title: Complexities of bloom dynamics in the toxic dinoflagellateAlexandrium fundyenserevealed through DNA measurements by imaging flow cytometry coupled with species-specific rRNA probes.
abstract: Measurements of the DNA content of different protist populations can shed light on a variety of processes, including cell division, sex, prey ingestion, and parasite invasion. Here, we modified an Imaging FlowCytobot (IFCB), a custom-built flow cytometer that records images of microplankton, to measure the DNA content of large dinoflagellates and other high-DNA content species. The IFCB was also configured to measure fluorescence from Cy3-labeled rRNA probes, aiding the identification ofAlexandrium fundyense(syn.A. tamarenseGroup I), a photosynthetic dinoflagellate that causes paralytic shellfish poisoning (PSP). The modified IFCB was used to analyze samples from the development, peak and termination phases of an inshoreA. fundyensebloom (Salt Pond, Eastham, MA USA), and from a rareA. fundyense'red tide' that occurred in the western Gulf of Maine, offshore of Portsmouth, NH (USA). Diploid or G2 phase ('2C')A. fundyensecells were frequently enriched at the near-surface, suggesting an important role for aggregation at the air-sea interface during sexual events. Also, our analysis showed that large proportions ofA. fundyensecells in both the Salt Pond and red tide blooms were planozygotes during bloom decline, highlighting the importance of sexual fusion to bloom termination. At Salt Pond, bloom decline also coincided with a dramatic rise in infections by the parasite genusAmoebophrya. The samples that were most heavily infected contained many large cells with higher DNA-associated fluorescence than 2C vegetative cells, but these cells' nuclei were also frequently consumed byAmoebophryatrophonts. Neither large cell size nor increased DNA-associated fluorescence could be replicated by infecting anA. fundyenseculture of vegetative cells. Therefore we attribute these characteristics of the large Salt Pond cells to planozygote maturation rather thanAmoebophryainfection, though an interaction between infection and planozygote maturation may also have contributed. The modified IFCB is a valuable tool for exploring the conditions that promote sexual transitions by dinoflagellate blooms but care is needed when interpreting results from samples in which parasitism is prevalent.
authors.affiliation: Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
authors.forename: Michael L
authors.initials: ML
authors.lastname: Brosnahan
authors.affiliation: Department of Entomology, University of California-Davis, Davis, CA 95616.
authors.forename: Shahla
authors.initials: S
authors.lastname: Farzan
authors.affiliation: Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
authors.forename: Bruce A
authors.initials: BA
authors.lastname: Keafer
authors.affiliation: Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
authors.forename: Heidi M
authors.initials: HM
authors.lastname: Sosik
authors.affiliation: Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Olson
authors.affiliation: Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
authors.forename: Donald M
authors.initials: DM
authors.lastname: Anderson
keywords: A. tamarense Group I; Alexandrium fundyense; algal bloom dynamics; imaging flow cytometry; microalgal life cycles