Abstract:
Thraustochytrids are particle-associated protists, ubiquitous in the marine environment. They are known to display a negative relationship with phytoplankton cells but a positive one with their exudates and detritus. Phytoplankton are known to express allelopathic effects against other organisms, but the exact mechanism by which phytoplankton cause inhibition to thraustochytrids is understudied. This is the first report of examination of bioactivity of lipid extracts of diatom cultures on thraustochytrids. Lipid extracts from four diatom cultures, viz. Cylindrotheca closterium (JB2), Skeletonema sp.1 (JB3), Thalassiosira sp. (JB4) and Skeletonema sp.2 (JB5), were tested against the growth of thraustochytrids: Oblongichytrium spp. (isolates VD4 and VD6), Parietichytrium sp. (isolate VD12) and Schizochytrium sp. (isolate VDC23b). Among the thraustochytrid isolates, Parietichytrium sp. was the most sensitive, whereas Schizochytrium sp. appeared to be the most resistant to the lipid extracts. The lipid extract from C. closterium possessed high amounts of C16:0, C16:1, C18:0, C18:1 and C20:5 fatty acids and bioactivity against the thraustochytrids. Lipids extracted during the late stationary phase of diatoms were inhibitory to thraustochytrid growth. Overall, these observations not only enhance our understanding of the diversity of allelopathic interactions between thraustochytrids and diatoms in the marine ecosystem, but also reveal the pivotal role of diatom lipids in such interactions across trophic levels.
