Different bacterial groups for biodegradation of three- and four-ring PAHs isolated from a Hong Kong mangrove sediment

Mangrove sediments have been found to degrade three- to four-ring PAHs extensively. In the present study, 11 strains from 4 genera Mycobacterium (3 strains), Sphingomonas (5), Terrabacter (2) and Rhodococcus (1) were isolated from a single surface sediment sample of a Hong Kong mangrove swamp, among which the Terrabacter strains were isolated to grow with fluoranthene for the first time. Although all four genera could degrade three- and four-ring PAHs, their in situ activities in natural sediment slurry were found to be different. A cultivable method showed that Sphingomonas strains grew rapidly under the induction of three-ring, but not four-ring PAHs, while only Mycobacterium degrading strains dominated in the four-ring PAHs spiked slurry. Culture-independent method using a reverse transcriptional PCR showed expressions of nahAc-like (mainly found in Gram-negative bacteria) and nidA-like (in Gram-positive bacteria) dioxygenase genes parallel with the degradation of three- and four-ring PAHs, respectively. The present study suggested that surface mangrove sediments harbored diverse PAH-degrading bacteria, which showed different importance for biodegradation of three- and four-ring PAHs in the sediment.