The use of chemical compounds to suppress the growth of competitors is a competitive strategy known as allelopathy that can be readily observed with many phytoplankton species in laboratory studies. However, it is unclear how these allelopathic interactions are altered when the complexity of the system is increased to more closely mimic natural conditions. In the present study, we conducted laboratory experiments to decipher how the identity, abundance, and growth stage of competitors affect the outcome of allelopathic interactions with the red tide dinoflagellate Karenia brevis. Multiple chemical compounds produced by K. brevis were found to inhibit the growth of 4 phytoplankton competitors, although these competitors were susceptible to different combinations of compounds. We found that physiological state and cell concentration of competitors were important determinants of allelopathy, with early-stage (lag phase) cells more vulnerable to allelopathic effects than later growth stages for the diatom Skeletonema grethae. Despite being allelopathic to multiple competitors in the laboratory, in a microcosm experiment using plankton field assemblages, extracellular extracts of 2 strains of K. brevis had no effects on some taxa although they stimulated the growth of some diatoms. This suggests that in a species-rich ecological community under oligotrophic conditions, the relative importance of K. brevis allelopathy may not be as high as most laboratory studies predict.

Summary1. Resource quality can have conflicting effects on the spread of disease. High-quality resources could hinder disease spread by promoting host immune function. Alternatively, high-quality food might enhance the spread of disease through other traits of hosts or parasites. Thus, to assess how resource quality shapes epidemics, we need to delineate mechanisms by which food quality affects key epidemiological traits. 2. Here, we disentangle effects of food quality on 'transmission potential' -a key component of parasite fitness that combines transmission rate and parasite production -using a zooplankton host and fungal parasite. We estimated the components of transmission potential (i.e. parasite encounter rate, susceptibility and yield of parasite propagules) for hosts fed a highquality green alga and a low-quality cyanobacterium. 3. A focal experiment was designed to disentangle food quality effects on various components of transmission potential. The low-quality resource decreased transmission potential by stunting host growth and altering foraging behaviour. Hosts reared on low-quality food were smaller and had lower size-corrected feeding rates. Due to their slower grazing, they encountered fewer parasite spores in the water. Smaller hosts also had lower risk of an ingested spore causing infection (i.e. lower susceptibility) and yielded fewer parasite propagules. Hosts switched from high-to low-quality food during spore exposure also had low transmission potentialdespite their large size -because the poor quality resource strongly depressed foraging. 4. A follow-up experiment investigated traits of the low-quality resource that might have driven those results. Cyanobacterial compounds that can inhibit digestive proteases of a related grazer likely did not cause the observed reductions in transmission potential. 5. Our study highlights the value of using mechanistic models to pinpoint how resource quality can change transmission potential. Overall, our results show that low-quality resources could inhibit the spread of disease through effects on multiple components of transmission potential. They also provide insight into how disease outbreaks in wildlife may respond to shifts in resource quality caused by eutrophication or climate change.

No abstract

No abstract

This report summarizes recent research advancements in the chemical ecology of pelagic (open water) marine ecosystems. With the goal of providing a comprehensive overview of new knowledge in this field, we have reviewed the recent literature, and have organized it by type of ecological interaction, starting with studies on chemically-mediated intra-species communication, followed by inter-species interactions, and leading up to ecosystem-level effects of plankton secondary metabolites.