Predation pressure on bacteria increases along aquatic productivity gradients

Abstract

To elucidate how predation pressure on bacteria is affected by increasing productivity in aquatic systems, we ran model simulations of a microbial food web. In addition, we conducted a meta-analysis of data from studies in seas and lakes. The model was run as a flow-through system simulating conditions ranging from oligotrophic to hypertrophic. The organisms included in the model were heterotrophic bacteria, phytoplankton, 3 size classes of protozoa and metazooplankton. Predation pressure was defined as flagellate and ciliate grazing per bacterial biomass. The food-web model simulations showed increasing predation pressure on bacteria with increasing productivity (estimated chlorophyll a concentration 0.2 to 112 µg l -1). This was explained by an increase in bacterial production and the ratio of protozoan to bacterial biomass with increasing productivity. The results of the model simulation were supported by the meta-analysis, which showed increasing protozoan predation pressure on bacteria with increasing productivity in aquatic systems (chlorophyll a concentration 0.1 to 250 µg l -1). The ratio of protozoan to bacterial biomass also increased with productivity in the meta-analysis of field data. Our results suggest that protozoa control the bacterial community by predation in high-productivity environments.KEY WORDS: Bacteria · Protozoa · Predation · Microbial food web · Productivity · Regulation · Model · Omnivory Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 52: [45][46][47][48][49][50][51][52][53][54][55] 2008 sources in the food web, i.e. phytoplankton and heterotrophic bacteria (Cole et al. 1988). Increases in basal productivity cascade up through the food web via the predator-prey chain, possibly resulting in increased predation on bacteria and phytoplankton. If the food web is linear and prey organisms do not develop predation defense mechanisms, the top predator would exhibit an increase in biomass with increasing productivity, while lower trophic levels might exhibit alternating responses in biomass levels (according to the exploitation ecosystem hypothesis) (Oksanen et al. 1981, Thingstad & Sakshaug 1990. Thus, mechanisms controlling the structure of the microbial food web would shift between predation and resource control at different trophic levels. However, several mechanisms are known to modify this simple pattern: omnivory is common in aquatic food webs, increasing the complexity of trophic interactions (Diehl 2003). Introducing heterogeneity of trophic levels alters the response to increasing productivity compared to the predictions made by the exploitation ecosystem hypothesis (Leibold 1989, Abrams 1993. Furthermore, predation defense mechanisms are common in phytoplankton and bacterial communities in nutrient-rich waters, possibly as a response to increased predation pressure (Jürgens & Güde 1994, Rousseau et al. 2000. In a microcosm study, Hall et al. (2006) showed that an inedible producer influenced the degree of top-down vers...

Response of heterotrophic stream biofilm ­communities to a gradient of resources

“…Meta-analyses of cross-site eutrophication gradients show that planktonic primary production increases linearly with nutrient enrichment, stimulating a linear, but less than 1:1, increase in bacterioplankton production (Cole et al 1988, Thelaus et al 2008. Bacterial biomass, however, increases more slowly than production as a result of increased predation (Thelaus et al 2008). Similar patterns have been observed using experimental mesocosms through the manipulation of primary production (Hobbie & Cole 1984, Oviatt et al 1986) and direct additions of C, N and P (Joint et al 2002, Smith & Prairie 2004, Jansson et al 2006.…”

“…Comparable results for cell density were reported by Mohamed et al (1998) who found that a 15% increment in available DOC led to a 3.6× increment in bacterial density in a heterotrophic biofilm (b~2). A synoptic comparison of 15 marine systems showed that planktonic bacterial biomass and abundance increased only 10× over a 2800× range in primary production (b~0.01) (Thelaus et al 2008). Bacterial productivity was more dynamic, showing a 1000× response in relation to the 2800× change in primary production (b~0.8).…”

“…Be cause they are simpler to manipulate, plankton communities have been the focus of most studies that examine the composition and metabolism of microbial communities in relation to the availability of resources. Meta-analyses of cross-site eutrophication gradients show that planktonic primary production increases linearly with nutrient enrichment, stimulating a linear, but less than 1:1, increase in bacterioplankton production (Cole et al 1988, Thelaus et al 2008. Bacterial biomass, however, increases more slowly than production as a result of increased predation (Thelaus et al 2008).…”

“…Metabolism is regulated by topdown (predator-prey) and bottom-up (resource availability) interactions. Bottom-up effects are dominant in oligotrophic systems and top-down interactions are most important in eutrophic environments (Dufour & Torreton 1996, Gasol et al 2002, Vargas et al 2007, Thelaus et al 2008. Within planktonic communities there is little evidence for cooperative or synergistic interactions among populations.…”

Effect of top-down control on cell-specific activity and diversity of active marine bacterioplankton

“…Furthermore, concurrent changes in nutrient availability and predation further complicate interpretion of the effect of predation on a bacterial community (Corno & Jürgens 2008). Modeling results have indicated top-down control of bacterial abundance is more important in eutrophic marine environments (Sanders et al 1992, Thelaus et al 2008). However, a compilation of model and experimental results indicated the opposite: topdown control of marine bacterioplankton abundance was greater in oligotrophic systems (Gasol et al 2002).…”