However, there have been few studies that have investigated the response mechanism of their toxic effects. It has been confirmed that NAs are an important factor that influence the composition of marine phytoplankton communities. Compared with environmental media, such as rivers and lakes, the hydrophilic carboxyl group makes NAs more soluble in slightly alkaline seawater, and it has been concluded that NAs will have a faster diffusion rate and wider spread in crude oil-contaminated seawater. The ecotoxicity of NAs to terrestrial ecosystems, such as rivers and soil, has been confirmed, and there is evidence that NAs show stronger toxicity in aquatic environments. The formula of this compound is C nH 2n+zO 2, where n is the number of carbon atoms, and z is zero or a negatively even integer that refers to the loss of a hydrogen. Īs a new pollutant, NAs have caused widespread concern due to their high level of ecotoxicity. In addition, the compounds have been detected in aquatic organisms, such as freshwater fish. Petroleum spills have resulted in large amounts of NAs entering the environment, and they are now being found in environmental media such as groundwater, rivers, lakes, and sediments. Naphthenic acids (NAs) are the primary acid component in high-acid crude oil, and its weight ratio in crude oil is 1–2%. The current production of high-acid crude oil accounts for approximately 5% of the total global crude oil production each year and is increasing at an annual rate of 0.3% on average. According to the current domestic demand, the use of high-acid crude oil is increasing. The frequent occurrence of oil spills in coastal areas makes marine crude oil pollution an important environmental problem. In addition, it provides prospective research data for the prediction and avoidance of ecological risk from NA pollution in marine environments. This study provides theoretical and practical data for the development of an important toxicological model of the toxicant’s excitement effects and antioxidant defense mechanisms. The behavior in the low-concentration treatment groups showed toxicant excitatory effects on the photosynthetic efficiency and antioxidant enzyme activity of the target organisms. The target organisms showed a staged oxidative stress response to NAs. The 16 mg/L NAs stress caused severe damage to the morphology and structure of the target biological cells in a short time (96 h), and the population growth decreased. In this study, the toxic effect of NAs on the target organism showed a clear concentration–dose relationship. Experiments confirmed the hormesis of low-concentration (0.5, 2, and 4 mg/L) NAs on Heterosigma akashiwo, and the indicators of high-concentration (8 and 16 mg/L) NA exposures showed inhibition. The mechanism of NAs’ damage effect was investigated in terms of the antioxidant enzyme activity, cell number, the chlorophyll positive fluorescence parameters, and the cell morphology of microalgae. It is important to study the mechanism of the toxic effect of NAs on the typical red tide algae, Heterosigma akashiwo, for the balance and stability of marine algae. Naphthenic acids (NAs) are the primary acidic component of crude oil, and red tide is of great concern due to its high diffusivity and strong destructive properties. Petroleum hydrocarbons can serve as a carbon source for marine phytoplankton so, marine high-acid crude oil pollution events are likely to result in algal outbreaks or harmful algal blooms (HABs) in surface waters.
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