Toxicity of violacein‐producing bacteria fed to bacterivorous freshwater plankton

Chemical defenses have been hypothesized to be widespread among aquatic bacteria, but few studies have assessed the susceptibility of pelagic consumers to bacterial secondary metabolites. In this study, we determined the effect of bacteria containing the indole alkaloid violacein on a range of freshwater plankton organisms that ingest bacteria. Growth and survival of two nanoflagellates ( Bodo saltans , Ochromonas sp.), two ciliates ( Colpidium campylum , Tetrahymena pyriformis ), two rotifers ( Keratella cochlearis , Brachionus calcyfloris ), and adults and juveniles of the cladoceran Daphnia magna were examined in laboratory experiments in which these organisms were fed bacterial diets supplemented with violacein‐producing bacteria such as Janthinobacterium lividum and Chromobacterium violaceum . Unselective uptake of violacein‐producing bacteria resulted in significantly lower survivorships, indicating acute toxicity to all zooplankton taxa tested. The presence of 10% C. violaceum in an otherwise nontoxic bacterial diet, equivalent to 9 ng violacein mL −1 , reduced growth rates and peak abundances of flagellates, ciliates, and rotifers by 18–100%. For D. magna , wild‐type C. violaceum was poisonous starting at concentrations <1 x 10 6 cells mL −1 , whereas a violacein‐deficient mutant did not have any lethal effect on the animals. Species‐specific variability in predator susceptibility was explained by the negative correlation between toxin sensitivity and biomass‐specific clearance rates. The susceptibility of a broad spectrum of planktonic bacterial consumers to violacein‐producing bacteria illustrates the potential effect of bacterial bioactives on the structure of aquatic food webs, biodiversity, and ecosystem functioning.