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.

To obtain otolith material from marine sediment, the latter should undergo a wet sieving process. This procedure consists in forcing the marine sediment through different sieves with the help of water pressure, with the sieves mounted one on top of one another and successively smaller in mesh size. I used a sieve with a mesh size of 125 µm for the coarse fraction and 32 µm for the fine fraction. As isotopic measurements are expected to be performed on the otoliths recovered from the coarse fraction, I used deionized (DI) water for sieving on the top sieve and tap water on the bottom (ions present in tap water may alter the results). By the time sample sieving is done, the clays that comprise most of the sediment are disintegrated and washed through the sink. The residue (which includes the otoliths) is separated by grain size.

Based on my personal experience, sieving is a harder task than it may seem. You should be very persistent, as, per sample, it will take a mean time of 1 ½ hr. to separate the clay from the rest of the material. Also, you must be very careful not to lose any material during the process, which can be a challenge when the sieves get clogged. Afterwards comes the picking procedure, on which we must select the otoliths from the rest of the same-size material. We do so with the help of a stereoscopic microscope that allows us to clearly distinguish the material despite their small size. From all the new techniques and procedures I am having to put into practice during my Master Thesis development, picking is my personal favorite. The reason for this is how relaxing picking feels to me. I just get my headphones on, put some jazz music (Find my favorite tracks for picking attached by the end of this post) and submerge into an iridescent universe where sponge spicules, foraminifera, and the tiniest of bivalve shells swarm around.

Working with MERS in my Final Master´s Thesis is being a greatly enjoyable challenge. Considering I have a mostly paleontological background, all the bibliographical approach has been quite new for me, however, I am currently learning about climatic proxies and events in a much deeper way than I have ever had. Also, fishes are a greatly overlooked group in paleontology (you mostly learn about their great Devonian radiation and little more), so getting to grasp deeper knowledge on their recent paleobiogeography has awakened my curiosity towards this resilient and antique clade. Finally, this opportunity it is giving me a much deeper and broad understanding of how climate and climate changes work, which is very exciting for me, as contributing to the climate change problem is one of my main focuses right now.

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.