One man's trash is another man's treasure—the effect of bacteria on phytoplankton–zooplankton interactions in chemostat systems

Abstract

Chemostat experiments are employed to study predator–prey and other trophic interactions, frequently using phytoplankton–zooplankton systems. These experiments often use population dynamics as fingerprints of ecological and evolutionary processes, assuming that the contributions of all major actors to these dynamics are known. However, bacteria are often neglected although they are frequently present. We argue that even without external carbon input bacteria may affect the experimental outcomes depending on experimental conditions and the physiological traits of bacteria, phytoplankton, and zooplankton. Using a static carbon flux model and a dynamic simulation model, we predict the minimum and maximum impact of bacteria on phytoplankton–zooplankton population dynamics. Under bacteria‐suppressing conditions, we find that the effect of bacteria is indeed negligible and their omission justified. Under bacteria‐favoring conditions, however, bacteria may strongly affect average biomasses of phytoplankton and zooplankton. The population dynamics may become highly complex, which may result in wrong interpretations when inferring processes (e.g., trait changes) from population dynamic patterns without considering bacteria. We provide suggestions to reduce the bacterial impact experimentally. Besides optimizing experimental conditions (e.g., the dilution rate) the appropriate choice of the zooplankton predator is decisive. Counterintuitively, bacteria have a larger impact if the predator is not bacterivorous as high bacterial biomasses and complex population dynamics arise via competition for nutrients with the phytoplankton. Only at least partial bacterivory minimizes the impact of bacteria. Our results help to improve the design of chemostat experiments and their interpretation, and advance the study of ecological and evolutionary processes in aquatic food webs.

Co‐adaptation impacts the robustness of predator–prey dynamics against perturbations

“…Taking a more comprehensive approach, we proposed the average warping distance AWD and the maximum warping distance MWD to measure the distance between the pre‐ and post‐perturbation attractors and quantify the resistance of the system against press perturbations. These account also for more complex changes in the shape of the attractor, which otherwise could unnoticedly become highly complex, for example, multiple local maxima of one species within one population cycle could arise (as e.g., in Raatz, Schälicke, Sieber, Wacker, & Gaedke, ).…”

“…These account also for more complex changes in the shape of the attractor, which otherwise could unnoticedly become highly complex, for example, multiple local maxima of one species within one population cycle could arise (as e.g., in Raatz, Schälicke, Sieber, Wacker, & Gaedke, 2018).…”

“… References: a Varied within this study, b Raatz et al. () and references therein, c van Velzen and Gaedke (), d van Velzen and Gaedke (), e As attack rates and handling times are rarely measured, they were calculated from the predator's maximum grazing rate g max and its half‐saturation constant K Pred via a = g max / K Pred and h = 1/ g max . …”

Biogenic Aggregation of Small Microplastics Alters Their Ingestion by a Common Freshwater Micro-Invertebrate

“…It is known from previous studies that the diet of B. calyciflorus includes not only algae but also bacteria to some extent. Bacteria may be utilized as food (Raatz et al, 2018) and, moreover, it seems likely that Brachionus can also ingest larger detrital particles and bacterial aggregates, deriving nutritional benefit from those cells as well (Starkweather et al, 1979).…”

Enhancing rotifer Brachionus calyciflorus population growth with commercial probiotics

“…It seems possible that these positive effects are due to the ability of zooplankton to feed on bacteria (Starkweather, Gilbert, & Frost, ); enzyme secretion by bacteria (Marques et al, ); nutrient recycling by bacteria (Raatz, Schaelicke, Sieber, Wacker, & Gaedke, ); antagonism against pathogenic organisms (Grotkjær et al, ); and water quality improvement by removal of toxic substances (Loka et al, ; Verschuere et al, ). Previous studies tested both bacterial isolates (Harzevili et al, ; Mejias et al, ) and commercially available probiotics (Loka et al, ; Zink et al, ).…”