Isolation and characterization of iron reducing bacteria from peninsular Malaysia East Coast marine environments

Iron-reducing bacteria (IRB) play significant roles in iron geochemical cycles. IRB are capable of reducing Fe(III) to Fe(II) for energy conservation and thrive in both freshwater and marine aquatics environments which contain abundance of Fe(III) oxides. In this study, IRB from three South China Sea marine environments which represent three different marine ecosystems (brackish water, coastal marine ecosystem, and ocean floor benthic ecosystem) were isolated and identified. Sediment samples collected from Sungai Geting (brackish water), Kuantan Port (coastal marine ecosystem), and Ekspedisi Pelayaran Saintifik Kebangsaan (ocean floor benthic ecosystem) were enriched in minimal medium supply with Fe(III) hydroxyoxide (FeOOH) as the sole electron acceptor and lactate as the sole electron donor. The iron reduction activities of IRB were measured with Ferrozine assay and the result showed that up to 1 - 4.5mM of Fe(III) were reduced after 14 days of incubation. Among the three samples, the one from Sungai Geting showed the highest Fe(III) reduction activities in both first and second enrichments which produced (≈4-4.5mM) of Fe(II) after 14 days of incubation. A total of 27 IRB strains were successfully isolated from all three locations with the same enrichment strategy. IRB strains from this enrichment strategy mainly belong to two major groups which are Gram-negative Gammaproteobacteria (Klebsiella, Pseudomonas, Shewanella, Escherichia, and Enterobacter genera) and Gram-positive Bacilli (Bacillus and Exiquobacterium genera). The findings showed that iron-reducing Klebsiella thrived in coastal marine and brackish environments but not on the ocean floor with 10 strains out of 27, and IRB that belong to the Bacillus genus are the dominant IRB group at the South China Sea benthic ecosystem with nine strains. Among all the isolates, two isolates; KP20 and ESPK21 which were identified as Klebsiella pneumonia and Bacillus licheniformis, respectively, showed higher reduction activities on both soluble and insoluble iron compared to widely studied IRB strain, Shewanella oneidensis MR-1. An E. coli strain that is capable of reducing both soluble and insoluble irons comparable to S. oneidensis was also successfully isolated. Furthermore, there is no indigenous E. coli strain capable of reducing iron reported previously. The results from this study indicated that diverse groups of IRB dominate different marine environment in South China Sea. Further understanding on the presence of IRB groups will provide more insights on geochemical cycle of iron and other nutrients in the marine environments. In addition, even though a number of Gram-positive bacteria are capable of reducing metals have been reported, the understanding regarding it still markedly less compared to Gram-negative IRB.