Diversity of protists and bacteria determines predation performance and stability

Abstract

Predation influences prey diversity and productivity while it effectuates the flux and reallocation of organic nutrients into biomass at higher trophic levels. However, it is unknown how bacterivorous protists are influenced by the diversity of their bacterial prey. Using 456 microcosms, in which different bacterial mixtures with equal initial cell numbers were exposed to single or multiple predators (Tetrahymena sp., Poterioochromonas sp. and Acanthamoeba sp.), we showed that increasing prey richness enhanced production of single predators. The extent of the response depended, however, on predator identity. Bacterial prey richness had a stabilizing effect on predator performance in that it reduced variability in predator production. Further, prey richness tended to enhance predator evenness in the predation experiment including all three protists predators (multiple predation experiment). However, we also observed a negative relationship between prey richness and predator production in multiple predation experiments. Mathematical analysis of potential ecological mechanisms of positive predator diversity-functioning relationships revealed predator complementarity as a factor responsible for both enhanced predator production and prey reduction. We suggest that the diversity at both trophic levels interactively determines protistan performance and might have implications in microbial ecosystem processes and services.

Trophic complexity in aqueous systems: bacterial species richness and protistan predation regulate dissolved organic carbon and dissolved total nitrogen removal

“…All bacteria were isolated from soil, and none of these strains fixes molecular nitrogen. The bacterial strains were maintained in Brunner CR-2 medium [26,27]. Further, we used three predators which differ in their feeding modes: the amoeba Acanthamoeba sp.…”

“…We selected a substitutive experimental design to test the impact of bacterial (prey) and protistan (predator) species richness on both carbon and nitrogen removal as described in [26,27]. Our experiment comprised three levels: (I) controls without predation, (II) predation by one out of three different predators and (III) multiple predation (all predators together).…”

“…A general linear regression accompanied by ANOVA was done to show the relationship between predator production and DTN removal (electronic supplementary material, figure S6). Finally, we performed Spearman's rank order correlations analysis to determine the correlation of complementarity and selection effects as determined in [26,27] with nutrient removal determined in this study (electronic supplementary material, table S1).…”

“…The protists were grown in the respective media one week before starting the experiments. They were concentrated by centrifugation [26,27] and washed with Brunner CR-2 medium before using them in the predation experiments. The overall total initial density of protists in monocultures and mixtures was kept the same (5 Â 10 4 cells ml 21 ).…”

“…Trophic interactions between bacteria and their predators have received some attention for instance in the context of organic matter decomposition and pollutant degradation [20][21][22]. Despite theoretical considerations that diversity changes at linked trophic levels simultaneously affect resource removal, resource use efficiency and ecosystem stability [23,24], only few studies have analysed the combined effect of predator and prey diversity on ecosystem function [25][26][27].…”