Abstract
Analysis of the total petroleum hydrocarbons (TPH) and the activities of hydrocarbon-utilizing bacteria (HUB) and diazotrophic bacteria (DB) in mangrove sediments of the Qua Iboe Estuary revealed that the organic-laden sediments have high hydrocarbon repository status. The epipelic sediment was relatively richer in total organic nitrogen (TON) content. Correlation analysis revealed positive relationship between TPH and HUB (r = 0.60) in the epipelic sediment, while no definite relationship (r = − 0.30) was established between the variables in benthic sediment. A strong negative relationship was observed between TPH and TON in the intertidal (r = − 0.746) and benthic (r = − 0.838) sediments. However, weak positive (r = 0.451) relations were established between TPH and log-transformed densities of DB in both the epipelic and benthic sediment. These indicate that although hydrocarbons contamination may have a direct negative effect on nitrogen availability in mangrove sediment, it has variable influence on the activities of nitrogen fixers. The effect on nitrogenase activity was much stronger in the “oxic” epipelic than the “hypoxic” benthic sediments. Of the 21 bacterial species isolated, only 7 – Azotobacter, Bacillus subtilis, Bacillus sp, Chromatium sp, Klebsiella sp, Moraxella sp, and Pseudomonas aeruginosa – have been implicated in nitrogen fixation. Chromatium sp, a microaerophilic nitrogen-fixing bacterium failed to utilize hydrocarbons while Bacillus megaterium, Micrococcus sp, Acetobacterium sp, Flavobacterium, Nocardia, and Alcaligenes species with no nitrogenase activity demonstrated oil-degrading capabilities. Among the free-living, facultative anaerobic nitrogen fixers encountered Bacillus subtilis, Bacillus sp, and Pseudomonas aeruginosa were prevalent, less affected, and exhibited strong ability to degrade hydrocarbons. These species of free-living nitrogen fixers are recommended for the remediation of oil-contaminated ecosystems in the Niger Delta of Nigeria.
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