2,6-Dimethylphenol (2,6-DMP), an important chemical intermediate and the monomer of plastic polyphenylene oxide, is widely used in chemical and plastics industry. However, the pollution problem of 2,6-DMP residues is becoming increasingly serious, which is harmful to some aquatic animals. Microbial degradation provided an effective approach to eliminate DMPs in nature, which is considered as a prospective way to remediate DMPs-contaminated environments. But the 2,6-DMP-degrading bacteria is not available and the molecular mechanism of 2,6-DMP degradation is unclear as well. Here, a 2,6-DMP-degrading bacterium named B5-4 was isolated and identified as Mycobacterium neoaurum. M. neoaurum B5-4 could utilize 2,6-DMP as the sole carbon source for growth. Furthermore, M. neoaurum B5-4 could degrade 2,6-DMP with concentrations ranging from 1 to 500 mg L⁻¹. Six intermediate metabolites of 2,6-DMP were identified and a metabolic pathway of 2,6-DMP in M. neoaurum B5-4 was proposed, in which 2,6-DMP was initially converted to 2,6-dimethyl-hydroquinone and 2,6-dimethyl-3-hydroxy-hydroquinone by two consecutive hydroxylations at C-4 and γ position; 2,6-dimethyl-3-hydroxy-hydroquinone was then subjected to aromatic ring ortho-cleavage to produce 2,4-dimethyl-3-hydroxymuconic acid, which was further transformed to citraconate, and subsequently into TCA cycle. In addition, toxicity bioassay of 2,6-DMP in water using zebrafish indicates that 2,6-DMP is toxic to zebrafish and M. neoaurum B5-4 could effectively eliminate 2,6-DMP in water to protect zebrafish from 2,6-DMP-induced death. This work provides a potential strain for bioremediation of 2,6-DMP-contaminated environments and lays a foundation for elucidating the molecular mechanism and genetic determinants of 2,6-DMP degradation.

Fluoroquinolones (FQs) present in water environments pose threats to aquatic organisms. The concentration of FQs adsorbed onto sediments is generally higher than that in water. Here, we studied the toxicities of two FQ antibiotics, norfloxacin (NOR) and ofloxacin (OFL), on the snail Bellamya aeruginosa, an adaptable benthic animal. For this, we performed acute toxicity experiments on young snails and sub-chronic exposure experiments on adult snails by exposure to the FQs for 1–28 days. The 96 h median effective concentrations showed that NOR toxicity was higher than OFL toxicity, although both NOR and OFL were low-toxicity substances (EC₅₀ > 100 mg L⁻¹). Four concentrations were used in the sub-chronic exposure experiments. Results of the measurement of bioconcentration factors (BCF) showed that OFL accumulation was higher than NOR accumulation in snail tissues. During the depuration period (28–49 days), at low concentrations (2 and 20 μg L⁻¹), both FQs required at least 7 days to reach the maximum residue limit (50 μg kg⁻¹). Conversely, at high concentrations (200 and 2000 μg L⁻¹), 14 days (NOR) and 21 days (OFL) were required. Our results could facilitate management of the environmental risks of antibiotics and conservation of aquatic animals.

The accumulation of ammonia and nitrite in natural water and aquaculture systems would suppresses the immune system of aquatic animal and reduces the fish growth. Nitrifying bacteria have been widely used to reduce the accumulation of ammonia and nitrite in aquaculture systems, but are still ineffective in many cases. An aquaculture model system consisting of red crucian carp, algae, nitrifying bacteria, and pond water from a natural fish culture was established to explore the limitation of algae and light to nitrifying bacteria content and bacterial nitrification in the presence of a predator. The concentrations of nitrifying bacteria and bacterial nitrification in the group containing algae and light were significantly limited, and addition of nitrifying bacteria in algae groups had little effect. In algae-free groups, the concentrations of ammonia and nitrite were decreased by nitrifying bacteria, and the potential ammonia oxidization rate was also increased. Our findings reveal that the combined effects of algae growth and light exposure are responsible for the observed ineffectiveness of nitrifying bacteria in natural aquaculture environments.

Steroid estrogens are natural hormonal compounds produced by various animals and humans. Estrone (E1), estradiol (E2), and estriol (E3) are the most commonly known estrogens that are released into the environment along with human and animal excreta, which end up polluting water bodies. While these estrogens are usually biotransformed into their respective by-products by various microbial strains, E2 could also be transformed into E1 by 17β-hydroxysteroid dehydrogenases (17β-HSDs) under reducing environmental conditions. However, due to limited further biotransformation of E1, it accumulates to higher levels in water bodies compared to other natural estrogens in the aquatic environment. Given that E1 is one of the potential endocrine-disrupting compounds (EDCs), with several adverse effects on aquatic animals and consequently on the seafood industry, it is vital to remove E1 from the environment via improved steroid bioremediation. In the present study, we successfully isolated a potential E1-degrading microbial strain (named as BH2-1) from soil sediments collected from the Bai Hai mangrove region of the South China Sea. The strain BH2-1 has excellent E1-degrading potential and could degrade 89.5% of E1 after 6 days of incubation in a MSM-E1 medium containing 1% NaCl at pH 6. Besides, after 3 h and 6 h of extraction, two non-accumulating intermediate compounds [3-hydroxyandrosta-5,7,9(11)-trien-17-one and androsta-1,4,6-triene-3,17-dione (ATD)], respectively, were successfully identified using GC-MS analysis. These non-accumulating intermediate compounds have not previously been reported during E1 biodegradation and might be new intermediate metabolites. The identification of these new compounds also gives more insight into the mechanism of E1 metabolism and helps to establish a clear E1 biodegradation pathway, which further enriches our knowledge on the overall microbial steroid degradation pathway. Furthermore, whole-genome sequence analysis of strain BH2-1 revealed the presence of 46 genes that belong to 6 major steroid-degrading gene classes.

Sediment is an endogenous pollution source, which often leads water systems to eutrophication due to the release of nutrients, especially phosphorus (P). Calcium nitrate (CN) was dosed to the water systems under different modes to control P release from the sediments in this study. A 63-day static laboratory test was conducted to explore the effects of intermittent dosing and one-time dosing modes of CN on P locking in the sediment and the concentrations of nitrogen (N) and P in waters. Results showed that 89% total phosphorus (TP) in the overlying water and 91% TP in the interstitial water of sediment were reduced in the intermittent dosing reactor, which were 4% and 13% higher than those in the one-time dosing reactor, respectively. Thus, the concentration of TP in the overlying water of the dosing reactors was both below 0.1 mg/L during the whole experiment. Meanwhile, the mean values of oxidation–reduction potential (ORP) in the sediment increased to − 110.7 ± 42.02 mV when CN was added intermittently, which were significantly higher than those of the one-time dosing reactor (− 158.3 ± 44.61 mV) and control reactor (− 320.7 ± 0.05 mV). Compared with one-time dosing mode, the intermittent dosing not only reduced the maximum concentrations of NO₂⁻-N from 9.21 to 1.79 mg/L and NO₃⁻-N from 92.42 to 27.58 mg/L but also shorten their retention time in the overlying water, which might depress the toxic threats to aquatic animals in water environments. Therefore, the intermittent dosing of CN could not only improve the P locking effect but also minimize the risks to aquatic animals in water environments under the premise of reasonable dosage selected. In a word, this research provided an effective operation mode for locking P with CN in the heavily polluted water bodies, which is also advantageous to avoid toxic threats to aquatic animals in water environment.

Cadmium (Cd) is a non-essential environmental endocrine-disrupting compound found in water and a potential threat to aquatic habitats. Cd has been shown to have various short-term effects on aquatic animals; however, evidence for long-term effects of Cd on vocal communications in amphibians is lacking. To better understand the long-term effects of low-dose Cd on acoustic communication in amphibians, male Xenopus laevis individuals were treated with low Cd concentrations (0.1, 1, and 10 μg/L) via aqueous exposure for 24 months. At the end of the exposure, the acoustic spectrum characteristics of male advertisement calls and male movement behaviors in response to female calls were recorded. The gene and protein expressions of the androgen receptor (AR) were determined using Western blot and RT-PCR. The results showed that long-term Cd treatment affected the spectrogram and formant of the advertisement call. Compared with the control group, 10 μg/L Cd significantly decreased the first and second formant frequency, and the fundamental and main frequency, and increased the third formant frequency. One and 10-μg/L Cd treatments significantly reduced the proportion of individuals responding to female calls and prolonged the time of first movement of the male. Long-term Cd treatment induced a downregulation in the AR protein. Treatments of 0.1, 1, and 10 μg/L Cd significantly decreased the expression of AR mRNA in the brain. These findings indicate that long-term exposure of Cd has negative effects on advertisement calls in male X. laevis.

The emerging pollutants cover a wide range of synthetic chemicals that are indispensable to modern society but with little known effects for aquatic animals and for people who consume polluted waters with these products. Gels manipulated with hormones are widely used for hormone replacement, muscle growth, among other purposes. However, only a small part of these hormones are absorbed into the skin, and so these can be transferred to the domestic sewage during the washing of exposed body regions. Thus, the aim of this study was to evaluate the toxicity levels for the bioindicator Artemia salina L., and cytotoxicity and mutagenicity for the bioindicator Allium cepa L. of gels handled with 1% testosterone and 0.1% estradiol. Data from immobile/dead A. salina numbers after 24 h of exposure showed that the highest concentrations of testosterone (10 and 25 μg/mL) and the highest concentration of estradiol (15 μg/mL) were toxic to this bioindicator. For the bioindicator A. cepa, mitotic indices and chromosomal aberrations did not indicate statistical differences between the groups treated with the testosterone gels (1, 10, 50 μg/mL) and estradiol (0.03, 0.30, 1.5 μg/mL) and the control group. However, all concentrations of the testosterone-containing gel decreased the percentage of cell division in relation to the time 0 h of each treatment and to the time 24 h of the negative control. Therefore, it is concluded that rivers or aquatic environments can be polluted if wastewater with the toxic concentrations found of these hormonal gels is discarded without previous treatment, compromising the life of organisms that live there. And, it encourages the development of techniques for treating sewage and water to reduce/eliminate the hormones present in them.