Diatom Frustules as a Mechanical Defense Against Predation by Heterotrophic Dinoflagellates

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