Factors Affecting Predation by Cyclidium sp. and Euplotes sp. on PAH-Degrading and Nondegrading Bacteria

Abstract If predators select for or against contaminant-degrading bacteria, it will affect bacterial survival and has important implications for bioremediation. Protozoa are important predators of bacteria. In order to determine whether protozoa preyed differentially on bacteria with different degradation abilities, two ciliates (Euplotes sp. and Cyclidium sp.) and three strains of PAH-degrading bacteria (Vibrio spp., degrading naphthalene, anthracene, or phenanthrene) were isolated from sediment from New York/New Jersey Harbor. By manipulating growth conditions, bacterial strains with different PAH-degradation abilities and different cell properties were produced. Stepwise regression models were used to analyze how clearance rates on suspended bacteria and grazing rates on bacteria attached to particles were affected by bacterial size, hydrophobicity, C:N ratio, protein content, and PAH-degradation ability. Clearance rates ranged from 0 to 49 nl ciliate-1 h-1 for Euplotes sp. and from 0 to 1.7 nl ciliate-1 h-1 for Cyclidium sp. Clearance rates of both ciliates were positively correlated with bacterial size, hydrophobicity, and protein content, and negatively correlated with C:N ratio. PAH degradation ability had no (for Euplotes sp.) or small (for Cyclidium sp.) effects on clearance rates. The models accounted for 63-75% of the variation in clearance rates on different bacteria. Only Euplotes sp. grazed on attached bacteria, at rates from 3 to 176 bacteria ciliate-1 h-1. A regression model with only C:N ratio and protein content explained 45% of the variation in grazing rates. These models indicate that multiple properties of bacteria affect their susceptibility to predation by ciliates, but PAH-degradation ability per se has little effect.