Anaerobic biodegradation of naphthalene, phenanthrene, and biphenyl by a denitrifying enrichment culture

In previous results [Rockne and Strand (1998) Environ. Sci. Technol. 32, 2962-3967], anaerobic biodegradation of the polycyclic aromatic hydrocarbons, naphthalene, phenanthrene, and biphenyl in a fluidized bed reactor (FBR) enrichment was demonstrated. In this paper, re-feeding and mineralization experiments with sub-cultures of the nitrate-reducing enrichment are described. The subcultures continued to remove the PAHs after three feedings. PAH biodegradation ceased when nitrate was depleted and resumed when the enrichment was fed nitrate, demonstrating that PAH biodegradation was dependent upon nitrate reduction. Tests with radiolabeled PAH confirmed that PAH was mineralized, although the extent of mineralization differed greatly with different PAHs. Only partial mineralization (17% of initial carbon) was observed when the culture was fed naphthalene, whereas nearly complete mineralization (96%) was observed with phenanthrene. PAH carbon was incorporated into cell mass and mineralized after complete biodegradation of the PAHs, with 78-102% recoveries of radiolabel for naphthalene and phenanthrene, respectively. PAH carbon incorporation into biomass also varied considerably. Minor assimilation of biphenyl or phenanthrene was observed in the culture, whereas extensive assimilation of naphthalene carbon (57%) was observed when the culture was challenged with naphthalene. PAH degradation was approximately stoichiometric with the amount of nitrate consumed. Headspace analysis showed production of N2O, suggesting the enrichment coupled the biodegradation of PAH to denitrification.