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To investigate the diversity of dioxygenase genes involved in polycyclic aromatic hydrocarbon (PAH)-degradation, a total of 32 bacterial strains were isolated from surface mangrove sediments, from the genera Mycobacterium, Sphingomonas, Terrabacter, Sphingopyxis, Sphingobium and Rhodococcus. Two sets of PCR primers were constructed to detect the nidA-like and nahAc-like sequences of the α subunit of the PAH ring-hydroxylating dioxygenase. PCR amplified the DNA fragments from all Gram-positive bacteria by using nidA-like primers and from all Gram-negative bacteria, except two, by using nahAc-like primers. The nidA-like primers showed three subtypes of nidA-like gene: (i) fadA1, clustering with nidA3 from M. vanbaalenii PYR-1, (ii) nidA, clustering with nidA from PYR-1, and (iii) fadA2 clustering with dioxygenase from Arthrobacter sp. FB24. The amplicons detected by nahAc-like primers had high sequence homologies to phnA1a from Sphingomonas sp. CHY-1 and were amplifiable from 8 of the 16 Gram-negative isolates. The primer also generated amplicons that had a 32-36% similarity to phnA1a and 53-93% identity to p-cumate dioxygenase. These results suggest that the nidA-like and nahAc-like genes are prevalent in the PAH-degrading bacteria and that they are useful for determining the presence of PAH-dioxygenase genes in environmental samples. © 2006 Federation of European Microbiological Societies Published by Blackwell Publishing Ltd. All rights reserved.

Surface sediments from two mangrove swamps in Hong Kong were enriched to obtain PAH-degrading microbial consortia. The consortium from sediments enriched with phenanthrene (Phe) in Ma Wan, a mangrove swamp next to a boat anchorage, was effective in degrading PAH with 96.7% and 89.9% Phe degradation at 0 and 10 ppt salinities, respectively. The Phe-enriched microbial consortium could also use pyrene (Pyr) as the sole carbon source but the degradation was much less (around 15%). On the other hand, the Phe-enriched consortium from sediments in Yi O, a mangrove swamp recently polluted by an oil spill, completely removed Phe and Pyr at both 0 and 10 ppt salinities. This suggests that different consortia were selected in two sediments using the same PAH substrate. Not only sediments, PAH substrate used for enrichment also selected different consortia. The Pyr- and Fla-enriched Yi O consortia only achieved 10.5 and 4.5% Phe degradation, respectively and the corresponding pyrene degradation was 7 and 40%. Among the three PAH compounds, fluoranthene (Fla) was most difficult to degrade, and 21.1,11.3, and 36.8% Fla were degraded by Phe-, Fla- and Pyr-enriched cultures, respectively.

Elevated concentrations of polycyclic aromatic hydrocarbons (PAHs) have been found in mangrove sediments due to anthropogenic pollution, and microbial degradation has been suggested as the best way to remove PAHs from contaminated sediments. The degradation of phenanthrene, a model PAH compound by bacteria, either the enriched mixed culture or individual isolate isolated from surface mangrove sediments was examined. The effects of salinity, initial phenanthrene concentrations and the addition of glucose on biodegradation potential were also investigated. Results show that surface sediments collected from four mangrove swamps in Hong Kong had different degree of PAH contamination and had different indigenous phenanthrene-degrading bacterial consortia. The enriched bacteria could use phenanthrene as the sole carbon source for growth and degrade this PAH compound accordingly. A significant positive relationship was found between bacterial growth and percentages of phenanthrene degradation. The phenanthrene biodegradation ability of the enriched mixed bacterial culture was not related to the degree of PAH contamination in surface sediments. The growth and biodegradation percentages of the enriched mixed culture were not higher than that of the individual isolate especially at low salinity (0 and 10 ppt). High salinity (35 ppt) inhibited growth and biodegradation of phenanthrene of a bacterial isolate but less inhibitory effect was found on the mixed culture. The inhibitory effects of salinity could be reduced with the addition of glucose. © 2002 Elsevier Science Ltd. All rights reserved.