sustentabilidad en zonas costeras de Chile”. Additional funding was through the Fondecyt grant 1211977 and 1240579. Additionally, the authors acknowledge the mobility program PCI ECOS-ANID [210007] (ECOS-Sud C21B01, “EMMA”) funded by the French ministries MAEDI (Ministère des Affaires Étrangères et du Développement International) and MENESR (Ministère de l'Éducation nationale, de l'Enseignement supérieur et de la Recherche), as well as the Chilean ANID (Agencia Nacional de Investigación y Desarrollo de Chile). Céline L. was funded by the projects ANID InES I + D 2021 grant number INID210013 and Marie Curie Postdoctoral Fellowship (HORIZON-MSCA-2022-PF-01 project, grant number 101106387) and acknowledges the Severo Ochoa Centre of Excellence accreditation (CEX2024-001494-S) funded by AEI 10.13039/501100011033. Macarena P. has been founded by the Excellence grant of the University of Playa Ancha, Chile and by the "Programa de Contratación de Ayudantes de Investigación de Postgrado de la Universidad de Playa Ancha (Proyecto de Fortalecimiento UPA 24991)", Peer reviewed

Special Issue Monitoring and Assessment of Environmental Quality in Coastal Ecosystems, 4th Edition.-- 16 pages, 7 figures.-- Data Availability Statement:
The data that support the results of this study can be obtained from the corresponding author upon request, Antarctica is experiencing one of the fastest warming rates globally, profoundly impacting seawater temperature and salinity, with direct consequences for marine life. The present study examined the combined effects of salinity fluctuations at 20, 33 (control salinity), and 41 psu, and temperatures of 2 °C (control temperature) and 8 °C (thermal stress) for 3 days, on the health and physiology of the Antarctic intertidal macroalga Adenocystis utricularis. Photosynthetic activity, photoinhibition, and photoprotective processes were assessed alongside biomarkers of oxidative stress/damage (total ROS, lipid peroxidation, and protein carbonylation) and antioxidant/osmotic response (ascorbate, free amino acids, and proline). The results showed that maximum quantum yield (Fv/Fm) remained stable under both salinity and thermal stress. However, productivity (ETRmax), the photoprotection index (NPQmax), and irradiance saturation (EkETR) were significantly decreased at 8 °C, remaining constant under salinity fluctuations. At 2 °C, oxidative stress and damage were significantly higher under hypo- and hypersalinity conditions. However, at 8 °C, oxidative stress indicators decreased, accompanied by increased ascorbate levels in both hypo- (20 psu) and hypersalinity (41 psu) treatments compared to the control salinity. While warming temperatures negatively altered the oxidative response of A. utricularis at a 33 psu, we report here an interactive effect between salinity and temperature, leading to an altered stress response to salinity fluctuations under thermal stress. This study provides key information to better understand the adaptation of Antarctic intertidal macroalgae to multifactor climate change consequences, This study was financed by the INACH RT_30-21 grant from the Instituto Antártico Chileno, Chile. VM was funded by Fondecyt Regular N°1211977 and COPAS COASTAL ANID FB210021. CL was funded by the projects ANID InES I + D 2021, grant number INID210013, and the Marie Curie Postdoctoral Fellowship HORIZON-MSCA-2022-PF-01 project, grant number 101106387, and CL acknowledges the Severo Ochoa Centre of Excellence accreditation (CEX2019-000928-S), funded by AEI 10.13039/501100011033. BC acknowledges Fondecyt Regular N°1221264 and ANID-Milenio-NCN2023_054. KG was financed by the UPLA inner doctoral scholarship UPA22991. Data obtained for total ROS quantification were granted by Cytation 5 equipment from the Fondequip EQM160131 project of the University of Playa Ancha, Chile, Peer reviewed

10 pages, 6 figures, supplementary material https://www.frontiersin.org/articles/10.3389/fpls.2025.1727961/full#supplementary-material.-- Data availability statement: The original contributions presented in the study are included in the article/Supplementary Material https://doi.org/10.20350/digitalCSIC/18067. Further inquiries can be directed to the corresponding author, Warming oceans are reshaping coastal ecosystems, yet the thermal sensitivity of many foundation macroalgae remains poorly constrained. Under SSP2-4.5, sea surface temperatures are expected to rise by ~2.7 °C by 2100, with unknown consequences for the canopy-forming kelp Lessonia spicata. We exposed thalli to control (15 °C) and warming (18 °C) conditions for 14 days to simulate near-future thermal scenarios, assessing photophysiology, oxidative stress and pigment composition. Warming caused marked declines in Fv/Fm and ETRmax and increased NPQmax, signaling reduced photochemical efficiency and enhanced thermal energy dissipation. Simultaneously, pigment concentrations decreased, ROS and MDA accumulated, and antioxidant capacity dropped, indicating rapid disruption of cellular redox homeostasis. Our results demonstrate that even modest warming consistent with mid-century projections triggers immediate photophysiological impairment and oxidative stress in L. spicata. These mechanistic insights highlight potential vulnerability of kelp forests in central Chile as ocean temperatures continue to rise, The financial and logistical support were support by the project FONDECYT 11180197, ANID, Chile. The authors acknowledges to supported by the ANID InES GÉNERO [INGE 220004] – ANID and the “Programa de Apoyo para el pago de Procesamiento de Artículos InES Género 2025, Dirección General de Investigación, Universidad de Playa Ancha, Chile”. CNA acknowledges to national doctoral scholarship 21231539 by ANID, Chile. Also, C.L. was supported by the ANID InES I + D 2021 [INID210013], Marie Curie Postdoctoral Fellowship HORIZON-MSCA-2022-PF-01 project [101106387] and the Severo Ochoa Centre of Excellence accreditation [CEX2019-000928-S] funded by AEI 10.13039/501100011033, Peer reviewed

14 pages, 8 figures, 1 table, supplementary data https://doi.org/10.1016/j.jenvman.2026.129386.-- Data availability: Data will be made available on request, Microorganisms play a crucial role in maintaining ecosystem functions by transforming matter and energy. The impact of environmental perturbations on the cycling and loss of aquatic microbial communities remains poorly characterized, particularly in the Southern Hemisphere. Here, we present the shifts in active microbial community and the maintenance of glacier-derived microbial core along the Aconcagua Basin, Central Chile. Through metabarcoding of the 16S rRNA transcripts and genes in water samples from 13 sites along the river continuum-including glacier-influenced headwaters, mining-proximal zones, agro-industrial reaches, and estuarine environments-distinct shifts in microbial community composition were observed. Glacier-fed waters harbored rich, cold-adapted microbial communities with elevated viral pressure, while mining and/or natural acid drainage promoted the dominance (up to 52% relative abundance) of sulfur-oxidizing bacteria (SOB) and reduced overall diversity. Among the SOB taxa, Thiovirga, Sulfuricurvum, and Thiobacillus-affiliated ASVs dominated together with minor taxa such as Thiomonas, Longilinea, Alishewenella, and Acidithiobacillus. Downstream, runoff and wastewater inputs from agricultural and urban areas altered microbial assemblages, introducing new taxa (e.g., C39 and Cellvibrio) and further diminishing glacier-derived core members. Estuarine sites exhibited pronounced eutrophication and significant loss of upstream microbial diversity. Along the basin, the virus-to-prokaryotes ratio gradually decreased (i.e., 95 to 5) from the glacier-fed lagoon with low prokaryotic biomass to the estuarine wetland with high virus and prokaryotic abundances. This study underscores the importance of integrating microbial diversity into water resource management and conservation policy to ensure ecosystem resilience amid climate change and intensifying human impacts, This research was funded by PCI ECOS-ANID [210007] (ECOS-Sud C21B01), ANID InES I + D [INID210013], FIC-R BIP [40046077] of Valparaíso Regional Government and the Plan de Fortalecimiento de Universidades del Estado (CUECH) [RED21992]. C.L. was funded by the Marie Curie Postdoctoral Fellowship HORIZON-MSCA-2022-PF-01 project [101106387]. E.M. and C.A.S. is funded by ANID FONDECYT [1240365 and 11201072]. M.P. has been founded by the Excellence grant of the University of Playa Ancha, Chile and by the "Programa de Contratación de Ayudantes de Investigación de Postgrado de la Universidad de Playa Ancha (Proyecto de Fortalecimiento UPA 24991)", This work contributes to the Institut de Ciències del Mar "Severo Ochoa Centre of Excellence" accreditation CEX2024-001494-S funded by AEI 10.13039/501100011033 of the Spanish Ministry of Science and Innovation, Peer reviewed

14 pages, 7 figures, 1 table, supporting information https://doi.org/10.1002/lno.70371.-- Data Availability Statement: Data described in this manuscript are available at Zenodo repository under 10.5281/zenodo.17045204. Any other data are available to the reader upon request to the corresponding authors, Submarine groundwater discharge (SGD) is a widely recognized pathway for nutrient transport to coastal systems. Prior studies overlook the unique biogeochemical signatures of different SGD pathways, neglecting their differences in biogeochemical transformations and spatiotemporal factors. Here, we present an integrated assessment of nutrient delivery through three SGD pathways (fresh SGD, long-scale recirculated SGD, and short-scale porewater exchange), alongside surface water inputs, to evaluate their seasonality and impact on nutrient dynamics in a coastal lagoon. We conducted five field surveys in March, July, and November 2021, July 2024, and March 2025 in the Mar Menor lagoon, an ecosystem facing severe ecological degradation, to analyze nutrient concentrations in stream water, groundwater, and lagoon water. Concentrations in discharging waters were shaped by watershed-scale land use and local redox-driven biogeochemical processes. Recirculated SGD (long-scale and short-scale), often overlooked, emerged as the dominant source of ammonium (NH+4; 100–160 kmol d−1), dissolved organic nitrogen (DON; 270–900 kmol d−1), dissolved inorganic phosphorus (DIP; 4–30 kmol d−1), and dissolved silica (DSi; 200–270 kmol d−1) across seasons, while continental discharge (streams and fresh SGD) was the main source of nitrate and nitrite (NOx; 30–210 kmol d−1). Short-scale recirculation flows controlled seasonal nutrient limitation by delivering additional DIP inputs (24 kmol d−1) in summer, increasing eutrophication risk. Identifying preferential transport pathways for each nutrient into coastal systems is crucial for predicting and managing ecosystem vulnerability, This work was funded by the Agencia Estatal de Investigación through the Origin and Pathways of Anthropogenic solutes into coastal Lagoons: groundwater, sediments, and episodic events (PID2019-110311RB-CXX) and WINDERS projects (TED2021-130710B-I00) and contributes to ICTA-UAB “María de Maeztu” Programme (CEX2024-001506-M funded by Ministerio de Ciencia, Innovación y Universidades (MICIU/Agencia Estatal de Investigación/10.13039/501100011033). Júlia Rodriguez-Puig acknowledges financial support from Grant FPU20/01369 and support from Marine and Environmental Biogeosciences Research Group (2021-SGR 00640). Clara Ruiz-González acknowledges funding from the grant (RYC2019-026758-I) and project MINIOM (PID2022-142480NB-I00) funded by MICIU, as well as the institutional support of the “Severo Ochoa Centre of Excellence” accreditation (CEX2024-001494-S). Marc Diego-Feliu acknowledges financial support from Grant JDC2022-050316-I funded by MCIN/Agencia Estatal de Investigación and by the NextGenerationEU/ Plan de Recuperación, Transformación y Resiliencia (PRTR) and the support from the Grup d'Hidrologia Subterrània- Universitat Politècnica de Catalunya fundings (2021-SGR 00609). AGB acknowledges (RyC2019-028400-I) and MerTerMar (PID2019-111722RJ-I00). Irene Alorda-Montiel acknowledges financial support from Grant FPI/PRE2020-092343. Daniel Romano-Gude acknowledges the funding of the FPI Grant (PRE2020-095468). Céline Lavergne was funded by Marie Curie Postdoctoral Fellowship HORIZON-MSCA-2022-PF-01 #101106387. Gemma Casas was funded by PTA2022-022535-I. This study also formed part of the THINKINAZUL programme and was supported by MICIU with funding from NextGenerationEU (PRTR-C17.I1) and by Fundación Séneca with funding from Comunidad Autónoma Región de Murcia with additional funds also provided by the D.G. Mar Menor of the Regional Government of Murcia, Peer reviewed

17 pages, 7 figures, 1 table, supplementary material https://doi.org/10.1093/ismeco/ycag208.-- Data availability: The datasets generated and/or analysed during the current study are available in the European Nucleotide Archive (ENA) repository (BioProject PRJEB109277) and the MetaboLights repository (accession MTBLS14103 https://www.ebi.ac.uk/metabolights/editor/MTBLS14103/files?reviewCode=937ea8b4-a4cf-4a49-9f8a-28a99e05577d), The Antarctic intertidal zone hosts ecologically pivotal holobionts such as the brown macroalga Adenocystis utricularis, whose response to climate-driven stressors remains poorly understood. Here, we applied an integrative holobiont framework combining microbial community profiling, host physiological assays, and phycosphere metabolomics to assess how thermal stress (2°C vs. 8°C) influences microbiome dynamics, host stress responses, and algal surface chemistry after 5 days under experimental conditions. Results showed that warming induced a transient reduction in microbial diversity, with Shannon diversity decreasing at Day 3 under 8°C and returning to levels comparable to the control by Day 5, accompanied by marked community structural reorganization. Elevated temperature enriched Campylobacteria and Gammaproteobacteria, while Bacteroidia and Verrucomicrobia decreased. Functional predictions revealed a shift from nutrient cycling and carbon turnover at 2°C toward heterotrophy, fermentation, and bacterivory-related pathways at 8°C. Microbiome disruption was associated with impaired photosynthetic performance, increased oxidative damage, reduced antioxidant defenses, and altered osmolyte accumulation, under warming conditions. Untargeted metabolomics uncovered pronounced thermal reprogramming of the algal surface metabolome, with >98% of detected features remaining chemically uncharacterized. Detected key metabolites included ceramides, N-acyl amino acids, and amphipathic compounds with putative cytotoxic or antioxidant activities, many of which increased under 8°C. Together, these findings demonstrate that A. utricularis response to thermal stress emerges from dynamic host–microbiome–metabolome interactions. By linking microbial restructuring, host physiology, and metabolomic plasticity, our study highlights the holobiont as the operative unit of adaptation in Antarctic coastal ecosystems facing climate change, This study was financed by INACH RT_30-21 grant from the Instituto Antártico Chileno, Chile. K.G.P. was financed by INACH postgraduate scholarship DG_02-25, UPLA inner doctoral scholarship MINEDUC UPA24991 and “Concurso Semilleros I+D+I y/o creación para estudiantes 2025 de la Universidad de Playa Ancha, UPA24991 SEIV 03-26”. V.M.T. was funded by Fondecyt Regular N°1262251 and COPAS COASTAL ANID FB210021. NN was financed by Fondecyt Iniciación N°11160520. C.L. was funded by the projects ANID InES I + D 2021 grant number INID210013 and Marie Curie Postdoctoral Fellowship HORIZON-MSCA-2022-PF-01 project grant number 101106387 and acknowledges the Severo Ochoa Centre of Excellence accreditation (CEX2024-001494-S) funded by AEI 101106387. B.C. acknowledges Fondecyt Regular N°1221264 and ANID-Milenio-NCN2023_054. J.P.R. was financed by ANID National Doctoral Scholarship N°21230403. G.P.H. was financed by ANID National Doctoral Scholarship N°21251006. PAM was funded by Postdoc FONDECYT grant N°3240498. Data obtained for total ROS quantification was granted by Cytation 5 equipment from the Fondequip EQM160131 project of University of Playa Ancha, Chile. FRR was funded by Fondecyt Regular N°1260700, Peer reviewed

Mercury loads from Submarine Groundwater Discharge (SGD) may represent an overlooked source of methylmercury (MeHg) to the ocean, affecting human and ecosystem health. The SGD process involves the flow of fresh, saline, or mixed groundwater from coastal aquifers into the oceans classified in different components. Existing studies rarely report the fluxes supplied by the different SGD components; therefore, the relevance of SGD as a source of mercury remains unclear. We aimed to quantify SGD-driven mercury/methylmercury fluxes to the coast, focusing on the largest coastal lagoon in the western Mediterranean. We measured total dissolved mercury and MeHg in surficial and porewaters during the summer and autumn. Porewaters were enriched in total dissolved Hg and MeHg compared to the lagoon. Lagoon shore waters and porewaters with a high concentration of labile dissolved organic matter were prone to MeHg formation and thus had higher MeHg concentrations. The mercury input through SGD to the Mar Menor lagoon (4300 mmol year–1) was similar to atmospheric deposition and 1 order of magnitude greater than the stream input. Among different components of the SGD, long-scale lagoon water recirculation dominated. These findings have substantial implications for the regional Hg budget and raise awareness of the importance of considering the different SGD components for an accurate estimation of SGD-based Hg input to the coastal ocean., This research was funded by the Marie Curie Postdoctoral Fellowship HORIZON-MSCA-2022-PF-01 project grant number 101106387. The research was also supported by MerTerMar (PID2019-lll722RJ-I00), the OPAL (PID2019-110311RB-C21), MINIOM (PID2022-142480NB-I00), WINDERS (TED2021-130710B-I00) funded by the Spanish Ministry of Science Innovation and Universities (MICINN) and by European Union NextGenerationEU/PRTR. This study also formed part of the THINKINAZUL program and was supported by MICIU with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Fundación Séneca with funding from Comunidad Autónoma Región de Murcia (CARM) with additional funds also provided by the D.G. Mar Menor (CARM). C. Ruiz-González. was further supported by a RyC contract (RYC2019-026758-I). A. G. Bravo was supported by RyC contract (RYC2019-028400-I). J. Rodriguez-Puig acknowledges financial support from FPU grant FPU20/01369, and the institutional support of the María de Maeztu Programme (CEX2019-000940-M) for Units of Excellence of the Spanish Ministry of Science and Innovation. M. Montero-Curiel acknowledges the funding from the FPI grant (PRE2020-096147), with the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S-20-5). D. Romano-Gude acknowledges the funding from the FPI grant (PRE2020-095468), with the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S-20-2). I. Alorda-Montiel acknowledges financial support from FPI grant PRE2020-092343. M. Diego-Feliu acknowledges financial support from grant JDC2022-050316-I funded by MCIN/AEI/10.13039/501100011033 and by the European Union - NextGenerationEU/PRTR, and support from the Grup d’Hidrologia Subterrània - GHS (Universitat Politècnica de Catalunya) funded by grant 2021-SGR 00609 from Generalitat de Catalunya. L-E. Heimbürger-Boavida acknowledges financial support from the French National Research Agency (ANR) through the project HydrOThermal Mercury (ANR-21-CE34-0026), and from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie GMOSTrain, grant agreement no. 860497., Peer Reviewed, Article signat per xx autors/es: Céline Lavergne,* Júlia Rodriguez-Puig, Clara Ruiz-González, María Montero-Curiel, Gemma Casas,Daniel Romano-Gude, Irene Alorda-Montiel, Júlia Dordal-Soriano, Aaron Alorda-Kleinglass,Marc Diego-Feliu, Javier Gilabert, Alex Campillo-de La Maza, Cristina Romera-Castillo, Natalia Torres-Rodriguez, Lars-Eric Heimbürger-Boavida, Jordi García-Orellana, Valentí Rodellas,and Andrea G. Bravo*

Mercury loads from Submarine Groundwater Discharge (SGD) may represent an overlooked source of methylmercury (MeHg) to the ocean, affecting human and ecosystem health. The SGD process involves the flow of fresh, saline, or mixed groundwater from coastal aquifers into the oceans classified in different components. Existing studies rarely report the fluxes supplied by the different SGD components; therefore, the relevance of SGD as a source of mercury remains unclear. We aimed to quantify SGD-driven mercury/methylmercury fluxes to the coast, focusing on the largest coastal lagoon in the western Mediterranean. We measured total dissolved mercury and MeHg in surficial and porewaters during the summer and autumn. Porewaters were enriched in total dissolved Hg and MeHg compared to the lagoon. Lagoon shore waters and porewaters with a high concentration of labile dissolved organic matter were prone to MeHg formation and thus had higher MeHg concentrations. The mercury input through SGD to the Mar Menor lagoon (4300 mmol year-1) was similar to atmospheric deposition and 1 order of magnitude greater than the stream input. Among different components of the SGD, long-scale lagoon water recirculation dominated. These findings have substantial implications for the regional Hg budget and raise awareness of the importance of considering the different SGD components for an accurate estimation of SGD-based Hg input to the coastal ocean.

Submarine groundwater discharge (SGD) is a widely recognized pathway for nutrient transport to coastal systems. Prior studies overlook the unique biogeochemical signatures of different SGD pathways, neglecting their differences in biogeochemical transformations and spatiotemporal factors. Here, we present an integrated assessment of nutrient delivery through three SGD pathways (fresh SGD, long-scale recirculated SGD, and short-scale porewater exchange), alongside surface water inputs, to evaluate their seasonality and impact on nutrient dynamics in a coastal lagoon. We conducted five field surveys in March, July, and November 2021, July 2024, and March 2025 in the Mar Menor lagoon, an ecosystem facing severe ecological degradation, to analyze nutrient concentrations in stream water, groundwater, and lagoon water. Concentrations in discharging waters were shaped by watershed-scale land use and local redox-driven biogeochemical processes. Recirculated SGD (long-scale and short-scale), often overlooked, emerged as the dominant source of ammonium ((Formula presented.); 100-160 kmol d-1), dissolved organic nitrogen (DON; 270-900 kmol d-1), dissolved inorganic phosphorus (DIP; 4-30 kmol d-1), and dissolved silica (DSi; 200-270 kmol d-1) across seasons, while continental discharge (streams and fresh SGD) was the main source of nitrate and nitrite (NOx; 30-210 kmol d-1). Short-scale recirculation flows controlled seasonal nutrient limitation by delivering additional DIP inputs (24 kmol d-1) in summer, increasing eutrophication risk. Identifying preferential transport pathways for each nutrient into coastal systems is crucial for predicting and managing ecosystem vulnerability.