www.nature.com/scientificreports OPEN Isolation and characterization of a salt-tolerant denitrifying bacterium Alishewanella sp. F2 from seawall muddy water Rui Cheng1,2,3, Xinyi Wang1,3,4, Hui Zhu1,3 ✉ , Baixing Yan1,3, Brian Shutes5, Yingying Xu6, Baorong Fu4 & Huiyang Wen1,3 A salt-tolerant denitrifying bacterium strain F2 was isolated from seawall muddy water in Dalian City, Liaoning Province, China. Strain F2 was identifed by morphological observations, physiological and biochemical characteristics and 16 S rDNA identifcation. The salt tolerance of strain F2 was verifed and – the factors afecting the removal ability of strain F2 to nitrous nitrogen (NO2 N) and nitrate nitrogen – (NO3 N) in saline conditions were investigated. Strain F2 was identifed as Alishewanella sp., named Alishewanella sp. F2. Strain F2 can tolerate NaCl concentrations up to 70 g/L, and its most efcient denitrifcation capacity was observed at NaCl concentrations of 0−30 g/L. In the medium with NaCl – – concentrations of 0−30 g/L, strain F2 exhibited high removal efciencies of NO2 N and NO3 N, with – – the removal percentages for both NO2 N and NO3 N of approximately 99%. In saline conditions with 30 g/L NaCl, the optimum culture pH, NaNO2 initial concentrations and inoculation sizes of strain F2 – were 8−10, 0.4−0.8 g/L and 5−7%, respectively. Strain F2 was highly efective in removing NO2 N and – NO3 N in saline conditions, and it has a good application potential in saline wastewater treatment. In the past 40 years, with the rapid development of aquaculture industry in coastal areas of China, a large amount of coastal aquaculture wastewater has been discharged, which brought about various negative impacts on the environment1. Te coastal aquaculture wastewater usually contains both inorganic salts (on average of 10−30 g/L NaCl) and many contaminants including nitrogen2. Large amounts of nitrogen pollutants (i.e., nitrous nitrogen – – (NO2 N) and nitrate nitrogen (NO3 N)) which are continuously released from the uneaten feed residue can result in the increase of inorganic nitrogen pollution in aquaculture water year by year1,3. Furthermore, with the rapid development of industrialization, a large number of saline wastewater sources, e.g., vegetable pickled wastewater4, textile wastewater5, and oily wastewater6, etc., with NaCl concentrations exceeding 30 g/L have been produced in various industrial processes. Most saline industrial wastewater also contains a large amount of nitrogen pollut- ants7,8. Te increasing discharge of saline wastewater from aquaculture and industry without any treatment has threatened the aquatic, terrestrial and wetland ecosystems9,10. Terefore, the removal of nitrogen pollutants from saline wastewater has become an urgent problem. Nitrite, an intermediate product of nitrifcation and denitrifcation, is frequently detected in water bodies and seriously threatens the aquatic organisms and human health11,12. Te concentration of nitrite in wastewater is up to 50 mg/L or more11. High concentrations of nitrite seriously endanger the growth and normal metabolism of aquatic organisms. Humans accidentally drinking water containing high concentration of nitrite could easily lead to impaired intelligence and form strong carcinogens in the human body (e.g., nitrosamines), and even lead to death. As the most stable form of nitrogenous compounds in the aerobic environment, nitrate has a high solubil- ity and can migrate and difuse rapidly in water, resulting in secondary pollution13,14. Te discharging of untreated 1Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun, 130102, P R China. 2University of Chinese Academy of Sciences, Beijing, 100049, P R China. 3Jilin Provincial Engineering Center of CWs Design in Cold Region & Beautiful Country Construction, Changchun, 130102, P R China. 4School of Environment, Liaoning University, Shenyang, 110036, P R China. 5Urban Pollution Research Centre, Middlesex University, Hendon, London, NW4 4BT, UK. 6Key Laboratory of Songliao Aquatic Environment, Ministry of Education, Jilin Jianzhu University, Changchun, 130118, P R China. ✉e-mail: [email protected] SCIENTIFIC REPORTS | (2020) 10:10002 | https://doi.org/10.1038/s41598-020-66989-5 1 www.nature.com/scientificreports/ www.nature.com/scientificreports – – – – – – NO2 N initial NO2 N efuent NO2 N removal NO3 N initial NO3 N efuent NO3 N removal Strain concentration(mg/L) concentration(mg/L) percentage(%) concentration(mg/L) concentration(mg/L) percentage(%) F1 115.90 ± 4.63 1.43 ± 0.12 98.76 42.47 ± 1.19 6.43 ± 1.98 84.85 F2 106.87 ± 4.58 1.73 ± 0.54 98.38 45.60 ± 1.77 1.77 ± 0.54 96.13 F3 105.50 ± 3.09 1.37 ± 0.45 98.70 45.00 ± 2.30 4.33 ± 1.89 90.37 – – Table 1. Removal efect of NO2 N and NO3 N by strains in denitrifcation medium afer 5 d cultivation. and unqualifed wastewater containing nitrate into surface water bodies will bring serious potential safety hazards 15 – and threaten the growth of plants, animals and human health . Terefore, efectively reducing the NO2 N and – NO3 N concentrations in water is of great practical signifcance for both ecosystem and human health. Te main technologies for removing nitrogen from wastewater include physical, chemical and biological methods16–19. Compared with physical and chemical methods, biological methods have the advantages of high efciency, low energy consumption, low cost, easy implementation, etc., and have been widely used to remove – – – NO2 N from wastewater. Denitrifcation can convert NO2 N and NO3 N into gaseous nitrogen (i.e., nitrogen (N2) 20 and nitrous oxide (N2O)), and fundamentally solve the nitrogen pollution problem in water . Several nitrite-type denitrifying bacteria, e.g., Acinetobacter baumannii21, Pseudomonas putida22, and Pseudomonas tolaasii23, etc., – have been screened and were proved to be efcient in removing NO2 N from wastewater. However, saline waste- water contains not only high concentrations of nitrogen but also a large amount of soluble salts. Te presence of salts can signifcantly and negatively afect the removal of pollutants by microorganisms24,25. When microorgan- isms are exposed to highly saline environment, osmotic pressure is increased, resulting in the excessive loss of water in general microbial cells, the separation of protoplasm, the inhibition of microbial growth and metabo- lism, and even death26. Te denitrifcation capacity of most denitrifying bacteria is inhibited by high salinity27. – – Terefore, salt-tolerant denitrifying bacteria with efcient NO2 N and NO3 N removal ability are required to be isolated for the treatment of saline wastewater. Te objectives of this study are to: 1) isolate and identify a salt-tolerant denitrifying bacterium strain F2 from seawall muddy water; 2) validate the salt tolerance as well as the denitrifcation capacity of strain F2; and 3) evaluate the efects of initial pH, NaNO2 initial concentration and inoculation size on the denitrifcation capacity of strain F2 in saline conditions. Te results of this study will provide efcient microbial resource and optimal process parameters for microbial denitrifcation of saline wastewater, which is of great signifcance for protecting the water environment safety and human health. Results Isolation of denitrifying bacteria. An anoxic condition was created by submerged culturing of denitrifca- tion medium with NaNO2 as the sole nitrogen source, but a small amount of NaNO2 was still oxidized and rapidly 28 converted into NaNO3 . Afer repeating three times of enrichment and isolation, three strains of denitrifying bacteria with nitrite as sole nitrogen source were obtained, and numbered F1, F2 and F3, respectively. Te three strains became turbid during the enrichment and cultivation in the denitrifcation medium that was accompanied 29 by diferent degrees of gas generation (i.e., N2, N2O and/or nitric oxide (NO)) . Te enrichment cultivation of each strain cultivated for 5 d is shown in Fig. S1 (in supplementary material). – – Table 1 shows the removal percentages of NO2 N and NO3 N by each respective strain. Afer a 5 d cultivation, – the removal percentages of NO2 N in denitrifcation medium by all the three strains were above 98%. However, – the removal percentages of NO3 N by strain F2 were higher than that of other strains. Te removal percentages of – – NO2 N and NO3 N by strain F2 were 98.38% and 96.13%, respectively, and gas was produced concomitantly in a + 5 d cultivation. During the 5 d of cultivation, ammonia nitrogen (NH4 -N) concentration in the medium of strain F2 increased slightly, and the total nitrogen (TN) removal percentage by strain F2 was 20.99% (Fig. S2 in supple- – – mentary material), which indicated that some of the NO2 N and NO3 N in the medium were transformed into + NH4 -N, and some were transformed into nitrogen-containing substances which are necessary for the growth of strain F2 through assimilation. Besides, according to the conservation of elements, the removal of TN in the medium was mainly due to the emission of gaseous nitrogen (i.e., N2, N2O and/or NO). Afer comprehensive analysis, strain F2, an ideal bacterium, was selected as the research object in the subsequent experiments. Identifcation of strain F2. Te physiological and biochemical characteristics of strain F2 are shown in Table 2. Te extracted bacterial DNA was amplifed using 16 S rDNA primers to a 1033 bp amplifed fragment (Fig. S3 in supplementary material). Te phylogenetic tree constructed by the MEGA 4.0 version sofware is shown in Fig. 1. Te 16 S rDNA identifcation reveals that strain F2 was 99.81% homology genetic related with Alishewanella sp. N5 (GenBank accession no. EU287929.1). Terefore, according to the morphological obser- vation and 16 S rDNA gene analysis, strain F2 was identifed as Alishewanella sp., named Alishewanella sp. F2 (GenBank accession no. MN396708). It was deposited at the China General Microbiological Culture Collection Center (CGMCC) on March 25, 2019, numbered CGMCC No: 17433.