Phosphorus cycling in a subterranean estuary seepage face: A seasonal view on inventory composition and linkage with nitrate transformations.

Seasonal field surveys (April 2018 to February 2019) were conducted in a subterranean estuary (STE) seepage face in Sanggou Bay (China) aiming to explore the transport and reactivity of phosphorus (P) and biogeochemical linkages with the cycling of nitrogen (N) prior to discharge. Porewater dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP) together with different fractions of sedimentary P were analyzed in the upper, middle and lower intertidal covering the top 20 cm of sediment (1-4 cm, 5-8 cm, 9-12 cm, 13-16 cm and 17-20 cm depth). The accumulation of sedimentary organic P stimulated the growth of phosphate-solubilizing microorganisms and led to porewater DOP enrichment during spring. During summer, total P (TP), porewater DIP and DOP concentrations decreased, potentially due to enhanced mineralization driven by high ambient temperature. From autumn to winter, pelagic organic matter into the STE lowered, triggering a drop of TP standing stocks. Compared with the significant seasonality, sedimentary P storage was statistically identical along the intertidal. Such spatial homogeneity likely results from the rebalance driven by P adsorption dynamics and pelagic organic matter delivered by tide and wave setup. The vertical distribution of DIP, DOP, and sedimentary TP were linked to nitrate transformations. In the sediment layer with active mineralization and nitrification, concentrations of DOP, sedimentary redox and clay P increased. In the layer with active nitrate removal (2-5 cm depth), both DIP and DOP concentrations decreased. The sedimentary loosely-bound and organic P were also lower there. Notably, a substantial quantity of soluble P seeped out, acting as an important contributor to the dissolved P pool of the receiving waters. The spatial and temporal overlap of high concentrations of N and P in STEs adds variabilities and uncertainties in P out-drainage fluxes and nutrient stoichiometry balances, which should draw attention from coastal researchers and stakeholders.

Radon prevalence in domestic water in the Ría de Vigo coastal basin (NW Iberian Peninsula).

The Ría de Vigo catchment is situated in the largest radon-prone area of the Iberian Peninsula. High local indoor radon (222Rn) levels are the preeminent source of radiation exposure, with negative effects on health. Nevertheless, information on radon levels of natural waters and the potential human exposure risks associated with their domestic use is very sparse. To elucidate the environmental factors increasing human exposure risk to radon during domestic water use, we undertook a survey of local water sources, including springs, rivers, wells, and boreholes, over different temporal scales. Continental waters were highly enriched in 222Rn: activities ranged from 1.2 to 20.2 Bq L-1 in rivers and levels one to two orders of magnitude higher were found in groundwaters (from 8.0 to 2737 Bq L-1; median 121.1 Bq L-1). The geology and hydrogeology of local crystalline aquifers support one order of magnitude higher 222Rn activities in groundwater stored in deeper fractured rock compared to that contained within the highly weathered regolith at the surface. During the mean dry season, 222Rn activities nearly doubled in most sampled waters in comparison to the wet period (from 94.9 during the dry season to 187.3 Bq L-1 during wet period; n = 37). Seasonal water use and recharge cycles and thermal convection are postulated to explain this variation in radon activities. The high 222Rn activities cause the total effective dose of radiation received from domestic use of untreated groundwaters to exceed the recommended 0.1 mSv y-1. Since more than 70% of this dose comes from indoor water degassing and subsequent 222Rn inhalation, preventative health policy in the form of 222Rn remediation and mitigation measures should be implemented prior to pumping untreated groundwater into dwellings, particularly during the dry period.

On the hidden diversity and niche specialization of the microbial realm of subterranean estuaries.

Subterranean estuaries (STEs) modulate the chemical composition of continental groundwater before it reaches the coast, but their microbial community is poorly known. Here, we explored the microbial ecology of two neighbouring, yet contrasting STEs (Panxón and Ladeira STEs; Ría de Vigo, NW Iberian Peninsula). We investigated microbial composition (16S rRNA gene sequencing), abundance, heterotrophic production and their geochemical drivers. A total of 10,150 OTUs and 59 phyla were retrieved from porewater sampled during four surveys covering each STE seepage face. In both STEs, we find a very diverse microbial community composed by abundant cosmopolitans and locally restricted rare taxa. Porewater oxygen and dissolved organic matter are the main environmental predictors of microbial community composition. More importantly, the high variety of benthic microbiota links to biogeochemical processes of different elements in STEs. The oxygen-rich Panxón beach showed strong associations of the ammonium oxidizing archaea Nitrosopumilales with the heterotrophic community, thus acting as a net source of nitrogen to the coast. On the other hand, the prevailing anoxic conditions of Ladeira beach promoted the dominance of anaerobic heterotrophs related to the degradation of complex and aromatic compounds, such as Dehalococcoidia and Desulfatiglans, and the co-occurrence of methane oxidizers and methanogens.

Organic carbon in a seepage face of a subterranean estuary: Turnover and microbial interrelations.

Subterranean estuaries, the mixing zone between terrestrial groundwater and coastal seawater, are important biogeochemical hotspots. In the present study, organic carbon cycling and related drivers, including the characterization of different organic carbon pools and sediment microbial community, were investigated in a subterranean estuary seepage face. Within the first 20 cm depth seepage face sediments, both production and removal of dissolved organic carbon (DOC) were observed, mainly driven by heterotrophic microbes. From spring to autumn, active DOC production occurred on the seepage face at the 15-20 cm depth, likely via aerobic degradation of sediment organic carbon (SOC) with subsequent release of dissolved fractions into the porewater. During winter, DOC production moved to a shallower depth of the seepage face due to increasing SOC content in the surface layer. DOC production rate depended on heterotrophic microbial biomass (e.g. Proteobacteria) and was enhanced by high microbial activity and porewater advection. DOC removal frequently occurred at the 0-5 cm depth layer except in winter. The seasonal shift in carbon source utilization (SOC to DOC) in this layer likely resulted from the decrease in SOC pool, especially the labile portion of SOC and the increased availability of DOC due to production in the deeper sediment (15-20 cm). Given the similarity in microbial community structure along the sediment profile, this shift suggests SOC as the preferential carbon source for benthic microbes as well as adaptive flexibility in microbial carbon source utilization. DOC removal was also significantly tied to microbial activity and advection rate. Because DOC production rates were higher compared to DOC consumption the seepage face acted as a net source of DOC to the coastal ecosystem.