Grazing of nonindigenous bacteria by nano-sized protozoa in a natural coastal system

✍ Scribed by K. Christoffersen; T. Ahl; O. Nybroe

• Publisher: Springer
• Year: 1995
• Tongue: English
• Weight: 746 KB
• Volume: 30
• Category: Article
• ISSN: 0095-3628
• DOI: 10.1007/bf00184514

✦ Synopsis

Mesocosms (-4.5 m 3) situated in a closed bay area were used to investigate the effect of protozoan predation on nonindigenous bacteria. Pseudomonasfluorescens strain Agl was released into mesocosms as a single inoculum of 1 × l0 s cells ml -~ (final concentration) or as four inocula (same concentration each) at intervals of 3 days. Mesocosms that had received growth media corresponding to the inoculum served as controls. Numbers of P. fluorescens Agl decreased rapidly whether released as single or multiple inocula. Direct estimation of protozoan predation using fluorescently labeled P. fluorescens from log phase and starved cultures, respectively, revealed that natural populations of heterotrophic nanoflagellates consumed substantial amounts of the nonindigenous bacterial strain. The volume of fluorescently labeled cells prepared from starved cells was 68% of log phase cell volume, but the individual clearance of the small cells was five to seven times higher than that of the log phase bacteria. The natural populations of nanoflagellates consumed 34-62% of P. fluorescens Agl daily if starved bacteria were offered as food, and 3-13% if the cells were in the logarithmic growth phase. This suggests that the effect of protozoan predation on nonindigenous bacterial strains is substantial because cultured bacteria are likely to starve in natural environments. The addition of P. fluorescens Agl and the growth medium enhanced the abundance of natural bacteria, chlorophyll a, heterotrophic nanoflagellates, and ciliates, but it did not improve the growth conditions for the released strain. The effects on the indigenous populations were more pronounced after addition of fresh medium than following inoculation with cells, which possibly was due to the lower nutrient content of spent medium. However, these results, based on direct estimation of protozoan predation on log phase and starved nonindigenous Correspondence to: K. Christoffersen. 68 K. Christoffersen et al. bacteria, point to the conclusion that mortality induced by bacterivorous predators is the key factor determining removal of nonindigenous bacteria introduced in natural aquatic systems.