 Corresponding author: Chidi Juliet Nwankwo Copyright © 2023 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution Liscense 4.0. Toxicity of bitumen on soil fertility bacteria - Nitrobacter and Nitrosomonas from Tekulu Waterside, Rivers State, Nigeria Chidi Juliet Nwankwo * , Lucky Obukowho Odokuma and Chimezie Jason Ogugbue Department of Microbiology, Faculty of Science, University of Port Harcourt, Rivers State, Nigeria. GSC Advanced Research and Reviews, 2023, 16(03), 053–061 Publication history: Received on 09 July 2023; revised on 27 August 2023; accepted on 29 August 2023 Article DOI: https://doi.org/10.30574/gscarr.2023.16.3.0342 Abstract Toxicity of bitumen on Nitrosomonas and Nitrobacter from Tekulu Waterside, Bodo in Gokana Local government area of Ogoni, Rivers State, Nigeria was carried out using standard analytical procedures. The density, API and flash point of the bitumen used in this study at 15°C was 0.9898 g/cm 3 , API at 60°F value of 11.46 and >200°C respectively. The total TPH and PAH constituent of the bitumen was 20,549.77 mg/L and 924.09 mg/L respectively. Analysis of the effect of different concentration (50 mg/L, 250 mg/L, 500 mg/L, 750 mg/L and 1000 mg/L) of bitumen on Nitrosomonas sp. over a period (24hrs, 48hrs. 72hrs and 96hrs) revealed that the mortality of the organism increased as the concentration of the toxicant. The mortality was also affected by the duration of exposure. The probit analysis on the lethal concentration (LC10, LC20 and LC50) of the bitumen on Nitrosomonas sp. was 223.32 mg/L, 369.682 mg/L and 969.622 mg/L respectively and Nitrobacter sp. 587.837 mg/L, 765.74 mg/L and 1269.841 mg/L respectively, when exposed for 96 hours revealed a varying mortality status. The LC50 of the bitumen on Nitrosomonas sp. peaked at 48hrs. The study shows that bitumen was more toxic to Nitrosomonas than Nitrobacter sp. based on the findings of the study, it is recommended that test organisms (Nitrosomonas sp. and Nitrobacter sp.) could serve as a potential tool for eco- toxicological assay and pollution biomarker. Keywords: Toxicity; Nitrosomonas; Nitrobacter; Probit; Mortality 1. Introduction Petroleum is still the principal energy source for industries and industrial uses; even for some domestic uses. Despite the importance in the society; petroleum is a major source of pollution in the environment. Certain petroleum hydrocarbons are carcinogenic and mutagenic [1;2] and pose serious threat to human; plants; and animals health. Pollution of ecosystem is a matter of global concern as it leads to contamination of the food chain. In the environment; microorganisms play a fundamental role in biogeochemical processes including nutrient cycling. One of the microbial groups that play vital functions especially in plant fertility is the nitrogen fixing (nitrifying) bacteria. The abundance of these groups of organisms sometimes are used as indicators of soil fertility status and as bioindicators. Environmental pollution especially those of hydrocarbon-based pollutants; are known to be toxic to nitrifying bacteria and other autochthonous soil microorganisms; thus; influencing their growth and survival in the ecosystem [3]. Nitrobacter is a genus of mostly rod-shaped; gram-negative; aerobic-nitrifying and chemoautotrophic bacteria and cells normally reproduce by budding [4]. Microbial monitoring specifically for hydrocarbon is the concurrent stimulation and inhibition effect of petroleum hydrocarbons on bacteria; which complicates toxicity assessments [5]. The number of organisms that die after the exposure can then be measured and the concentration of a substance that kills half the test population calculated. This is the basis of the 24hour LC50 (Lethal concentration that kills 50% of the test population) toxicity test method [3]. The harmful effect of chemicals has on organism depend on many different factors such as the type of organism; age; size;