Unraveling the link between microbial biogeochemical cycles and mercury transformations in oxygen-depleted submarine groundwater discharges

Project Information

MIMOA

Grant agreement ID: 101106387

DOI

10.3030/101106387

Project closed

EC signature date 25 July 2023

Start date 1 January 2024

End date 30 June 2026

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 220 743,20

Coordinated by

AGENCIA ESTATAL CONSEJO SUPERIOR DE INVESTIGACIONES CIENTIFICAS
Spain

Periodic Reporting for period 1 - MIMOA (Unraveling the link between microbial biogeochemical cycles and mercury transformations in oxygen-depleted submarine groundwater discharges)

Reporting period: 2024-01-01 to 2026-06-30

Summary of the context and overall objectives of the project

Coastal aquifers are important transition zones between land, freshwater and the sea. When fertilisers and other nutrients enter these systems, it can stimulate microbial activity and reduce oxygen availability. This groundwater can reach the coastal ocean through a process called submarine groundwater discharge (SGD). In coastal waters, these low-oxygen conditions can influence the transformation of mercury, including the formation of methylmercury — a highly toxic compound that can accumulate in marine food webs and ultimately affect human health via the consumption of seafood. MIMOA investigates how nutrient-rich SGD carry mercury and modulates its transformation in coastal areas, addressing an important gap in current mercury budgets. By combining microbial ecology, mercury chemistry, biogeochemistry, molecular biology and science education, the project will provide new evidence to support better monitoring and management of vulnerable coastal zones. It will also raise public awareness of the need to protect freshwater from fertiliser pollution, and of the role healthy freshwater systems play in safeguarding coastal ecosystems from mercury impacts. In line with EU priorities to restore ocean, coastal and inland waters and prevent pollution, MIMOA connects scientific knowledge with policy needs and public engagement. Its expected impact is significant: improved understanding of mercury pathways can help protect marine ecosystems, seafood safety and coastal communities, while contributing in the longer term to reducing environmental and economic losses linked to oxygen depletion, degraded ecosystem services, fisheries impacts and methylmercury exposure.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Mercury pollution is a major concern for public health and coastal ecosystems, yet important sources are still missing from current mercury budgets. Until now, most assessments have focused on atmospheric deposition, while rivers and submarine groundwater discharge (SGD) has received little attention. The MIMOA project helps fill this gap by showing that SGD can be an important pathway for mercury and methylmercury entering Mediterranean coastal waters, including both detrital aquifers such as the Mar Menor lagoon in southern Spain and karstic systems such as the Marseille-Cassis Calanques in southern France.
In the Mar Menor, the project provides the first measurement of total mercury and methylmercury fluxes from different groundwater pathways. Results show that coastal groundwater was enriched in both forms of mercury. SGD inputs were about 70 times higher than river inputs and comparable to atmospheric deposition, demonstrating that groundwater must be included in future mercury assessments for this vulnerable lagoon.
In the Marseille-Cassis Calanques, mercury and methylmercury fluxes from natural karstic sources were similar to atmospheric deposition and up to five times higher than releases from one of Europe’s largest submarine wastewater treatment plants. Overall, MIMOA highlights SGD as a previously overlooked source that should be considered in EU coastal pollution monitoring and management.
To date, this project mainly resulted in three scientific articles published or under revision:
1. Gana B., et al. & Lavergne C.* (2024) Geoenvironmental Effects of the Hydric Relationship between the Del Sauce Wetland and the Laguna Verde Detritic Coastal Aquifer, Central Chile. Hydrology, doi: 10.3390/hydrology11100174;
2. Lavergne C.*, et al. & Bravo A.* (2025) Unveiling hidden mercury and methylmercury sources: the role of submarine groundwater discharge in the largest western Mediterranean Sea coastal lagoon. Environmental Science and Technology, doi: 10.1021/acs.est.5c07191;
3. Rodriguez‐Puig, J., et al. (2026) Recirculated submarine groundwater discharge dominates nutrient inputs and enhances eutrophication risk in a coastal lagoon. Limnology & Oceanography, doi: 10.1002/lno.70371

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The Mar Menor coastal lagoon is subject to environmental pressures resulting from domestic and agricultural discharges. It has traditionally been described that the main source of water entering the Mar Menor—and, consequently, of pollutants—came from the Albujón River. However, the impact of groundwater flow from an aquifer, which acts as an invisible connection beneath the surface, was unknown. During the MIMOA project we deciphered that this underground connection contributes largely to mercury release to the Mar Menor. It represents 70 times more dissolved mercury than the Albujón River—specifically, nearly 1 kg per year. Using isotopic tracers, we demonstrated that the mercury released into the Mar Menor through this pathway consists mostly of mercury emitted in the past (called legacy mercury) and stored in the aquifer. Finally, the study indicates that this underground connection, reaching the coastal zone, creates conditions conducive to the formation of methylmercury by the microorganisms inhabiting this ecosystem, which thrive due to high nutrient levels and a significant lack of oxygen. These findings highlight the importance of monitoring all sources of pollution in the coastal lagoons and particularly in the Mar Menor, including groundwater, which plays a key role in transporting microorganisms, nutrients, and pollutants to coastal ecosystems. This phenomenon is consistent with what may occur in other coastal ecosystems around the world and has implications for environmental management, conservation, and food security on a global scale.
Further research is needed to determine the hotspots of methylmercury formation within the subterranean estuaries, and to deepen understanding of both the diversity of subsurface microorganisms capable of mercury methylation and the cellular processes involved. We also identified spatial variability in SGD fluxes, highlighting the need to better integrate local data into global and regional models in order to improve predictions of mercury pollution in vulnerable coastal areas.