Metabolic stability of freshwater Nitzschia palea strains under silicon stress associated with triacylglycerol accumulation

Si is an essential nutrient for diatom growth. It is involved in frustule formation and is directly linked to the cell cycle. Despite numerous studies analyzing Si starvation in diatoms, the interaction between Si and central metabolism remains poorly understood. Accordingly, we analyzed the metabolite profiles of two freshwater strains of Nitzschia palea, BR006 and BR022 (a fast and a slow-growing strain, respectively), under different Si concentrations (0, 0.25, 1.25, and 5 mM) in different harvesting points. The strains exhibited similar metabolic responses under Si starvation, such as high pigment and total lipid, as well as high primary metabolite accumulation; only ornithine level was low in both strains in the Si-starved condition. However, the response of each strain differed under different Si concentrations. Triacylglycerol (TAG) accumulation in cells was inversely proportional to growth rate. Unlike BR006, slow-growing BR022 cells exhibited high TAG production even under high Si conditions. These results suggest that under optimal Si conditions, in the fast-growing BR006 strain, carbon from photosynthesis is directed toward new cell formation, while in the slow-growing BR022 cells, carbon is stored as TAG as a priority.