Microzooplankton grazers induce chain length plasticity in colonial diatoms

Many diatoms form long chains and the distribution of chain lengths within a species depends on several environmental factors, among them grazing risk. Larger grazers, such as copepods, efficiently handle and ingest even very long chains but are less efficient with individual cells, whereas most smaller grazers are unable to feed on chains exceeding a few cells in length. Copepod cues make several species shorten their chain length, and theory predicts that cues from small grazers should induce increased, but this remains to be tested, despite the importance of diatoms in marine food webs. Here, we expose three species of chain-forming diatoms, Skeletonema marinoi, Chaetoceros affinis, and Thalassiosira rotula to cues from various actively feeding micrograzers and record their response. The effect of grazer presence on chain length varies depending on both the type of grazer and the diatom species. For example, S. marinoi increased its chain length when exposed to grazing cues from the heterotrophic dinoflagellate Gyrodinium dominans and the ciliate Euplotes sp. but not to a copepod nauplii (Temora longicornis). C. affinis also responded to cues from grazing G. dominans by increasing chain length, and the response increased with exposure time. Finally, T. rotula did not respond to grazing cues from G. dominans, but rather inhibited the dinoflagellates' ability to feed, presumably through the release of chemical compounds. Our results suggest that some chainforming diatoms can sense and appropriately respond to fast-growing micrograzers, thus contributing to the success of diatoms in marine environments.