When we think of the hydrological connectivity between land and ocean, we immediately think of rivers. However, hidden from the eye, there is a much less known pathway that also contributes enormous quantities of water and chemicals (nutrients, pollutants) to the sea: the so-called submarine groundwater discharge, or SGD. Beneath our feet, a vast volume of groundwater flows through subterranean rivers, some of which discharge into oceans through submarine springs or diffusing slowly through coastal sediments. Over the last decades, multiple hydrogeological or geochemical studies have shown the relevance of SGD for the ocean. However, major knowledge gaps remain about the microbial dimension of this process, both regarding the important microbially-mediated chemical transformations of groundwater occurring in the terrestrial-marine interface, but also the effects of SGD on marine microbial communities. In this study, we review for the first time the state of the art of the microbial dimension of SGD, identifying current challenges and future research directions. We show that, despite the growing interest in microbial communities of coastal aquifers, there is still a need to improve the description of groundwater microbiota, which is characterized by ultrasmall cells, inactive and unknown taxa, and by high ratios of sediment-attached versus free-living cells. Studies should also explore the functional role of microbial communities along the entire groundwater–marine continuum, the bidirectional dispersal of microorganisms from groundwater and seawater, and the responses of marine bacterioplankton to SGD. Interactions between traditionally independent disciplines (e.g., hydrogeology, microbial ecology) are needed to fully understand SGD and its influence in coastal biogeochemical cycles.

Read the full study  here:

C. Ruiz-González, V. Rodellas and J. Garcia-Orellana (2021) The microbial dimension of submarine groundwater discharge: current challenges and future directions. FEMS Microbiology Reviews, fuab010, https://doi.org/10.1093/femsre/fuab010

Text written by Clara Ruiz-González and edited by Clara Ruiz and Félix Picazo