The sterol compositions of 14 species of marine diatoms were determined by gas chromatography and gas chromatography-mass spectrometry. A variety of sterol profiles were found. The sterols 24-methylcholesta-5,22E-dien-3 beta-ol, cholest-5-en-3 beta-ol, and 24-methylcholesta-5,24(28)-dien-3 beta-ol, previously described as the most common sterols found in diatoms, were major sterols in only a few of the species. In light of this and other recent data, it is clear that these three sterols are not typical constituents of many diatom species. Most of the centric species examined had 24-methylcholesta-5,24(28)-dien-3 beta-ol and 24-methylcholest-5-en-beta-ol as two of their major sterols. The exception was Rhizosolenia setigera, which possessed cholesta-5,24-dien-3 beta-ol as its single major sterol. In contrast to the centric species, the pennate diatoms examined did not have any particular sterols common to most species. Minor levels of Delta(7)-sterols, rarely found in large amounts in diatoms, were found in four species. C-29 sterols were found in many species; seven contained 24-ethylcholest-5-en-3 beta-ol and three contained 24-ethylcholesta-5,22E-dien-3 beta-ol, reinforcing previous suggestions that C-29 sterols are not restricted to higher plants and macroalgae. 24-Ethylcholesta-5,22-E-dien-3 beta-ol may prove to be useful for taxonomy of the genus Amphora and the order Thalassiophysales. A major sterol of Fragilaria pinnata was the uncommon algal sterol 23,24-dimethylcholesta-5,22E-dien-3 beta-ol. Cholesta-5,24-dien-3 beta-ol was the only sterol found in the culture of Nitzschia closterium. This differed from previous reports of 24-methylcholesta-5,22E-dien-3 beta-ol as the single major sterol in N. closterium. Two C-28 sterols possessing an unusual side chain were found in Thalassionema nitzschioides, a C-28:2 sterol (16%) and a C-28:1 sterol in lower abundance (2.5%), which may be 23-methylcholesta-5,22E-dien-3 beta-ol and 23-methyl-5 alpha-cholest-22E-en-3 beta-ol, respectively. The species Cylindrotheca fusiformis, T. nitzschioides, and Skeletonema sp. may be useful as direct sources of cholesterol in mariculture feeds due to their moderate to high content of this sterol.
