Geochemical partitioning and possible heavy metal(loid) bioaccumulation within aquaculture shrimp ponds

Cadmium exposure elicited dynamic RNA m6A modification and epi-transcriptomic regulation in the Pacific whiteleg shrimp Litopenaeus vannamei

N6-methyladenosine (m6A) methylation is the most prevalent post-transcriptional RNA modification in eukaryotic organisms, but its roles in the regulation of physiological resistance of marine crustaceans to heavy metal pollutants are poorly understood. In this study, the transcriptome-wide m6A RNA methylation profiles and dynamic m6A changes induced by acute Cd2+ exposure in the the pacific whiteleg shrimp Litopenaeus vannamei were comprehensively analyzed. Cd2+ toxicity caused a significant reduction in global RNA m6A methylation level, with major m6A regulators including the m6A methyltransferase METTL3 and the m6A binding protein YTHDF2 showing declined expression. Totally, 11,467 m6A methylation peaks from 6415 genes and 17,291 peaks within 7855 genes were identified from the Cd2+ exposure group and the control group, respectively. These m6A peaks were predominantly enriched in the 3' untranslated region (UTR) and around the start codon region of the transcripts. 7132 differentially expressed genes (DEGs) and 7382 differentially m6A-methylated genes (DMGs) were identified. 3186 genes showed significant changes in both gene expression and m6A methylation levels upon cadmium exposure, and they were related to a variety of biological processes and gene pathways. Notably, an array of genes associated with antioxidation homeostasis, transmembrane transporter activity and intracellular detoxification processes were significantly enriched, demonstrating that m6A modification may mediate the physiological responses of shrimp to cadmium toxicity via regulating ROS balance, Cd2+ transport and toxicity mitigation. The study would contribute to a deeper understanding of the evolutionary and functional significance of m6A methylation to the physiological resilience of decapod crustaceans to heavy metal toxicants.

Potentially toxic element bioaccumulation in consumed indoor shrimp farming associated with diet, water and sediment levels

Shrimp production is an important industry for many countries and shrimp consumption is increasing worldwide. Shrimps are a highly nutritional food, but can pose a risk for human health if subject to high levels of environmental contaminants. This work studies the presence of As, Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn in shrimps from Ecuador and compares them to such contents noted in other shrimp-production areas in the world to evaluate the possible risks associated with these elements for consumer health, and to relate them to potentially toxic element (PTE) contents in water, sediments and diets, and also to animal biometric parameters. The PTE levels (mg kg-1 DM) obtained are as follows: in the head-As (3.52-6.11), Cd (0.02-0.10), Co (0.14-0.49) Cr (0.23-4.89), Cu (99.9--233.0), Ni (0.52-1.86), Pb (0.24-1.09), Zn (51.8-100.5) and Hg (μg kg-1 DM) (10.00-66.81); in the tail-(0.91-3.21), Cd (0.01-0.02), Co (0.01-0.43) Cr (0.01-6.52), Cu (20.0-72.44), Ni (0.15-2.03), Pb (0.01-0.69), Zn (31.2-66.1) and Hg (μg kg-1 DM) (10.00-67.18). The concentration of all the PTEs is generally lower than the limits set for seafood by European regulations, except for As in the cephalothorax (4.63 mg kg-1). Different behaviours for PTE accumulation in shrimps were found, which preferentially tend to accumulate in the cephalothorax, except for Hg (40.13 μg kg-1 DM), which accumulates in muscle (body) and is associated with contents of proteins, lipids and total shrimp weight. Nonetheless, the target hazard quotient (THQ) values for PTEs indicate that the consumption of shrimp muscles from Ecuador does not pose a human health risk because the values of these indices are below 1 in all cases.

Effect of cage culture on sedimentary heavy metal and water nutrient pollution: Case study in Sansha Bay, China

The aquaculture area in China's coastal waters has increased rapidly from 6000 km2 in 1990 to 22,000 km2 in 2020. Despite extensive research regarding the effect of coastal aquaculture on water and sediment pollution, evaluating the quantitative relationship between aquaculture and pollutants remains challenging. Sansha Bay, the world's largest cage aquaculture base for Pseudosciaena crocea, is a typical enclosed bay used for investigating aquaculture pollution. A cage culture database is established from 2000 to 2020 in Sansha Bay. Meanwhile, 236 sediment samples from 3 sediment cores and 67 water samples from 4 transects are obtained from the bay for experiments. The main indicators are five nutrients (NO3-, SiO32-, PO43-, NH4+, and NO2-) in the water samples, the grain size, the heavy metal (Zn, Cu, Pb, Cr, Cd, and As) content, and the 210Pb radioactivity in sediment samples. Based on data obtained and a new calculation method, the annual increment in Zn, Cu, As, Cd, Pb, and Cr contents in the cultured zone is shown to increase by 2137 %, 1881 %, 506 %, 300 %, 202 %, and 118 % in 2000-2018, respectively, as compared with the levels in a noncultured zone. The activities of the cage culture increased NO3- by 9 %, PO43- by 30 %, NH4+ by 115 %, and NO2- by 232 %, compared with natural conservative mixing processes, such as the mixing of SiO32-, in 2020. A novel quantitative approach with broad applicability is proposed to evaluate the magnitude of anthropogenically induced environmental contamination. The effectiveness of the proposed technique is demonstrated through a case study conducted in Sansha Bay, China.

Assessing pathways of heavy metal accumulation in aquaculture shrimp and their introductions into the pond environment based on a dynamic model and mass balance principle

The impact of heavy metals (HMs) on the quality of aquaculture products has attracted worldwide attention. Since Litopenaeus vannamei is a popular aquaculture product among consumers worldwide, it is of great importance to guarantee its dietary safety. An in-situ monitoring program lasting for three months in a typical Litopenaeus vannamei farm found that Pb (100 %) and Cr (86 %) in the adult shrimp were higher than the safety guidelines. In the meantime, Cu (100 %), Cd (100 %) in the water and Cr (40 %) in the feed exceeded the corresponding thresholds. Therefore, quantification of different exposure pathways of shrimp and contamination origins in pond is valuable to improve the dietary safety of the shrimp. Based on Optimal Modeling for Ecotoxicological Applications (OMEGA), Cu was primarily from the ingestion of feed, accounting for 67 % of bioaccumulation, while Cd, Pb and Cr primarily entered shrimp through the adsorption from overlying water (53 % for Cd and 78 % for Pb) and porewater (66 % for Cr), respectively. The HMs in the pond water were further tracked based on a mass balance analysis. The main source of Cu in the aquaculture environment was feed, being responsible for 37 % of the total input. Pb, Cd and Cr were primarily from the inlet water with contributions of 84 %, 54 % and 52 %, respectively. In summary, the proportions of different exposure pathways and origins of HMs in pond-cultured shrimp and its living environment varied widely. To keep end-consumers eating healthily, species specific treatment is required. Feed should be regulated more for Cu. Aimed pretreatments for Pb and Cd in influent water are needed and an additional immobilization for Cr in sediment porewater should be investigated. After implementation of these treatments, the food quality improvement could be further quantified based on our prediction model.

Hazardous toxic metal(loid)s in top- and deep-soils during the transformation of aquaculture ponds restored to farmland

The pollution risks due to the soil migration of toxic metal(loid)s (TMs) are a greatly hazard to ecological environment as well as animal and human health. Previous studies have primarily focused on surface contamination while deep soil layers often contain dangerous levels of TMs. We used restored wheat and rice farmlands from aquaculture ponds as a case study to examine the ecological risk and distribution of TMs in soil profiles. The elements Cu, Zn, Cr, Cd, Hg and As were markedly enriched in the 60-180 cm soil layers of restored farmland, and their concentrations decreased in the several depths as follows: 120-180 cm > 60-120 cm > 0-60 cm. Concentrations of TMs were 9.5-128 % greater in the restored farmlands relative to farmlands not exposed to aquaculture practices. Cadmium and mercury were the most serious contaminants and increased the overall ecological risk. The subsoil of wheat farming system had the highest pollution risk versus the restored rice farmland at 60-120 cm due to elevated levels of Cu, Zn and Pb. Toxic metal(loid)s might be derived from natural sources in deep soil of conventional farmland whereas aquaculture practices were found to constitute the major contribution in the subsoil of restored farmland. Our results indicated that the TMs that were buried in deep soil layers migrated upward and were a significant pollution risk. Urgent actions should be taken to identify and alleviate the contamination sources of these deep soils in addition to the conventional leaching and migration processes of surface contaminants.

Exploring the variation of bacterial community and nitrogen transformation functional genes under the pressure of heavy metals in different coastal mariculture patterns

Equilibrium in microbial dynamics and nitrogen transformation in the sediment is critical for maintaining healthy mariculture environment. However, our understanding about the impact of heavy metals on the bacterial community and nitrogen transformation functional genes in different mariculture patterns is still limited. Here, we analyzed 30 sediment samples in the vertical distribution from three different mariculture patterns mainly include open mariculture zone (K), closed mariculture pond (F) and pristine marine area (Q). Illumina MiSeq Sequencing was applied to investigate the bacterial community and structure in the sediment. Quantitative polymerase chain reaction (qPCR) was used to determine the effect of heavy metals on nitrogen transformation functional genes. Results showed that bacterial community and structure varied greatly in different mariculture patterns. Chloroflexi, Proteobacteria and Desulfobacterota were predominant phyla in the coastal mariculture area. High concentrations of heavy metals mainly enriched in the up layer (5-40 cm) of the sediment in the mariculture zone. The abundance of functional genes in the closed mariculture pond was much higher than the open mariculture zone and pristine marine area. And the high abundance of nitrification and denitrification functional genes mainly accumulated at the depth from 5 cm to 40 cm. Heavy metals content such as Fe, Cr, Mn, Ni, As, Cd, Pb and nutrient content NH₄⁺-N, NO₃^--N and NO₂^--N were highly associated with bacterial community and nitrogen transformation functional genes. This study comprehensively elaborated the effect of heavy metals on the bacterial community and nitrogen transformation functional genes in different coastal mariculture patterns, indicating the possible role of closed mariculture pond in reducing nitrogen transformation efficiency, which will provide useful information for preventing pollution risk in the mariculture area.

Heavy metal(loid)s in multiple media within a mussel mariculture area of Shangchuan Island, China: Partition, transfer and health risks

Limited research has been conducted on the heavy metal (loid) (HM) exchanges among seawater, sediments and cultured mussels in mariculture areas. To determine the impacts of mussel mariculture on the transfer and risks of HMs in coastal environments, samples of seawater, sediments and cultured mussels from a typical mariculture area in the South China Sea were analysed. The logarithmic partition coefficients of sediment/water indicated that HMs were preferentially retained by sediments. There were relatively low ecological risks in mussel raft mariculture areas. Principal component analysis and correlation analysis revealed the influences of terrestrial input and ocean circulation on the HMs in seawater, and the bioaccumulation factors reflected the possible accumulation of HMs in seawater by cultured mussels. Mussel farming was the indirect source of HMs in sediments, and the biochemical processes of cultured mussels promoted the transfer and accumulation of HMs into sediments. Health risk assessment revealed that children were more susceptible than adults, and Cd, As and Cr were among the threatening elements. Nonetheless, the health risks that were possibly caused by the daily consumption of farmed mussels were still within safe limits. Thus, the potential pollution remediation and biomonitoring functions of mussel aquaculture in coastal water environments will be further studied in the future.

Arsenic Bioaccumulation and Identification of Low-Arsenic-Accumulating Food Fishes for Aquaculture in Arsenic-Contaminated Ponds and Associated Aquatic Ecosystems

Arsenic-contaminated food including farmed fish is one of the main routes of human exposure. Fish farmed in contaminated environment accumulates arsenic in different tissues with great variability. Thus, it is utmost important to quantify the risk associated with different farmed fish species in arsenic-contaminated aquaculture systems. In the present study, arsenic content was measured in twelve fish species (Labeo rohita, L. catla, Cirrhinus mrigala, Oreochromis niloticus, O. mossambicus, Liza tade, Puntius javanicus, L. calbasu, Glossogobius giuris, Macrobrachium rosenbergii, Ctenopharyngodon idella, and Bellamya bengalensis (gastropod)) collected from arsenic-contaminated aquaculture systems. Among the studied finfishes, C. idella was found to accumulate the lowest amount of arsenic (< 0.05 ± 0.00 mg kg^-1) whereas the highest accumulation was noticed in O. mossambicus (1.0 ± 0.18 mg kg^-1). However, the estimated carcinogenic and non-carcinogenic risks of human were found to be low for all the studied fishes. The calculated target hazard quotient (THQ) value for adults ranged from 0.01 to 0.08 whereas for children it ranged from 0.05 to 0.27 for low-arsenic-accumulating fishes (arsenic conc. < 0.5 mg kg^-1). Based on these findings, C. mrigala, C. idella, and M. rosenbergii could be recommended as the candidate species for aquaculture in the arsenic-contaminated areas as farming of the low-arsenic-accumulating food fishes would also lower the risk of human exposure through food chain.

The response of sediment microbial communities to temporal and site-specific variations of pollution in interconnected aquaculture pond and ditch systems

Sediment microbial communities play critical roles in the health of fish and the biogeochemical cycling of elements in aquaculture ecosystems. However, the response of microbial communities to temporal and spatial variations in interconnected aquaculture pond and ditch systems remains unclear. In this study, 61 sediment bacterial samples were collected over one year from 11 sites (including five ponds and six ditches) in a 30-year-old fish aquaculture farm. The 16S rRNA approach was used to determine the relative abundances of microbial communities in the sediment samples. The relationships among nutrients, heavy metals, and abundant microorganisms were analyzed. Our results showed that Proteobacteria, Bacteroides and Chloroflexi were the predominant phyla in the sediments of aquaculture pond, with average abundances of 36.33%, 18.60%, and 14.58%, respectively. The microbial diversity in aquaculture sediments was negatively correlated (P < 0.05) with the concentrations of total nitrogen and total phosphorus in sediments, indicating that the microbial diversity is highly associated with the remediation of nutrients in sediments. The sediment samples with high similarities were discovered by the t-distributed stochastic neighbor embedding (t-SNE) method. The site-specific correlations between specific microorganisms and heavy metals were explored. The network analysis revealed that the microbial diversities in aquaculture ponds were more stable than that in aquaculture ditches. The network analysis also illustrated that the microbial genera with low relative abundances may become key groups of microbial communities in sediment ecosystems. Our work deepens the understanding of the relationships between microbial communities and the spatiotemporal characteristics of surface water and sediments in aquaculture farms.