The Beach Aquifer Microbiome: Research Gaps and Data Needs

Abstract

Beach aquifers, located in the subsurface of sandy beaches, are unique ecosystems with steep chemical and physical gradients resulting from the mixing of terrestrial fresh groundwater and saline groundwater from the sea. While work has rapidly progressed to understand the physics and chemistry in this environment, much less is known about the microorganisms present despite the fact that they are responsible for vital biogeochemical processes. This paper presents a review of the current state of knowledge of microbes within beach aquifers and the mechanisms that control the beach aquifer microbiome. We review literature describing the distribution and diversity of microorganisms in the freshwater-saltwater mixing zone of beach aquifers, and identify just 12 papers. We highlight knowledge gaps, as well as future research directions: The understanding of beach aquifer microorganisms is informed primarily by 16S ribosomal RNA gene sequences. Metagenomics and metatranscriptomics have not yet been applied but are promising approaches for elucidating key metabolic and ecological roles of microbes in this environment. Additionally, variability in field sampling and analytical methods restrict comparison of data across studies and geographic locations. Further, documented evidence on the migration of microbes within the beach aquifer is limited. Taking into account the physical transport of microbes through sand by flowing groundwater may be critical for understanding the structure and dynamics of microbial communities. Quantitative measurements of rates of elemental cycling in the context of microbial diversity need further investigation, in order to understand the roles of microbes in mediating biogeochemical fluxes from the beach aquifer to the coastal ocean. Lastly, understanding the current state of beach aquifers in regulating carbon stocks is critical to foster a better understanding of the contribution of the beach aquifer microbiome to global climate models.

High spatial heterogeneity and low connectivity of bacterial communities along a Mediterranean subterranean estuary

“…and Bacteroidota were among the dominant phyla, in agreement with most available studies on subterranean estuaries (Archana et al, 2021;Ruiz-González et al, 2021). However, Patescibacteria was the second most dominant phylum reaching up to 23% of local sequences.…”

“…All this suggests that the spatial distribution of prokaryotic groups observed here will vary seasonally following the temporal variations in aquifer physicochemistry, but whether such seasonal changes will be due to a spatial redistribution of the same microbial taxa or to the growth of new groups peaking at different moments of the year remains unknown. Moreover, there are multiple evidences that a large of the groundwater microbial biomass and activity is concentrated on aquifer surfaces, and that these communities differ taxonomically and functionally from free-living cells (Archana et al, 2021;Ruiz-González et al, 2021), so it is likely that our data, based only in groundwater samples, are providing an incomplete view of the microbial complexity hidden within the Argentona aquifer.…”

“…Moreover, despite that the number of microbial studies in coastal aquifers has increased in recent years (Archana et al, 2021;Ruiz-González et al, 2021), most have been based on amplicon sequencing and thus very little is known regarding basic features such as the cell abundance and size, metabolic status or activity rates of coastal groundwater microbiota.…”

Microbial Diversity in Groundwater and Its Response to Seawater Intrusion in Beihai City, Southern China

“…This provided us with a valuable opportunity to study the spatial variation of the microbial community structure for different degrees of seawater intrusion. Previous research has shown that groundwater is often rich in inorganic nutrients, including nitrate, phosphate and silicate, while seawater provides more abundant oxygen, salt, sulfate and organic matter ( Archana et al, 2021 ). Therefore, with the different degrees of seawater intrusion, an environmental gradient was formed among G1, G2 and G3, and the corresponding microbial community structure also showed obvious spatial differences.…”

Ideas and perspectives: Land-ocean connectivity through groundwater

“…FSGD sites on the seafloor can be identified through identification of morphologic depressions (pockmarks, sinkholes) through high frequency acoustic seafloor bathymetry mapping and identification of anomalous seafloor fauna and flora associated with a change in water salinity and nutrients input (e.g. Lecher and Mackey, 2018;Archana et al, 2021). Other approaches used to search FSGD sites on a regional scale include mapping radiogenic isotopes that are associated with groundwater (Burnett, 2006;Paldor et al, 2020), shallow physical imaging of resistivity anomalies, survey of small-scale magnetic susceptibility anomalies caused by preservation or diagenetic alteration of iron oxides in sediments (Müller et al, 2011), satellite infrared imagery using e.g.…”