ISSN 2449-8955 European Journal of Biological Research Research Article European Journal of Biological Research 2018; 8 (1): 7-13 Effects of cobalt and manganese on biomass and nitrogen fixation yields of a free-living nitrogen fixer - Azotobacter chroococcum Justina Orji, Chima Ngumah*, Hanna Asor, Anulika Anuonyemere Federal University of Technology Owerri, Department of Microbiology, P.M.B 1526 Owerri, Nigeria *Corresponding author: Chima Ngumah; E-mail: ccngumah@yahoo.com ABSTRACT The effects of different concentrations of cobalt and manganese on the biomass and the ability of Azotobacter chroococcum to fix nitrogen were investigated. In vitro trials were conducted in Jensen’s (nitrogen free) broth (half strength) under continuous air flow, incubated at ambient room temperatures for seven days. Results obtained showed that 12.5 mg/l, 25 mg/l, 50 mg/l, 100 mg/l, and 200 mg/l concentrations of cobalt and manga- nese respectively enhanced microbial growth of Azotobacter chroococcum concomitantly. However, nitrogen fixation was enhanced only at 12.5 mg/l and 25 mg/l concentrations for cobalt, and only at 12.5 mg/l concentration for manganese. Statistical analysis revealed no significant difference in the specific growth rates and nitrogen fixations respec- tively, between the cobalt and manganese trials. Kinetic modeling revealed that nitrogen fixation was associated with biomass concentration, and not with cell mass growth. Keywords: Diazotroph; Micronutrients; Biostimu- lation; Toxicity; Luedeking-Piret model. 1. INTRODUCTION Nitrogen is the most abundant element in the earth’s atmosphere. However, due to the fact that atmospheric nitrogen is extremely un-reactive, it is also the most limiting nutrient to most plants [1]. Nitrogen is a constituent of proteins, enzymes, chlorophyll, and plant growth regulators; and its deficiency causes reduced growth and increased functional stress [2]. Biological nitrogen fixation is the process of converting elemental nitrogen into the forms, ammonium (NH + 4 ) and nitrates (NO - 3 ), available to plants [3], which are subsequently incorporated into amino acids [4]. Nitrogen fixation is the second most important biological process after photosynthesis, and it is mediated exclusively by prokaryotes [2]. Some bacteria fix nitrogen in a free living state (non-symbiotic fixation). Others are closely associated with plant roots (associative nitrogen fixation), and still others form a mutualistic symbiosis [4]. Although free living nitrogen fixing orga- nisms are widely distributed in soils, the quantity of nitrogen they fix seldom approaches that of the symbiotic systems. It is not that they are inherently incapable of vigorous nitrogen fixation, but abundant substrates to support their growth and fixation are commonly lacking in the soil; Received: 27 November 2017; Revised submission: 15 January 2018; Accepted: 22 January 2018 Copyright: © The Author(s) 2018. European Journal of Biological Research © T.M.Karpiński 2018. This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial 4.0 International License, which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited. DOI: http://dx.doi.org/10.5281/zenodo.1157098 brought to you by CORE View metadata, citation and similar papers at core.ac.uk