Zooming in on microbes from coastal aquifers to understand the effects of groundwater discharge in the ocean

A new study by the Institute of Marine Sciences (ICM-CSIC) and the Institute of Environmental Science and Technology of the Autonomous University of Barcelona (ICTA-UAB) published in the journal FEMS Microbiology Reviews points out the importance of investigating the microorganisms from coastal aquifers to understand and predict the effects of groundwater discharge to the sea.

Submarine groundwater discharge (SGD) plays a key role in the maintenance of coastal brackish ecosystems such as lagoons or wetlands, but, more importantly, it constitutes a very important source of nutrients, metals and pollutants to the coastal ocean.

For this reason, threats to coastal aquifers, such as excessive water pumping of water or the use of fertilizers, can endanger the sustainability and vulnerability of marine systems, as well as the services and goods that they provide, such as leisure activities or water and food provisioning.

In this context, it is essential to quantify the SGD-mediated transfer of chemicals to the ocean, which requires a detailed knowledge of the geochemical and microbial processes occurring at the land-ocean interface.

The ICM-CSIC and ICTA-UAB study, carried out in the framework of the GRAMMI project and published in the FEMS Microbiology Reviews journal, contains a review of the available studies on the microbial aspects associated with submarine groundwater discharge and identifies current challenges and future research directions in this field of research.

“Microbial activity is a key controller of both the fluxes and the fate of chemical compounds delivered by SGD, but very little is known regarding the microbial aspects related to this hydrological pathway”, explains the ICM-CSIC researcher Clara Ruiz González.

Studies on inland aquifers: the key to progress

Another of the main conclusions of this literature review is that some of the current methodologies used in coastal aquifers can be limiting for the achievement of an accurate description of groundwater microbiota.

In this regard, the authors explain that while coastal aquifers have been much less studied than inland aquifers, recent advances in inland groundwater microbiology are providing a valuable basis for anticipating the challenges that that the study of coastal aquifers might face.

“For example, we know that inland groundwater microbial communities are characterized by ultra-small cells that are not captured by common sampling strategies or some sequencing techniques, and hence most recent studies have probably missed this important microbial component of coastal aquifers”, details Ruiz González.

Learning from previous investigations in inland aquifers and in the ocean, the article proposes tools or approaches that could be tackled for achieving a more accurate understanding of microbes inhabiting the terrestrial-marine interface and the effects of submarine groundwater discharge on the functioning of marine plankton communities.

The need for collaboration between microbial ecology and hydrogeology

The authors point to the potential of new molecular techniques for the discovery of microbial species, for a better understanding on their influence on the chemical composition of groundwater and for deciphering whether coastal aquifers represent a source of microbial biodiversity for the ocean.

“The article is addressed not only to microbiologists but also to researchers working on SGD-related processes, such as oceanographers and hydrologists. The study of SGD has traditionally been approached from either a geochemical or a hydrogeological point of view, so we lack a biological understanding of the underlying processes”, regrets the ICTA-UAB researcher Jordi Garcia-Orellana.

From his part, Valentí Rodellas, also from the ICTA-UAB, explains that “Over the next few years, we expect to conduct multiple sampling campaigns on aquifers from Mediterranean and Mar Menor coasts, as well as in different Spanish national parks (Timanfaya, Cabrera, Cíes) to keep advancing towards a comprehensive understanding of microbial diversity and ecology at the terrestrial-marine interface”.

All in all, the authors are convinced that the incorporation of this microbial dimension in SGD investigations will help not only constrain the magnitude of solute fluxes supplied by this groundwater flow, but also gain insight into the ecological and biogeochemical consequences of SGD in the marine environment.

Reference Article

Ruiz-González, C., Rodellas, V., & Garcia-Orellana, J. (2021). The microbial dimension of submarine groundwater discharge: current challenges and future directions. FEMS Microbiology Reviews.