Use of biolumescent bacteria, Xenorhabdus luminescens , to measure predation on bacteria by freshwater microflagellates *

Abstract Ligh emitted by the bioluminescent bacterium, Xenorhabdus luminescens (isolated from a nematode host), can be measured to monitor reductions of these bacteria in the presence of phagotrophs. X. luminescens is relatively large (0.6 ? 3 μm), but comparable in size to many cyanobacteria. We used the light emission method to examine phagotroph feeding on X. luminescens using uni‐specific cultures of two chrysomonads, Ochromonas sp. and Spulemma sp. From light decay rates in control and experimental vials, we computed an apparent filtration rate (FR); then, for a concentration (C) of 1 ? 10 6 bacteria ml −1 , we estimated capture rate (CR) as FR ? C. The Ochromonas sp. did not ingest the bacterium. The maximum estimated FR for Spumella , observed at 6.0 ? 10 3 flagellates ml −1 (medium density), was 0.37 ml h −1 , for a volume‐specific clearance rate (FR/cell volume) of 7.9 ? 10 5 h −1 and a CR of 62 bacteria ? flagellate −1 h −1 . Video microscopy indicated these were accurate estimates of capture rates. Microscopic counts were used to monitor growth of a flagellate, Spumella sp., on X. luminescens as the sole food supply. The flagellate doulbed in number every 3.2, while consuming bacteria at a rate of 23 bacteria flagellate −1 h −1 . The Spumella grazed the bacteria to a minimum of 5 ? 10 5 cells ml −1 , a concentration comparable to observed field densities of other bacteria.