Abstract
Anthropogenic activities such as mining, and the use of pesticides in agriculture have led to an uncontrollable accumulation of heavy metals in the environment. Heavy metals are widespread pollutants which affect soil properties and are hazardous for all biosphere unless removed from the environment. Bioremediation employs use of biological species in the removal of heavy metals from the environment. Plant endophytes are affected by the accumulation of heavy metals and are known to develop mechanisms to deal with these pollutants by means of prevention, moderation and nullification of the toxicity of such metals. Therefore, these heavy metals are then removed from the environment or converted to products that are less toxic and are utilised by the endophytes or the plant host, reducing what would otherwise be a pollutant. Thus, this study set out to isolate, characterize, and identify bacterial endophytes from the leaves of the metal hyperaccumulator Solanum nigrum, the endophytes were further screened for plant-growth-promoting (PGP) traits to assess their role as endophytes and their contribution to plant health. Their tolerance to heavy metals (Lead [Pb], Zinc [Zn] and Copper [Cu]) was evaluated by determining the minimum inhibitory concentration (MIC). The PGP tests included indole-3-acetic acid (IAA) production, phosphate solubilisation, siderophore production, -Aminocyclopropane-1-Carboxylate (ACC) deaminase, and nitrogen fixation. The ability of the strains to potentially bioremediate heavy metals was also screened using different metal concentrations, with the best candidate chosen for further experimental studies under variable temperature and pH level. Five bacterial endophytes belonging to the Bacillus genus (Bacillus sp. strain 34 (OR046313), Bacillus sp. strain 35 (OR046314), Bacillus sp. strain 36 (OR046315), Bacillus sp. strain 37 (OR046316) and Bacillus sp. strain 38 (OR046317) were characterized, and two strains (Bacillus sp. strain 36 (OR046315), Bacillus sp. strain 37 (OR046316)]) were positive for plant-growth promoting properties such as production, nitrogen fixation, siderophore production, with the exception of Bacillus sp. strain 34 (OR046313), Bacillus sp. strain 35 (OR046314) and Bacillus sp. strain 38 (OR046317) which were negative only for IAA production. All the strains exhibited high tolerance for Pb while showing minimum resistance to Cu. Bacillus sp. strain 36 (OR046315) and Bacillus sp. strain 37 (OR046316) were the only two species that showed high tolerance to Pb. The two strains further indicated high affinity
III
to sequester metals, and results indicated that their ability for sequestration was influenced by pH and temperature of the growth media, thus the two species are recommended for further studies in microbial bioremediation.
Keywords: Solanum nigrum, Bacterial Endophytes, Heavy metals, Bioremediation, Minimum Inhibitory Concentration (MIC).