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Date of Award
Master of Science (MS)
Sulkin, Stephen D.
Strom, Suzanne L., 1959-
Miner, Benjamin G., 1972-
Olson, M. Brady (Michael Brady)
Marine phytoplankton are important players in the global ecosystem, contributing up to 50% of global primary productivity. Predation by microzooplankton is one of the most important sources of mortality for phytoplankton. However, phytoplankton defenses against predators are not well understood despite their expected importance. I tested for inducible defenses in the coccolithophore Emiliania huxleyi, an abundant and ecologically important bloom-forming species with a heteromorphic haploid-diploid life cycle, against the common ciliate predator, Strombidinopsis acuminatum. The calcifying diploid and non-calcifying, flagellated haploid phases of E. huxleyi likely occupy different ecological niches, potentially explaining the maintenance of haploid-diploidy in this species. The role of the haploid phase in the ecology of E. huxleyi remains little studied. I hypothesized that the two phases are differently defended against predation. To test this, I first exposed haploid and diploid E. huxleyi to ciliate predators for 24 hr. I then compared ciliate ingestion rates on predator-exposed prey to rates on naïve prey over the course of 30 min. I quantified ingestion by counting the prey inside ciliate food vacuoles under epifluorescence microscopy at three time points (5, 10, and 30 min). Prey were considered to have a defense response when ingestion rates on naïve cells were higher than ingestion rates on predator-exposed cells. Haploid E. huxleyi, although ingested at a lower rate than the larger diploid cells, had the strongest defense response. Diploid E. huxleyi had little to no defense response. The defense response in haploid E. huxleyi was dependent on nutrient availability. Ingestion rates on nutrient-replete haploid cells were reduced by 25 - 43% when previously exposed to the ciliate predator. In contrast, nutrient-limited haploid cells showed no evidence of a defense response within 30 min. I found that this defense was specific to haploid E. huxleyi and that the presence of defended E. huxleyi did not reduce ciliate ingestion on another prey species (Heterocapsa triquetra). To my knowledge this is the first demonstration of an inducible defense system in the Coccolithophoridae, although the mechanism of defense remains unknown. The question of this species' success as a common and bloom-forming phytoplankter may be answered in part by the haploid-diploid life cycle and an accompanying ability to separate traits for rapid growth/bloom formation and predator defense into two morphologically distinct phases, allowing this species to enjoy the best of both worlds and a wide ecological range.
Western Washington University
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Kolb, Amelia, "An inducible predator defense in the marine microalga Emiliania huxleyi (Prymnesiophyceae) is linked to its heteromorphic haploid-diploid life cycle" (2012). WWU Graduate School Collection. 211.