A Systematic Investigation into the Environmental Fate of Microcystins and The Potential Risk: Study in Lake Taihu

Single microcystin exposure impairs the hypothalamic-pituitary-gonadal axis at different levels in female rats

Microcystin (MC) is most common cyanobacterial toxin. Few studies have evaluated the MC effects on the hypothalamic-pituitary-gonadal (HPG) axis and metabolic function. In this study, we assessed whether MC exposure results in HPG axis and metabolic changes. Female rats were exposed to a single dose of MC at environmentally relevant levels (5, 20 and 40 μg/kg). After 24 h, we evaluated reproductive and metabolic parameters for 15 days. MC reduced the hypothalamic GnRH protein expression, increased the pituitary protein expression of GnRHr and IL-6. MC reduced LH levels and increased FSH levels. MC reduced the primary follicles, increased the corpora lutea, elevated levels of anti-Müllerian hormone (AMH) and progesterone, and decreased estrogen levels. MC increased ovarian VEGFr, LHr, AMH, ED1, IL-6 and Gp91-phox protein expression. MC increased uterine area and reduced endometrial gland number. A blunted estrogen-negative feedback was observed in MC rats after ovariectomy, with no changes in LH levels compared to intact MC rats. Therefore, these data suggest that a MC leads to abnormal HPG axis function in female rats.

Stress response of Microcystis aeruginosa to chlorine during transportation: The significance of surface-adsorbed organic matter

The desorption of surface-adsorbed organic matter (S-AOM) without damaging algal cells was reported to be the key to destabilizing Microcystis aeruginosa (M. aeruginosa) cells while avoiding intracellular organic matter (IOM) release in our previous study. However, a temporal effect was found from spontaneous and continuous damage to algal cells even after quenching. This study aims to demonstrate the mechanism of the temporal inactivation effect and the stress response exhibited by chlorine-oxidized algal cells, and finally guide the prechlorination process for algae-laden water at water sources. Chlorine was proved to cause oxidative stress to M. aeruginosa cells, and result in a rapid increase in intracellular reactive oxygen species (ROS) levels. S-AOM appeared to have a protective effect on algal cells against oxidative damage, as evidenced by the maintenance of algal cell integrity and activated antioxidant enzymes. In addition, the activity of Caspase 3, a key protease for the execution of programmed cell death (PCD), was significantly enhanced during prechlorination. Cellular chromatin condensation and DNA fragmentation occurred in the early stages of PCD in algal cells. Therefore, the pre-treatment of algae-laden water at water sources, even with low chlorine doses, can induce a risk of significant algal cell death during the water transfer process due to activation of the PCD process, resulting in a higher health risk for drinking water. These findings indicate that both the dosage of chlorine and the duration of the transportation process should be considered during the prechlorination of algae-laden water, which can provide an important basis for avoiding increasing the risk to drinking water safety.

Combined inhibitory effects of microcystin-LR and microcystin-RR on growth and development in zebrafish larvae

Microcystins (MCs) are the most widespread, frequently found, and seriously toxic cyanobacterial toxins in aquatic environments. Microcystin-leucine-arginine (MCLR) and microcystin-arginine-arginine (MCRR) are the most studied MCs. Normally, their levels are low and they coexist in the environment; however, they may also interact with each other. The developmental toxicity of MCLR in the presence of MCRR in the early life stage of zebrafish (from 2 to 120 h post fertilization) was investigated for the first time in this study. Our findings revealed that MCRR treatment marginally elevated thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels, whereas MCLR treatment alone resulted in a significant increase in T3 and T4 levels, indicating a cooperative effect. Furthermore, clear changes in the expression levels of genes involved in growth and development, accompanied by growth inhibition, were observed after co-treatment with MCRR and MCLR. In addition, zebrafish larvae subjected to MCRR and/or MCLR treatment showed increased levels of superoxide dismutase, glutathione, and malondialdehyde, and decreased levels of catalase in the MCRR + MCLR group, indicating oxidative stress and lipid peroxidation. Thus, we investigated the synergistic developmental toxicity of MCRR and MCLR during the early life stages of zebrafish development.

Effects of microcystin-LR on mammalian ovaries

The ovaries play critical roles in regulating oocyte maturation and sex steroid hormone production and thus are critical for female reproduction. Ovarian function relies on hormone receptors and signaling pathways, making the ovaries potential targets for environmental factors, such as microcystins (MCs). MCs are a diverse group of cyanobacterial toxins generally found in eutrophic water or algal blooms. Here, we review relevant research on the associations between MC exposure and ovarian dysfunction, including their effects on ovarian morphology, folliculogenesis, steroid production, oxidative stress, endoplasmic reticulum stress, apoptosis, autophagy, and fertility. This review covers the most recent in vitro and in vivo studies in mammals. We also discuss important gaps in the literature. Overall, current evidence indicates that MC exposure causes impairments in ovarian function, but further studies are needed to elucidate the mechanisms through which MCs affect ovarian function and other female endocrine functions.

Effects of Sulfamethoxazole and Florfenicol on Growth, Antioxidant Capacity, Immune Responses and Intestinal Microbiota in Pacific White Shrimp Litopenaeus vannamei at Low Salinity

Antibiotic residue may pose a serious risk to aquaculture, and the culture of Litopenaeus vannamei in a low-salinity environment is a growing trend over the world. Here, we aimed to understand the combined effect of low salinity and sulfamethoxazole (SMZ) and florfenicol (FLO) antibiotics on L. vannamei. The growth performance, immune functions, antioxidant capacity and intestinal microbiota were investigated. Compared with the control group, the weight gain and survival rate significantly decreased (p < 0.05) in shrimp after they were exposed to low-salinity (salinity 3) water and the mixture of antibiotics and low-salt conditions for 28 days. The antioxidant activities of SOD and T-AOC, shown at low salinity and in the higher concentration of the SMZ treatment group (SMZH), were significantly decreased, while the GST activity was significantly increased in each treatment group in comparison with the control group. The expression of immune-related genes, including TOLL, LvIMD, PPO and HSP, in the low concentration of the SMZ treatment group (SMZL) was higher than that in the other groups. The diversity of intestine microbiota was disturbed with a lower Shannon index in the low-salinity and SMZH groups, and a higher Simpson index in the SMZH group. Proteobacteria, Actinobacteria and Bacteroidetes were the dominant phyla in the gut of L. vannamei. At the genus level, Microbacterium, Shewanella, Aeromonas, Acinetobacter, Gemmobacter, Paracoccus and Lysobacter were significantly decreased in the low-salinity group. However, the abundance of opportunistic pathogens belonging to the genus Aeromonas in the FLO group was increased. The predicted microbe-mediated functions showed that the pathway for “amino acid metabolism” and “replication and repair” was significantly inhibited in both the low-salinity and antibiotic-exposed groups. All the findings in this study indicate that the combined effect of antibiotics and low salinity on L. vannamei negatively impacted the physiological and intestinal microbiota functions.

Aquatic ecosystem responds differently to press and pulse nutrient disturbances as revealed by a microcosm experiment

Due to climate change and increasing anthropogenic activities, lakes are disturbed frequently, usually by press (e.g., diffused pollution, rising temperatures) or pulse (e.g., storms, rainfall, pollution events) disturbances. Both press and pulse disturbances can affect abiotic and biotic environments, changing the structure of ecosystems and affecting ecosystem services. To confront with the effects of climate change and increasing anthropogenic activities, understanding the different effects of press and pulse disturbances on lake ecosystems is essential. This study assessed the effect of press and pulse disturbances of phosphorus on a microcosmic aquatic ecosystem by measuring the total phosphorus (TP), algae density, and physiological indicators of submerged macrophytes. We found that the microcosmic aquatic ecosystem responded differently to press and pulse disturbances. Our results suggested that it had a lower resistance to pulse phosphorus disturbances than to press phosphorus disturbances. There were significantly higher nutrient concentrations and algal densities in the pulse treatment than in the press treatment. Positive feedback was found between the biomass of submerged macrophytes and the water quality. There was a higher submerged macrophytes biomass at low TP concentration and algal density. In the context of climate change, press and pulse disturbances could have severe impacts on lake ecosystems. Our findings will provide some insight for further research and lake management.

Dietary recombinant human lysozyme improves the growth, intestinal health, immunity and disease resistance of Pacific white shrimp Litopenaeus vannamei

The present study was conducted to investigate the effects of dietary recombinant human lysozyme (RHL) on the growth, immune response, anti-oxidative activity, intestinal morphology, intestinal microflora and disease resistance of shrimp Litopenaeus vannamei. Shrimps with an initial body weight of 2.36 ± 0.02 g were fed diets supplemented with 0 (control group, R0), 0.0025% (R1), 0.005% (R2), 0.01% (R3), 0.02% (R4) and 0.04% (R5) of RHL, respectively. After a 10-week feeding trial, the final body weight, survival rate, weight gain ratio and protein efficiency rate of the shrimps in dietary RHL supplemented groups were significantly higher than that in the control group, while feed conversion ratio was significantly lower (P < 0.05). The total haemocyte count, total anti-oxidative capacity, respiratory burst, activities of phagocytosis, nitric oxide synthase, phenol oxidase and lysozyme in serum were significantly higher in dietary RHL supplemented groups than those in the control group (P < 0.05). Meanwhile, the intestinal pile height and wall thickness were significantly higher in dietary RHL supplemented groups than those in the control group (P < 0.05). Dietary RHL significantly improved the expressions of immune-related genes in gill, such as lipopolysaccharide-β-glucan binding protein, Toll, immune deficiency, heat shock protein 70 and Crustin (P < 0.05). The abundance of proteobacteria and bacteroidetes in intestine was higher, while the abundance of firmicutes and cyanobacteria was lower than those in the control group at the phylum level. In addition, dietary RHL supplementation significantly improved the protective ability of shrimp against V. parahaemolyticus infection (P < 0.05). Based on the broken-line model analysis for weight gain ratio after the feeding trial, the optimal level of dietary RHL supplementation for shrimp was estimated to be 0.006375%.

Effects of Lysophospholipid Supplementation in Feed with Low Protein or Lipid on Growth Performance, Lipid Metabolism, and Intestinal Flora of Largemouth Bass (Micropterus salmoides)

The largemouth bass (Micropterus salmoides) were fed diets with three experimental feeds, a control diet (Control, crude protein (CP): 54.52%, crude lipid (CL): 11.45%), a low-protein diet with lysophospholipid (LP-Ly, CP: 52.46%, CL: 11.36%), and a low-lipid diet with lysophospholipid (LL-Ly, CP: 54.43%, CL: 10.19%), respectively. The LP-Ly and LL-Ly groups represented the addition of 1 g/kg of lysophospholipids in the low-protein and low-lipid groups, respectively. After a 64-day feeding trial, the experimental results showed that the growth performance, hepatosomatic index, and viscerosomatic index of largemouth bass in both the LP-Ly and LL-Ly groups were not significantly different compared to those in the Control group (P > 0.05). The condition factor and CP content of whole fish were significantly higher in the LP-Ly group than those in the Control group (P < 0.05). Compared with the Control group, the serum total cholesterol level and alanine aminotransferase enzyme activity were significantly lower in both the LP-Ly group and the LL-Ly group (P < 0.05). The protease and lipase activities in the liver and intestine of both group LL-Ly and group LP-Ly were significantly higher than those of the Control group (P < 0.05). Compared to both the LL-Ly group and the LP-Ly group, significantly lower liver enzyme activities and gene expression of fatty acid synthase, hormone-sensitive lipase, and carnitine palmitoyltransferase 1 were found in the Control group (P < 0.05). The addition of lysophospholipids increased the abundance of beneficial bacteria (Cetobacterium and Acinetobacter) and decreased the abundance of harmful bacteria (Mycoplasma) in the intestinal flora. In conclusion, the supplementation of lysophospholipids in low-protein or low-lipid diets had no negative effect on the growth performance of largemouth bass, but increased the activity of intestinal digestive enzymes, enhanced the hepatic lipid metabolism, promoted the protein deposition, and regulated the structure and diversity of the intestinal flora.

Co-occurrence of co-contaminants: Cyanotoxins and microplastics, in soil system and their health impacts on plant - A comprehensive review

Cyanotoxins (CTX) and micro/nanoplastics (M/NP) are ubiquitously distributed in every environmental compartment. But the distribution, abundance and associated ecological risks of CTX are still poorly understood in soil system. On the other hand, M/NP could serve as vectors for persistent organic/inorganic pollutants in the natural environment through the sorption of pollutants onto them. Thus, co-occurrence of CTX and M/NP in soils suggests the sorption of CTX onto M/NP. So, major aim of this review is to understand the relevance of CTX and M/NP in soils as co-contaminants, possible interactions between them and ecological risks of CTX in terms of phytotoxicity. In this study, we comprehensively discuss different sources and fate of CTX and the sorption of CTX onto M/NP in soil system, considering the partition coefficient of different phases of soil and mass balance. Phytotoxicity of CTX, CTX mixture and co-contaminants has also been discussed with insights on the mechanism of action. This study indicates the need for the evaluation of sorption between co-contaminants, especially CTX and M/NP, and their phytotoxicity assessment using environmentally relevant concentrations.

Long-term environmental exposure to microcystins increases the risk of nonalcoholic fatty liver disease in humans: A combined fisher-based investigation and murine model study

Microcystins (MCs) produced by cyanobacteria pose serious threats to human health. However, the contribution of long-term exposure to MCs to the development of nonalcoholic fatty liver disease (NAFLD) remains poorly documented. In this study, we estimated the environmental uptake of MCs by a small population of fishers who have lived for many years on Meiliang Bay of Lake Taihu, where cyanobacterial blooms occur frequently. Serum biochemical indices of liver function and their relationships with MC contamination in these people were also investigated. Moreover, to mimic the long-term effects of MC on the livers of fishers, an animal model was established in which mice were exposed to MC-LR at an environmentally relevant level, a reference level (the no-observed adverse effect level, NOAEL), and three times the NOAEL through drinking water for 12 months. We estimated the total daily intake of MCs by fishers through contaminated lake water and food to be 5.95 μg MC-LReq, far exceeding the tolerable daily intake (2.40 μg MC-LReq) proposed by the World Health Organization (WHO). More than 80% of participants had at least one abnormal serum marker. The indices of aspartate aminotransferase (AST)/alanine aminotransferase (ALT), triglyceride (TG), globulin (GLB), and lactate dehydrogenase (LDH) had close positive associations with MC contamination, indicating that both liver damage and lipid metabolism dysfunction were induced by chronic MC exposure. Furthermore, the animal experimental results showed that long-term exposure to MC-LR at the environmentally relevant level led to hepatic steatosis with molecular alterations in circadian rhythm regulation, lipid metabolic processes, and the cell cycle pathway. Exposure to MC-LR at or above the NOAEL worsened the pathological phenotype towards nonalcoholic steatohepatitis disease (NASH) or fibrosis. These results suggest that prolonged exposure to the reference level (NOAEL) of MC-LR could cause severe liver injury to mammals. People with long-term environmental exposure to MCs might be at high risk for developing NAFLD.

Biodegradation of microcystin-RR and nutrient pollutants using Sphingopyxis sp. YF1 immobilized activated carbon fibers-sodium alginate

A novel biological material named activated carbon fibers-sodium alginate@Sphingopyxis sp. YF1 (ACF-SA@YF1) was synthesized for microcystin-RR (MC-RR) and nutrient pollutant degradation in eutrophic water. The synthesized biomaterial was characterized by scanning electron microscopy (SEM). Box-Behnken design and response surface methodology (RSM) were utilized for the optimization of conditions during the MC-RR degradation. The degradation of MC-RR and nutrient pollutants was dynamically detected. The results revealed that the optimal conditions were temperature 32.51 °C, pH 6.860, and inoculum 14.97%. The removal efficiency of MC-RR, nitrogen, phosphorus, and chemical oxygen demand were 0.76 μg/mL/h, 32.45%, 94.57%, and 64.07%, respectively. In addition, ACF-SA@YF1 also performed satisfactory cyclic stability, while the MC-RR removal efficiency was 70.38% after seven cycles and 78.54% of initial activity after 20 days of storage. Therefore, it is reasonable to believe that ACF-SA@YF1 is an effective material which has a great prospect in removing MC-RR and nutrients from freshwater ecosystems.

Multimedia distributions, bioaccumulation, and trophic transfer of microcystins in the Geum River Estuary, Korea: Application of compound-specific isotope analysis of amino acids

To determine distributions, bioaccumulation, and trophic transfer of freshwater cyanobacterial toxins such as microcystins (MCs), surface water, suspended solids, sediments, and coastal organisms were collected from seven stations in inner and outer regions of the estuary dam in the Geum River Estuary in June and July 2017. Concentrations of MC variants (MC-LR, -RR, and -YR) in the multimedia samples were analyzed using a HPLC-MS/MS. Trophic position (TP) of organisms (fish, bivalve, gastropod, decapod, and polychaete) was determined by nitrogen stable isotope analyses of both bulk tissues and amino acids. From July to August 2017, great concentrations of MCs were detected in discharged freshwater ranging from 0.4 to 75 μg L^-1. Considerable amounts of MCs are delivered to the Geum River Estuary in summer season. MCs spread far away as dissolved phases (18.7-49.5 ng L^-1) in July when large amount of freshwater was discharged during the rainy season. Concentrations of MCs in marine organisms varied among species, ranging from 40 to 870 ng g^-1 dw. Bioaccumulated MCs tend to decrease with increasing TP of organisms, suggesting that MCs are biodiluted through the marine food web. Compound-specific isotope analysis (nitrogen of amino acids) provides more reliable TPs compared with those by bulk isotope analysis in a closed estuary (such as the Geum River Estuary) with large fluctuations in the isotope ratio of primary producers.

Effects of different concentrations of Microcystis aeruginosa on the intestinal microbiota and immunity of zebrafish (Danio rerio)

Microcystis aeruginosa is a primary species of toxin-producing cyanobacteria. This study explored the effects of short-term exposure (96 h) to M. aeruginosa on the intestinal microflora variation and immune function of zebrafish. After exposure to different cell concentrations of M. aeruginosa, marked histological variation was observed in the intestine, such as goblet cells proliferation and intestinal desquamation. In addition high-concentration M. aeruginosa treatment (initial concentration: 1.59 × 10⁵ cells mL^-1) induced a significant increase in cytokine levels compared with other groups. Low-concentration M. aeruginosa treatment (initial concentration: 0.88 × 10⁵ cells mL^-1) promoted the transcription of inflammatory genes, while high-concentration treatment restrained the transcription of these genes. Moreover, M. aeruginosa exposure also changed the intestinal microbial diversity. At the phylum level, bacteria belonging to Proteobacteria were the most abundant in all groups, and Gammaproteobacteria were the dominant bacteria with major changes. Pathogenic microorganisms such as Shewanella, Plesiomonas, Halomonas, Pseudomonas, and Lactobacillus increased greatly after treatment with different cell concentrations of M. aeruginosa. This study indicates that M. aeruginosa induces an increase in zebrafish goblet cells and enhances the inflammatory response, which may produce detrimental effects in zebrafish, resulting in a greater proportion of pathogenic bacteria and intestinal injury. The results of this study will help improve the understanding of the effects of M. aeruginosa on the intestines of aquatic organisms.

Microcystin-RR: Occurrence, content in water and food and toxicological studies. A review

Microcystins (MCs) are hepatotoxins, produced by various species of cyanobacteria, whose occurrence is increasing worldwide owing to climate change and anthropogenic activities. More than 100 variants have been reported, and among them MC-LR is the most extensively studied, but there are other MC congeners that deserve to be investigated. The need for data to characterize the toxicological profile of MC variants other than MC-LR has been identified in order to improve risk assessment in humans and wildlife. Accordingly, the aim of this study was to evaluate the information available in the scientific literature dealing with MC-RR, as this congener is the second most common cyanotoxin in the environment. The review focuses on aspects such as occurrence in water and food, and toxicity studies both in vitro and in vivo. It reveals that, although MC-RR is a real hazard with a high exposure potential in some countries, little is known yet about its specific toxicological properties that differ from those of MC-LR, and important aspects such as genotoxicity and chronic effects have not yet been sufficiently addressed.

Occurrence and toxicity of microcystin congeners other than MC-LR and MC-RR: A review

The occurrence of cyanobacterial toxins is being increasingly reported. This is a reason for concern as they can induce toxic effects both in humans and in the environment. Among them, microcystins (MCs) are the best described and most diverse group of cyanobacterial toxins, and MC-LR and MC-RR are the congeners most widely investigated. However, the number of MC variants has also increased in recent years. Some of these minority variants have been shown to have a different toxicokinetic and toxicodynamic profile, but research focused on them is still limited. Moreover, in some water bodies these minority variants can be the predominant toxins. Nonetheless, MC-LR is the only one used for risk evaluation purposes at present. In order to contribute to more realistic risk assessments in the future, the aim of this review was to compile the available information in the scientific literature regarding the occurrence and concentration of minority MCs in water and food samples, and their toxic effects. The data retrieved demonstrate the congener-specific toxicity of MCs, as well as many data gaps in relation to analytical or mechanistic aspects, among others. Therefore, further research is needed to improve the toxicological characterization of these toxins and the exposure scenarios.

Serum microcystin levels positively linked with risk of hepatocellular carcinoma: A case-control study in southwest China

Microcystins have been reported to be carcinogenic by animal and cell experimentation, but there are no data on the linkage between serum microcystins and hepatocellular carcinoma (HCC) risk in humans. We conducted a clinical case-control study to investigate the association between serum microcystins and HCC risk after controlling several known risk factors, such as hepatitis B virus, alcohol, and aflatoxin. From December 2013 to May 2016, 214 patients newly diagnosed with HCC along with 214 controls (frequency-matched by age and sex) were recruited from three hospitals in Chongqing, southwest China. Basic information on lifestyle and history of disease was obtained by questionnaire. Blood samples were collected and analyzed for serum microcystin-LR (MC-LR) and aflatoxin-albumin adduct by enzyme-linked immunosorbent assay and for hepatitis B surface antigen status by chemiluminescence assay. Binary logistic regression analyses were performed to assess the independent effects of MC-LR and its joint effects with other factors on HCC risk. The adjusted odds ratio for HCC risk by serum MC-LR was 2.9 (95% confidence interval [CI], 1.5-5.5) in all patients. Notably, a clear relationship between increased MC-LR level (Q2, Q3, and Q4) and HCC risk was observed with elevated adjusted odds ratios (1.3, 2.6, and 4.0, respectively). Positive interactions with the additive model were investigated between MC-LR and hepatitis B virus infection (synergism index = 3.0; 95% CI, 2.0-4.5) and between MC-LR and alcohol (synergism index = 4.0; 95% CI, 1.7-9.5), while a negative interaction was found between MC-LR and aflatoxin (synergism index = 0.4; 95% CI, 0.3-0.7). Additionally, serum MC-LR was significantly associated with tumor differentiation (r = -0.228, P < 0.001).

CONCLUSION

We provide evidence that serum MC-LR was an independent risk factor for HCC in humans, with an obvious positive interaction with hepatitis B virus and alcohol but a negative interaction with aflatoxin. (Hepatology 2017;66:1519-1528).

Clinical effects of chemical exposures on mitochondrial function

Mitochondria are critical for the provision of ATP for cellular energy requirements. Tissue and organ functions are dependent on adequate ATP production, especially when energy demand is high. Mitochondria also play a role in a vast array of important biochemical pathways including apoptosis, generation and detoxification of reactive oxygen species, intracellular calcium regulation, steroid hormone and heme synthesis, and lipid metabolism. The complexity of mitochondrial structure and function facilitates its diverse roles but also enhances its vulnerability. Primary disorders of mitochondrial bioenergetics, or Primary Mitochondrial Diseases (PMD) are due to inherited genetic defects in the nuclear or mitochondrial genomes that result in defective oxidative phosphorylation capacity and cellular energy production. Secondary mitochondrial dysfunction is observed in a wide range of diseases such as Alzheimer's and Parkinson's disease. Several lines of evidence suggest that environmental exposures cause substantial mitochondrial dysfunction. Whereby literature from experimental and human studies on exposures associated with Alzheimer's and Parkinson's diseases exist, the significance of exposures as potential triggers in Primary Mitochondrial Disease (PMD) is an emerging clinical question that has not been systematically studied.

Response of gut health and microbiota to sulfide exposure in Pacific white shrimp Litopenaeus vannamei

Sulfide is a natural and widely distributed toxicant. It can be commonly found on the interface between water and sediment in the aquatic environment. The Pacific white shrimp Litopenaeus vannamei starts life in the benthic zone soon after the mysis stage, an early stage of post larvae. Therefore, L. vannamei is inevitably affected by exposure to sulfide released from pond sediment. This study explored the toxicant effect of different concentrations of sulfide on the intestinal health and microbiota of Pacific white shrimp by monitoring the change of expression of inflammatory, immune related cytokines, and the structure of the intestinal microbiota. The gut histology, expressions of inflammatory and immune related cytokines (tumor necrosis factor-alpha, C-type lectin 3, myostatin and heat shock transcription factor 1), and the microbiota were determined in L. vannamei after exposure to 0 (control), 425.5 (1/10 LC 50-96 h), and 851 μg/L (1/5 LC 50-96 h) of sulfide for 21 days. With the increase of sulfide concentration, intestinal injury was aggravated and the inflammatory and immune related cytokines generated a range of reactions. The expression of myostatin (MSTN) was significantly down-regulated by the concentration of sulfide exposure. No difference in the expression of heat shock transcription factor 1 (HSF1) was found between the control and shrimp exposed to 425.5 μg/L, but significantly higher HSF1 expression was found in shrimp exposed to 851 μg/L of sulfide. Significantly higher values of tumor necrosis factor-alpha (TNF-α) and C-type lectin 3 (CTL3) were found in the shrimp exposed to 425.5 μg/L of sulfide compared to the control, but a lower value was found in the shrimp exposed to 851 μg/L (P < 0.05). Sulfide also changed the intestinal microbial communities. The abundance of pathogenic bacteria, such as Cyanobacteria, Vibrio and Photobacterium, increased significantly with exposure to the increasing concentration of sulfide. The abundance of some anti-stress bacteria, such as Chlorobi and Fusobacterium, increased. Nitrospirae which can alleviate nitrite toxicity decreased. Microbacterium, Parachlamydia, and Shewanella were all commonly found and down-regulated in both sulfide groups, which is associated with an adaptation to sulfide stimulation. This study indicates that chronic exposure to sub-lethal levels of sulfide could lead to damage of the gut structure, stimulate the response of the inflammatory and immune systems, and shape the structure of the gut microbiota in L. vannamei.