Protozoan predation drives adaptive divergence in Pseudomonas fluorescens SBW25; ecology meets experimental evolution.

Protozoan predators can affect the structure of bacterial communities, but investigations of how predation might influence bacterial evolution and antagonistic behaviours are scarce. Here, we performed a 20-day predator-prey evolution experiment on solid media to investigate the effect of continuous protozoan predation on bacterial traits using Pseudomonas fluorescens SBW25 as prey and Naegleria gruberi as an amoeboid predator. We observed the divergence of colony morphotypes coincident with an increase in bacterial grazing resistance and relative prey fitness in selected bacterial isolates. When subjected to these resistant prey, N. gruberi show reduced activity (increased encystment) and limited replication. An investigation of the mutations responsible for predation resistance reveals mutations in wspF and amrZ genes, affecting biofilm formation and motility. The bacterial mutants in the wspF gene successfully colonise the air-liquid interface and produce robust cellulose biofilms that prevent predation. The mutation in the amrZ mutant withstands predation but this variant produces low levels of cellulose and limited swarming motility. Our findings suggest that protozoan predation can profoundly influence the course of genetic and phenotypic evolution in a short period.

Grazing resistance in phytoplankton

Phytoplankton is confronted with a variable assemblage of zooplankton grazers that create a strong selection pressure for traits that reduce mortality. Phytoplankton is, however, also challenged to remain suspended and to acquire sufficient resources for growth. Consequently, phytoplanktic organisms have evolved a variety of strategies to survive in a variable environment. An overview is presented of the various phytoplankton defense strategies, and costs and benefits of phytoplankton defenses with a zooming in on grazer-induced colony formation. The trade-off between phytoplankton competitive abilities and defenses against grazing favor adaptive trait changes—rapid evolution and phenotypic plasticity—that have the potential to influence population and community dynamics, as exemplified by controlled chemostat experiments. An interspecific defense–growth trade-off could explain seasonal shifts in the species composition of an in situ phytoplankton community yielding defense and growth rate as key traits of the phytoplankton. The importance of grazing and protection against grazing in shaping the phytoplankton community structure should not be underestimated. The trade-offs between nutrient acquisition, remaining suspended, and grazing resistance generate the dynamic phytoplankton community composition.

Effects of Harmful Blooms of Large-Sized and Colonial Cyanobacteria on Aquatic Food Webs

Cyanobacterial blooms are the most important and best studied type of harmful algal blooms in fresh waters and brackish coastal seas. We here review how and to which extent they resist grazing by zooplankton, how zooplankton responds to cyanobacterial blooms and how these effects are further transmitted to fish. Size, toxicity and poor nutritional value are widespread mechanisms of grazing defense by cyanobacteria. In some cases, defenses are inducible, in some they are obligate. However, to some extent zooplankton overcome grazing resistance, partly after evolutionary adaptation. Cyanotoxins are also harmful to fish and may cause fish kills. However, some fish species feed on Cyanobacteria, are able to reduce their abundance, and grow on a cyanobacterial diet. While reduced edibility for crustacean zooplankton tends to elongate the food chain from primary producers to fish, direct feeding by fish tends to shorten it. The few available comparative studies relating fish yield to nutrients or phytoplankton provide no indication that cyanobacteria should reduce the ratio fish production: primary production.

Species-specific effects of herbivorous fishes on the establishment of the macroalga Lobophora on coral reefs

Herbivory is a key ecosystem function that influences ecosystem trajectories. However, interactions between plants and herbivores are species-specific and change throughout the plants’ lifetime. On coral reefs, herbivorous fishes reduce competition between corals and macroalgae through their grazing activity, thereby regulating the ecosystem state. Grazing vulnerability of marine algae generally decreases with increasing algal size. Therefore, the removal of newly settled recruits by herbivorous fish is likely important in preventing macroalgal blooms and reducing competition with corals. We studied the grazing susceptibility of recruits of the brown macroalga Lobophora to multiple fish species through a combination of feeding observations and manipulative in situ and ex situ experiments. Further, we recorded short-term Lobophora growth patterns and adult survival over 9 wk. Lobophora recruits were more susceptible to herbivory than adults, likely owing to their smaller size. However, recruit mortality was driven by only 3 of the studied species: Acanthurus nigrofuscus, Scarus niger and Chlorurus spilurus, whereas other common herbivores did not remove any Lobophora recruits. Our data also suggest variable growth and recruitment among months. These findings point to a possible increase in grazing resistance with age for Lobophora. As such, a decrease in grazing pressure by key fish species controlling Lobophora recruits could permit Lobophora to establish more grazing-tolerant adult populations.