Abstract
This study examined the effect that silica content in diatom cells has on the behavior of protists. The diatoms Thalassiosira weissflogii and T. pseudonana were cultured in high or low light conditions to achieve low and high silica contents, respectively. These cells were then fed to a heterotrophic dinoflagellate Noctiluca scintillans and a ciliate Euplotes sp. in single and mixed diet experiments. Our results showed that in general, N. scintillans and Euplotes sp. both preferentially ingested the diatoms with a low silica content rather than those with a high silica content. However, Euplotes sp. seemed to be less influenced by the silica content than was N. scintillans. In the latter case, the clearance and ingestion rate of the low silica diatoms were significantly higher, both in the short (6-h) and long (1-d) duration grazing experiments. Our results also showed that N. scintillans required more time to digest the high silica-containing cells. As the high silica diatoms are harder to digest, this might explain why N. scintillans exhibits a strong preference for the low silica prey. Thus, the presence of high silica diatoms might limit the ability of the dinoflagellate to feed. Our findings suggest that the silica content of diatoms affects their palatability and digestibility and, consequently, the grazing activity and selectivity of protozoan grazers.
Highlights
Diatoms are one of the most prevalent groups of phytoplankton in the world oceans
The diatom prey T. weissflogii and T. pseudonana grown in low light consistently contained significantly higher levels of biogenic silica than those grown in the high light throughout our study (Table 1)
A further comparison of the diatoms grown at high and low light intensity levels showed no significant difference in their equivalent spherical diameter (Table 1), but the cells grown under low light conditions yielded higher contents of cellular carbon (C), nitrogen (N) and phosphorus (P) (Table 2)
Summary
Diatoms are one of the most prevalent groups of phytoplankton in the world oceans. They are responsible for ∼40% of the oceanic primary productivity (Nelson et al, 1995; Smetacek, 1999), which makes them an important component of marine food webs and a major source of carbon export. The silica content of diatoms varies both within and among species, according to size (Paasche, 1973; Durbin, 1977; Waite et al, 1997) and growth phase (Brzezinski et al, 1990; Martin-Jézéquel et al, 2000; Hildebrand, 2006), as well as in response to environmental factors, such as light, temperature, salinity and nutrients (Bienfang et al, 1983; Martin-Jézéquel et al, 2000; Claquin et al, 2002; Vrieling et al, 2007; Shatwell et al, 2013). Other studies have shown that the dominant grazers of marine diatoms are microzooplankton (
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