We examined a biological control agent isolated from a bacteria species that may provide a mechanism for halting the growth of certain types of toxic dinoflagellate harmful algal blooms. We characterized the activity of the algicidal agent and its impact on dinoflagellate physiology. We also evaluated the effect of the algicide on microbial community structure and function to determine which natural bacteria work best.

Why We Care
Harmful blooms of toxic dinoflagellates cause a wide variety of environmental, economic, and human health problems in the United States. The growing frequency and magnitude of outbreaks has created a pressing need for ways to control or mitigate harmful algal bloom (HAB) impacts in coastal waters. Biological control agents like viruses or bacteria may play a natural role in controlling blooms. Research is needed to understand this phenomenon and find specific compounds that can be adapted to HAB control mechanisms without harming the environment. This work explores a promising control agent from the bacterium Shewanella sp. IRI-160.

What We Are Doing
The investigators characterized the algicidal agent IRI-160AA—produced by the marine bacterium Shewanella sp. IRI-160—and examined the effects of this compound on dinoflagellates and non-target species of plankton. The team characterized the mechanism of toxicity on several species of dinoflagellates and used mixed cultures to study the broader effects of this agent on community composition and function. The investigators tested hypotheses regarding the effect of IRI-60AA on photosynthesis, cell cycle progression, and programmed cell death in target algal species.

What We Found
Results show the algicide causes autocatalytic (non-necrotic) cell death in dinoflagellates, while having no negative impacts on other phytoplankton species tested in laboratory culture experiments. Algicidal effects were greatest during active growth stages of dinoflagellates, with minimal mortality in the stationary (S) phase. Analysis of algicide-treated dinoflagellate Karlodinium veneficum cultures shows that karlotoxins were below the level of detection, suggesting that treatment of K. veneficum blooms will not result in the release of toxins by this species.

We also examined the biochemical and physiological changes in dinoflagellates exposed to the algicide. Our research points to the nucleus as the primary organelle targeted by the algicide, with a loss of chromosomal structure in all species examined. This may be due to interactions between the algicide and metal ions that stabilize chromosomes in dinoflagellate species. Analysis of cell cycle progression (growth curve) in dinoflagellates exposed to the algicide also revealed a loss of DNA content. For those cells that retained DNA, there was an increase in the number of cells in the stationary (S) phase over time, suggesting that biochemical signals triggering progression to a second generation (G2) were inhibited. We also observed effects on the cell chloroplast, resulting in a decrease in photochemical function, but these effects varied depending on species.

We conducted small-scale, natural community, microcosm experiments during dinoflagellate blooms to evaluate the effects of the algicide on the total microbial community. Our results from these experiments reveal a restructuring of the microbial community in response to the algicide, with decreases in dinoflagellate species abundance and increases in the abundance of heterotrophic protists.

Furthermore, a survey of Shewanella sp. IRI-160 reveals that it is broadly distributed in Delaware’s lnland Bays—with higher cell concentrations in the sediments or associated with particles—and at higher salinities. Data from small-scale, mesocosm experiments indicated an increase in the abundance of Shewanella sp. IRI-160 in natural bacterioplankton communities with increased dinoflagellate abundance.
This work is part of the Prevention, Control, and Mitigation of Harmful Algal Blooms (PCMHAB) program. The project team was led by Kathryn Coyne, University of Delaware.

Benefits of Our Work
Our results suggest that control of dinoflagellate blooms by Shewanella sp. IRI-160 may be a natural phenomenon. Our findings indicate that application of the algicidal compound for control of dinoflagellate HABs will likely have the greatest impact in the early stages of a bloom, with minimal impact on the environment. More study is needed to confirm that the compound can be safely used in the environment as a means to control HABs.