Effect of nutrient availability on the biochemical and elemental stoichiometry in the freshwater diatom Stephanodiscus minutulus (Bacillariophyceae)

The objective of this studs was to examine the differences im the biochemic al and elemental stoichiometry of a freshwater centric diatom, Stephanodisc us minutulus (Grun.), under various nutrient regimes. Stephanodiscus minutu lus was grown at mu(max) or 22% of mu(max) under limitation by silicon, nit rogen, or phosphorus. Cell sizes for nutrient-limited cultures were signifi cantly smaller than the non-limited cell sizes, with N-limited cells being significantly smaller than all other treatments. Compared with the nutrient -replete treatment, both carbohydrates and lipids increased in Si- and P-li mited cells, whereas carbohydrates increased but proteins decreased in N-li mited cells. All of the growth-limited cells showed an increase of carbohyd rate and triglyceride, and a decrease of cell size and polar lipids as a pe rcentage of total lipids. The non-limited cells also had a signifi cantly h igher chl a concentration and,galactolipids as a percentage of total lipids than any of the limited treatments, and the low-Si and low-P cells had sig nificantly higher values than the low-N cells. The particulate C concentrat ions showed significant differences behtween treatments, with the Si- and P -limited treatments being significantly higher than the N- and non-limited treatments. Particulate Si did not show a strong- relationship with any of the parameters measured, and it was the only parameter with no differences between treatments. The low-Si cells had a significantly higher P content ( about two times more) than any other treatment, presumably owing to the lux ury consumption of P, and a correspondingly high phospholipid concentration . The elemental data showed that S. minutulus had a high P demand with low optimum N:P (4) and Si:P (10) ratios and a C:N:P ratio of 109:16:2.3. The p articulate C showed a positive relationship with POM (r = 0.93), dry weight (r = 0.88), lipid (r = 0.87) and protein (r = 0.84, all P < 0.0001). Parti culate N showed a positive relationship with galactolipids (r = 0.95), prot ein (r = 0.90), dry weight (r = 0.78), lipid (r = 0.75), and cell volume (r = 0.64, all P < 0.0001). It is evident that nutrient limitation in the fre shwater diatom S. minitulus has pronounced effects on its biochemical and e lemental stoichiometry.