Characterization of bacterial algicide IRI-160AA: insights on programmed death pathways and organelle impacts in harmful dinoflagellates
Date
2014
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Publisher
University of Delaware
Abstract
With increases in anthropogenic eutrophication, phytoplankton assemblages are shifting from healthy communities dominated by diatoms to communities dominated by flagellated phytoplankton. Among these are the dinoflagellates, a group of algae that includes more toxic species than any other phytoplankton lineage. The increase in harmful dinoflagellate blooms poses a threat to the health of marine environments and directly impacts recreational and commercial use of the oceans. This study evaluated the effects of an algicidal bacterial filtrate, produced by Shewanella sp. IRI-160, on dinoflagellate physiology. The algicide, IRI-160AA, was specific to dinoflagellates, with greatest algicidal activity during logarithmic growth. Changes in physiology were then examined in three dinoflagellate species: Karlodinium veneficum , Gyrodinium instriatum and Prorocentrum minimum . Markers for programmed cell death (PCD), including production of endogenous reactive oxygen species (ROS), extracellular H2 O 2 concentrations, caspase-like enzyme activity and the inversion of phosphatidylserine in the plasma membrane, were elevated in all three dinoflagellates exposed to the algicide. The elevation of these markers suggests that an autocatalytic cell death pathway is activated as a consequence of IRI-160AA exposure. Morphology and organelle ultrastructure were also examined in dinoflagellages exposed to the algicide using epifluorescence, super resolution-structured illumination (SR-SIM) and transmission electron (TEM) microscopy. A major finding of this research was that the algicide IRI-160AA targeted the nucleus, specifically the chromosome packaging system. Dinoflagellates have unique chromosome packaging in that they stabilize the negative charge of DNA with divalent cations and transition metals. Results of this investigation suggest that algicidal activity is due to the chelation of, or transfer of electrons to/from, these stabilizing metal ions in the nucleus, which induces the decompaction or ejection of chromosomes. Effects of IRI-160AA on the cell cycle were also evaluated, and revealed an accumulation of cells in S phase in all dinoflagellates examined. Results of this investigation suggest that the algicide could be of value in future biochemical studies of dinoflagellates, or for the investigation of signaling processes that occur in mixed microbial communities. Results of this dissertation also support the application of IRI-160AA as a management tool for the prevention of harmful dinoflagellate blooms in coastal environments. However, further purification and structural determination of IRI-160AA are necessary before this algicide can be applied to natural populations or in biochemical and signaling studies.