Effects of ammonium sulfate on stress physiology and innate immunity of Western mosquitofish (Gambusia affinis)

The mechanism of reactive oxygen species generation, DNA damage and apoptosis in hemocytes of Litopenaeus vannamei under ammonia nitrogen exposure

Ammonia-N poses a significant threat to aquatic animals. However, the mechanism of ROS production leading to DNA damage in hemocytes of crustaceans is still unclear. Additionally, the mechanism that cells respond to DNA damage by activating complex signaling networks has not been well studied. Therefore, we exposed shrimp to 0, 2, 10, and 20 mg/L NH4Cl for 0, 3, 6, 12, 24, 48, and 72 h, and explored the alterations in endoplasmic reticulum stress and mitochondrial fission, DNA damage, repair, autophagy and apoptosis. The findings revealed that ammonia exposure led to an increase in plasma ammonia content and neurotransmitter content (DA, 5-HT, ACh), and significant changes in gene expression of PLC and Ca2+ levels. The expression of disulfide bond formation-related genes (PDI, ERO1) and mitochondrial fission-related genes (Drp1, FIS1) were significantly increased, and the unfolded protein response was initiated. Simultaneously, ammonia-N exposure leads to an increase in ROS levels in hemocytes, resulting in DNA damage. DNA repair and autophagy were considerably influenced by ammonia-N exposure, as evidenced by changes in DNA repair and autophagy-related genes in hemocytes. Subsequently, apoptosis was induced by ammonia-N exposure, and this activation was associated with a caspase-dependent pathway and caspase-independent pathway, ultimately leading to a decrease in total hemocytes count. Overall, we hypothesized that neurotransmitters in the plasma of shrimp after ammonia-N exposure bind to receptors on hemocytes membrane, causing endoplasmic reticulum stress through the PLC-IP3R-Ca2+ signaling pathway and leading to mitochondrial fission. Consequently, this process resulted in increased ROS levels, hindered DNA repair, suppressed autophagy, and activated apoptosis. These cascading effects ultimately led to a reduction in total hemocytes count. The present study provides a molecular support for the understanding of the detrimental toxicity of ammonia-N exposure to crustaceans.

Ecosystem complexity explains the scale-dependence of ammonia toxicity on macroinvertebrates

The toxic effect of unionized ammonia (NH₃) on aquatic organisms is receiving increasing attention due to the excessive nitrogen discharge to various surface waters. Researches have suggested the scale-dependence of NH₃ toxicity, being lower in field than under lab conditions. Such scale-dependence of toxicity is a big challenge to water quality criteria setting as the results solely from lab tests might not apply to natural ecosystems. Therefore, it is necessary to explore the underlying mechanism to understand the difference of toxicity across various spatial scales. In this study, we used the widely distributed gastropod Bellamya aeruginosa as the test animal and performed two 192-h microcosm experiments. Each experiment included a control and an ammonia addition treatment: N0(LC₅₀) & N+(LC₅₀), N0(LC₁₀₀) & N+(LC₁₀₀) (96-h LC₅₀ = 0.8 mg NH₃N/L, 96-h LC₁₀₀ = 18.1 mg NH₃N/L). Besides water-only, three potential key components (food, sediment, and submersed macrophytes) were included in the various treatments to mimic different complexity levels of aquatic ecosystems (Water, Water + Food, Water + Sediment, Water + Sediment + Macrophytes). The results showed that: 1) food directly improved the survival and growth of gastropods under expected lethal concentration of ammonia (96-h concentration of NH₃N = LC₂₀ of the 96-h acute test); 2) sediment and macrophyte quickly decreased the ammonia concentration, mainly by sediment adsorption and macrophyte uptake, to alleviate the ammonia stress to gastropods and permitted them to survive and grow under expected lethal concentration of ammonia (96-h concentration of NH₃N = LC₁₀∼LC₂₀ of the 96-h acute test); 3) sediment and macrophyte also provided additional food for gastropods; 4) under extremely high ammonia stress (i.e., 96-h LC₁₀₀, food was left uneaten and macrophyte died, and gastropods could, therefore, not be released from ammonia stress. Our results demonstrate that under moderate ammonia stress, the introduction of extra ecosystem elements (food, sediment, and macrophytes) significantly improved the survival and growth of gastropods, mainly by enhancing their tolerance and by quickly decreasing the NH₃ concentration and thus toxicity. However, these introduced elements had little effect at very high concentration of ammonia (i.e., 96-h LC₁₀₀). Our findings add to the understanding of the reasons behind the previous observed scale-dependent toxicity of NH₃ on aquatic organisms and contribute to better decisions on the role of NH₃ in relation to water quality management.

Decreasing toxicity of un-ionized ammonia on the gastropod Bellamya aeruginosa when moving from laboratory to field scale

Along with a steady increasing use of artificial nitrogen fertilizer, concerns have been raised about the effects that high nitrogen loading may have on ecosystems. Due to the toxicity of unionized ammonia (NH₃), tolerance criteria have been proposed for ambient water management in many countries; however, these are mainly based on acute or chronic tests carried out under lab conditions run with purified water. Aiming at understanding the responses of organisms to natural exposure to high ammonia concentrations, a Viviparidae gastropod, Bellamya aeruginosa, was tested at three experimental scales: standard 96-h lab test, one-month cage test in 6 experimental ponds with continuous nitrogen inputs, and intensive investigation of the B. aeruginosa from these ponds in spring and winter. The results were: 1) 96-h LC₅₀ in the standard lab test was 0.56 mg NH₃-N/L and 343.3 mg TAN/L (total ammonia expressed as N, standardized at pH 7 and 20 ℃). 2) In the one-month cage test, the survival rate was 97% when NH₃-N was 0.61 mg/L (i.e., a higher concentration than the lab 96-h LC₅₀) and the body size of the gastropods actually increased with increasing NH₃-N concentrations. 3) In the winter-spring investigation, little effect of ammonia on the standing crops of gastropods was found, and the body size of the gastropods tended to increase with increasing ammonia concentrations (NH₃-N concentration range of 0.05 ~ 2.06 mg/L). Thus, B. aeruginosa showed higher tolerance to ammonia exposure (NH₃-N concentration < 0.81 mg/L) in the field than under laboratory conditions. Our study points to the necessity of considering the relevant scale when determining criteria for ammonia toxicity in water management.