Rapid economic expansion poses serious problems for groundwater resources in arid areas, which typically have high rates of groundwater depletion. In this study, integration of hydrochemical investigations involving chemical and statistical analyses are conducted to assess the factors controlling hydrochemistry and potential pollution in an arid region. Fifty-four groundwater samples were collected from the Dhurma aquifer in Saudi Arabia, and twenty-one physicochemical variables were examined for each sample. Spatial patterns of salinity and nitrate were mapped using fitted variograms. The nitrate spatial distribution shows that nitrate pollution is a persistent problem affecting a wide area of the aquifer. The hydrochemical investigations and cluster analysis reveal four significant clusters of groundwater zones. Five main factors were extracted, which explain >77% of the total data variance. These factors indicated that the chemical characteristics of the groundwater were influenced by rock–water interactions and anthropogenic factors. The identified clusters and factors were validated with hydrochemical investigations. The geogenic factors include the dissolution of various minerals (calcite, aragonite, gypsum, anhydrite, halite and fluorite) and ion exchange processes. The anthropogenic factors include the impact of irrigation return flows and the application of potassium, nitrate, and phosphate fertilizers. Over time, these anthropogenic factors will most likely contribute to further declines in groundwater quality.

Microplastics are extremely widespread aquatic pollutants that severely detriment marine life. In this study, the influence of microplastics on biomineralization was investigated. For the first time, multiple forms and types of microplastics were detected and isolated from the shells and pearls of Pinctada fucata. According to the present study, the abundance of microplastics in shells and pearls was estimated at 1.95 ± 1.43 items/g and 0.53 ± 0.37 items/g respectively. Interestingly, microplastics were less abundant in high-quality round pearls. Microplastics may hinder the growth of calcite and aragonite crystals, which are crucial components required for shell formation. During the process of biomineralization microplastics became embedded in shells, suggesting the existence of a novel pathway by which microplastics accumulate in bivalves. After a 96-h exposure to microplastics, the expression level of typical biomineralization-related genes increased, including amorphous calcium carbonate binding protein (ACCBP) gene which experienced a significant increase. ACCBP promotes the formation of amorphous calcium carbonate (ACC), which is the pivotal precursor of shell formation-related biominerals. ACCBP is highly expressed during the developmental stage of juvenile oysters and the shell-damage repair process. The increased expression of ACCBP suggests biomineralization is enhanced as a result of microplastics exposure. These results provide important evidence that microplastics exposure may impact the appearance of biominerals and the expression of biomineralization-related genes, posing a new potential threat to aquatic organisms.

Microbial carbonate precipitation is known as an efficient process for the remediation of heavy metals from contaminated soils. In the present study, a urease positive bacterial isolate, identified as Bacillus cereus NS4 through 16S rDNA sequencing, was utilized on a large scale to remove nickel from industrial soil contaminated by the battery industry. The soil was highly contaminated with an initial total nickel concentration of approximately 900 mg kg−1. The soluble-exchangeable fraction was reduced to 38 mg kg−1 after treatment. The primary objective of metal stabilization was achieved by reducing the bioavailability through immobilizing the nickel in the urease-driven carbonate precipitation. The nickel removal in the soils contributed to the transformation of nickel from mobile species into stable biominerals identified as calcite, vaterite, aragonite and nickelous carbonate when analyzed under XRD. It was proven that during precipitation of calcite, Ni2+ with an ion radius close to Ca2+ was incorporated into the CaCO3 crystal. The biominerals were also characterized by using SEM-EDS to observe the crystal shape and Raman-FTIR spectroscopy to predict responsible bonding during bioremediation with respect to Ni immobilization. The electronic structure and chemical-state information of the detected elements during MICP bioremediation process was studied by XPS. This is the first study in which microbial carbonate precipitation was used for the large-scale remediation of metal-contaminated industrial soil.

The potential of using mollusk shell powder in aragonite (razor clam shells, RCS) and calcite phase (oyster shells, OS) to remove Pb²⁺, Cd²⁺ and Zn²⁺ from contaminated water was investigated. Both biogenic sorbents displayed very high sorption capacities for the three metals except for Cd on OS. XRD, SEM and XPS results demonstrated that surface precipitation leading to crystal growth took place during sorption. Calcite OS displayed a remarkably higher sorption capacity to Pb than aragonite RCS, while the opposite was observed for Cd. However, both sorbents displayed similar sorption capacities to Zn. These could be due to the different extent of matching in crystal lattice between the metal bearing precipitate and the substrates. The initial pH of the solution, sorbent’s dosage and grain size affected the removal efficiency of the heavy meals significantly, while the organic matter in mollusk shells affected the removal efficiency to a lesser extent.

The current study identifies groundwater quality issues and investigates the most important geochemical processes that control seawater intrusion using various ionic ratios, hydrochemical facies evolution, and geochemical modelling. Cl₋/Br ratio is an important indicator to identify the origin of groundwater salinity in coastal aquifers. Nineteen percent of the groundwater samples with Cl⁻/Br⁻ ratio similar to that of Standard Mean Ocean Water (SMOW) are affected by seawater intrusion in the study area. Particularly, nine groundwater samples have high chloride values and are similar to SMOW, and it may derived salinity from seawater sources from the Bay of Bengal due to the over-pumping of production wells in the Uvari zone. Five samples are similar to SMOW, which is due to the presence of salt pan activities. The bivariate plots such as Ca²⁺ + Mg²⁺ vs Cl⁻, EC vs Cl⁻, and Na⁺/Cl⁻ ratio indicate that seawater intrusion is the primary source for groundwater salinisation. Evaporation is the dominant process controlling groundwater chemistry, rather than rock-water interaction and precipitation, according to mechanisms controlling groundwater chemistry. Direct ion exchange and converse ion exchange are the critical controlling factors for groundwater salinisation, according to the hydrochemical facies evolution diagram (HFED). The water quality index (WQI) shows that most groundwater belongs to the poor to the marginal category. The saturation indices show that the groundwater samples are saturated with minerals such as dolomite, calcite, aragonite and magnesite. Therefore, these minerals are susceptible to precipitation due to the effective leaching of calcareous minerals from the bedrocks. Compiled hydrogeochemical analysis and multivariate statistical analysis revealed that the Tiruchendur and Uvari zone was affected by the seawater intrusion and led to an increase in the salinity of the groundwater.

This study evaluated interannual variation in the subsurface aragonite undersaturation zone (ΩAᵣ<₁ layer) in the Pacific Arctic Ocean, using data from the 2016–2019 period. The upper boundary (DEPΩ<₁ᵁᴮ) of the ΩAᵣ<₁ layer generally formed at a depth where the contribution of corrosive Pacific water was approximately 98 %. The intensity of the Beaufort Gyre associated with freshwater accumulation mainly determined interannual variation in DEPΩ<₁ᵁᴮ, but the direction of its effect was opposite west and east of ~166°W. The lower boundary (DEPΩ<₁ᴸᴮ) of the ΩAᵣ<₁ layer was generally found at a depth range where equal contributions of Pacific and Atlantic water were expected. An Atlantic-origin cold saline water intrusion event in 2017 caused by an anomalous atmospheric circulation pattern significantly lifted the DEPΩ<₁ᴸᴮ, thus the thickness of the ΩAᵣ<₁ layer decreased.

The aragonite saturation state (Ωₐᵣₐg) was determined to assess its seasonal variations and the major controlling factors in the southeastern Yellow Sea (YS) over four seasons. Ωₐᵣₐg showed large seasonal variation in the surface waters, with dissolved inorganic carbon (DIC) as a major factor controlling the seasonal variation. In the bottom waters, Ωₐᵣₐg exhibited only small seasonal variation compared with the surface waters; DIC and total alkalinity were the main factors contributing to the variation. The bottom water of the southeastern YS was undersaturated with aragonite during the fall, even though the southeastern YS was not typically associated with upwelling, freshwater discharge, or eutrophication processes. Aragonite undersaturation was most likely due to ocean dumping of organic materials. Therefore, ocean pumping should be prohibited in shallow marginal seas to prevent aragonite undersaturation.

The effects of increasing CO2 concentrations and temperature on microalgal assemblages were examined in Taiwan using mesocosms that simulate coral reef ecosystem. We assessed changes in abundance and diversity of benthic algae grown at 25°C and 28°C, under ambient (~400μatm) and at high CO2 conditions (800–1000μatm). Total alkalinity, pCO2, and the aragonite saturation state, were all significantly different between control and high CO2 treatments in both temperature treatments. Chl a concentration increased significantly in CO2-treated groups at 25°C, but benthic microalgal abundance was not significantly different. The number of microalgal species and the microalgal community structure did not differ between control and CO2-treated groups at both temperatures. Our results suggest that increasing CO2 may boost benthic microalgal primary productivity if sufficient nutrients are available, although site-specific responses are difficult to predict.

Coral skeletons record historical trace metal levels in the environment, however, the use of coral skeletal records for biomonitoring studies mostly fail to consider the influence of metal regulation by the living components of coral and subsequent incorporation into the skeleton. This study presents Exaiptasia pallida as a representative of the living components of coral and shows metal partitioning between the tissue and zooxanthellae after chronic exposure to Zn. A strong tendency for preferential accumulation in the zooxanthellae occurred after 32days exposure and Zn concentrations in tissue and zooxanthellae were 123.3±0.7mgkg−1 and 294.9±8.5 respectively. This study shows zooxanthellae density plays an important role in controlling Zn loading in whole anemones and must be considered when investigating metal uptake and loading in zooxanthellate organisms. Further studies that investigate links between aragonite deposition rates and zooxanthellae density and incorporation pathways of metals into skeleton are warranted.

We investigated the aragonite saturation state (Ωarag) during all four seasons in a coastal region of southern Korea that receives considerable freshwater input. The surface Ωarag values were higher during productive seasons with enhanced freshwater influences, likely due to an increased net removal of dissolved inorganic carbon (DIC) from the water column (i.e., biological control). In addition, during the productive seasons, enhancement of Ωarag was observed with decreasing salinity within a linear mixing zone present between river-influenced surface and saltier bottom waters. DIC appeared to be effectively sequestered from the warmer, less salty surface water by downward flux of organic matter, but not significantly affected by the relatively DIC-rich, cooler and saltier bottom waters under strong stratification conditions during these seasons (i.e., physical control). Low phytoplankton productivity and seasonal breakdown of the stratification caused reduced saturation in other seasons and made the study area a weak sink for atmospheric CO2.