Award: OCE-1434842

This project investigated the interactions between environmental stress, chemical signaling, and predator-prey interactions within the communities of single-celled organisms that make up most of the ocean’s plankton. Experimental studies showed that even closely related microalgal species have evolved contrasting responses to high light stress, showing that the light stress response is an important aspect of the ecological niche or habitat that can by occupied by a given species. Exposing microalgae or their protist consumers to high light stress can change how food webs function. Light stress in one algal species (Emiliania huxleyi,the most common and abundant coccolithophore) reduced predation on those cells, while other types of microalgae did not show this effect. Similarly, light stressing protist grazers (heterotrophic dinoflagellates) was detrimental to one species, but not to another. In summary, light stress can shape planktonic communities through species differences in tolerance, and by modulating predation rates. Some microalgal groups produce mineral shells or armor-like plates whose thickness depends on environmental conditions. These shells and plates have been hypothesized to act as defenses against predators. We found that thicker silica shells (frustules) on diatoms did not reduce predation by dinoflagellates, but did slow digestion. Similarly, the calcium carbonate plates on coccolithophores did not consistently reduce dinoflagellate predation rates, but might have affected digestion. In summary, mineral structures on microalgal cells did not deter feeding by predators, but could decrease their ability to digest the cells that they eat, reducing community-level predation and increasing the likelihood that these microalgal species will form blooms. We undertook extensive methods development and experimental study of hydrogen peroxide (H2O2) production by microalgae. H2O2 is known to be a signaling molecule in other biological systems, and can be produced in response to stress. All microalgal species tested, including eukaryotes and cyanobacteria, could produce H2O2 and most could also degrade this reactive oxygen species (ROS). In some cases, bacteria associated with the microalgal cultures could also produce H2O2, although little bacterial degradation was seen. Although microalgal production of ROS is often reported to be associated with high light stress in the scientific literature, we found that almost no previous studies controlled for abiotic photochemical ROS production. In our experiments, all high light-associated H2O2 production was due to abiotic photochemistry, with none attributable to the microalgal cells themselves. We found evidence that both H2O2 and another, sulfur-containing algal signaling molecule (dimethylsulfoniopropionate, or DMSP) are produced at higher per-cell levels when concentrations of algal cells are low. This suggests a potential quorum sensing strategy, whereby microalgal cells adjust output of signal molecules as a means of assessing community cell density and/or of maintaining a particular environmental concentration of the signal molecule. Development of the H2O2 measurement assay allowed us to investigate H2O2 production by a toxic microalga, the fish-killing raphidophyte Heterosigma akashiwo. It has long been hypothesized that H2O2 is the agent of toxicity for this species, although the data remain inconclusive. We found that H. akashiwo does produce moderate amounts of H2O2, with the per-cell production rates dependent on strain (i.e. isolate), and on salinity (with higher production rates at higher salinities). H2O2 production rates increase when H. akashiwo is in the presence of competitor microalgal species or predators. Adding catalase, an enzyme that degrades H2O2, to experimental systems reduced the toxicity of H. akashiwo to two different ciliate predators, demonstrating conclusively that H2O2 is, at least in part, responsible for the toxic effects of this harmful bloom-forming (HAB) species. In summary, H2O2 appears to act as a signal molecule for a wide range of microalgal species, which have the ability to dynamically regulate its concentration through variations in production and decomposition rate. Production of relatively high H2O2 concentrations makes Heterosigma akashiwo toxic to predators; this HAB species can dynamically regulate H2O2 production in response to environmental conditions and to the presence of competitor and predator species. Broader impacts include training of graduate and undergraduate students (3 Masters theses were supported) and maintenance of a protist culture collection that was shared with researchers nationally and internationally. One of the main dinoflagellate predator species used in the above studies has recently been sequenced by our collaborators at the University of British Columbia. With its phylogenetic position now established, a redescription is in process that will place the organism into a newly described genus, Deanodinium. Seven data sets describing the chemical composition, environmental stress responses, and chemical signal production of various microalgae were added to a public data repository (BCO-DMO). Last Modified: 12/14/2020 Submitted by: Suzanne L Strom