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Date Permissions Signed


Date of Award

Winter 2016

Document Type

Masters Thesis

Degree Name

Master of Science (MS)



First Advisor

Strom, Suzanne L., 1959-

Second Advisor

Miner, Benjamin G., 1972-

Third Advisor

Kodner, Robin


Diatoms contribute up to 40% of the total primary production in the ocean and heavily influence the cycling of carbon and silica. Much of their success results from their silica frustule, which may provide a mechanical defense against grazers. In this study, I sought to determine the possible defense mechanism of the diatom’s frustule in the presence of one of their dominant grazers, heterotrophic dinoflagellates. I grew two species of diatoms, Thalassiosira rotula and Coscinodiscus radiatus, in semicontinuous culture with 80 μM or 20 μM silicic acid. Based on a 2-fold higher BSi cell-1, BSi:C, and BSi:N, this culture method successfully resulted in diatoms of both species with thick and thin frustules. I performed three predation experiments to determine if frustule thickness would affect predator ingestion, digestion or growth rate. I first fed thick and thin T. rotula to Gyrodinium spirale and measured ingestion and growth rate over 48 hr. I found no difference in ingestion rate between the thick and thin treatments, however G. spirale grew significantly slower on the thick-frustuled diatoms. I then fed thick and thin C. radiatus to Noctiluca scintillans, measuring ingestion rate in one experiment and digestion rate in a second. I found no difference in predator ingestion rate at the end of the 4 hr experiment. However, I did observe a significantly lower predator digestion rate when feeding on the thick-frustuled diatoms. The results strongly suggest that the frustule is providing a defense to the diatoms by slowing the predator’s digestion rate, which then decreases their population growth rate. This is the first reported evidence in favor of diatom frustules as a means of defense against microzooplankton. As such, this proposed mechanism has important implications for diatom bloom dynamics as well as global carbon and silica cycling.





Western Washington University

OCLC Number


Subject – LCSH

Diatoms--Frustules; Diatoms--Defenses; Dinoflagellates--Food




masters theses




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