| Studies on the interactions between organic contaminants and protozoa are rare, and our knowledge of the role of protozoa in polycyclic aromatic hydrocarbon (PAH) degradation is very incomplete. An examination of the possible roles of protozoa is crucial for understanding the in situ microbial ecology of PAH degradation and for developing bioremediation approaches. Predation by protozoa is known to have significant effects on bacterial abundance and, therefore, bacterial activity rates. Desirable bacteria may not persist because of predation. Alternatively, predation may stimulate bacterial activity and growth in some instances. Large numbers of protozoa (1,000 ciliates cm{dollar}sp{lcub}-3{rcub}{dollar} and 5 {dollar}times{dollar} 10{dollar}sp6{dollar} flagellates cm{dollar}sp{lcub}-3{rcub}){dollar} measured in chronically contaminated sediment from the New York/New Jersey Harbor suggested that protozoan grazing would have significant effects on bacterial community and activity rates.; In order to determine whether protozoa preyed selectively on bacteria with different degradation abilities, two ciliates (Euplotes sp. and Cyclidium sp.), and three strains of PAH-degrading bacteria (Vibrio spp., each strain could degrade either naphthalene, anthracene or phenanthrene) were isolated from the contaminated sediment. By manipulating the culture media, pairs of bacteria with different PAH-degradation abilities, morphological and physiological properties were produced. These bacteria were live-stained with a fluorescent dye and fed to the ciliates. Both ciliates showed selective predation, but selectivity was not directly related to the PAH degradation abilities of the bacteria. Bacterial size was the primary factor affecting predation: large bacteria were selectively grazed. Protozoa also avoided eating bacteria with more hydrophobic cell surfaces, and this factor had greater effect when bacteria were attached to particles. Protozoa had a positive effect on mineralization of naphthalene, which was not due to regeneration of limiting nutrients or bioturbation of the sediment. Selective predation on non-PAH-degrading bacteria was probably the working mechanism. Results from a model incorporating selective predation matched the patterns derived from field and laboratory data. Selective predation would enhance biodegradation of PAHs provided that degrading bacteria of competitive growth rates and densities were present. |
