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

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 · 10⁶ 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 · 10³ flagellates ml^-1 (medium density), was 0.37 ml h^-1, for a volume-specific clearance rate (FR/cell volume) of 7.9 · 10⁵ 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 · 10⁵ cells ml^-1, a concentration comparable to observed field densities of other bacteria.