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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‐3β‐ol, cholest‐5‐en‐3β‐ol, and 24‐methylcholesta‐5,24(28)‐dien‐3β‐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‐3β‐ol and 24‐methylcholest‐5‐en‐3β‐ol as two of their major sterols. The exception was Rhizosolenia setigera, which possessed cholesta‐5,24‐dien‐3β‐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 ofΔ7‐sterols, rarely found in large amounts in diatoms, were found in four species. C29sterols were found in many species; seven contained 24‐ethylcholest‐5‐en‐3β‐ol and three contained 24‐ethylcholesta‐5,22E‐dien‐3β‐ol, reinforcing previous suggestions that C29 sterols are not restricted to higher plants and macroalgae. 24‐Ethylcholesta‐5,22E‐dien‐3β‐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‐3β‐ol. Cholesta‐5,24‐dien‐3β‐ol was the only sterol found in the culture of Nitzschia closterium. This differed from previous reports of 24‐methylcholesta‐5,22E‐dien‐3β‐ol as the single major sterol in N. closterium. Two C28 sterols possessing an unusual side chain were found in Thalassi‐onema nitzschioides, a C28:2 sterol (16%) and a C28:1 sterol in lower abundance (2.5%), which may be 23‐methylcholesta‐5,22E‐dien‐3β‐ol and 23‐methyl‐5α‐cholest‐22E‐en‐3β‐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.

The effects of irradiance on the biochemical composition of the prymnesiophyte microalga, Isochrysis sp. (Parke; clone T‐ISO), a popular species for mariculture, were examined. Cultures were grown under a 12:12 h light: dark (L:D) regime at five irradiances ranging from 50 to 1000 μE·m 2·s−1 and harvested at late‐logarithmic phase for analysis of biochemical composition. Gross composition varied aver the range of irradiances. The highest levels of protein were present in cells from cultures grown at 100 and 250 μE·m 3·s1, and minimum levels of carbohydrate and lipid occurred at 50 μE·m−2·s−1. Because the cell dry weight was reduced at lower irradiances, different trends were evident when results were expressed as percentage of dry weights. Protein percentages were highest at Wand 100 μE·m−2·s−1 and carbohydrate at 100 μE·m−2·s−1. The composition of amino acids did not differ over the range of irradiances. Glutamate and aspartate were always present in high proportions (9.0–13.5%); histidine. methionine, tryptophan, cystine, and hydroxy‐proline were minor constituents (0.0–2.6%). Glucose was the predominant sugar in all cultures, ranging from 23.0% (50 μE·m−2·s−1) to 45.0% (100 μE·m−2·s−1) of total polysaccharide. No correlation was found between the proportion of any of the sugars and irradiance. The proportions of the lipid class components and fatty acids showed little change with irradiance. The main fatty acids were 14:0, 16:0, 16:1(n‐7), 18:1(n‐9), 18:3(n‐3). 18:4(n‐3), 18:5(n‐3), and 22:6(n‐3). Proportions of 22: 6(n‐3) increased, whereas l8:3(n‐3). 18:3(n‐6). and 18:4(n‐3) decreased, with increasing irradiance. Pigment concentrations were highest in cultures grown at 50 μE·m−2·s−1, except for fucoxanthin and diadinoxanthin (100 μE·m−2·s−1). The concentrations of accessory pigments correlated with chlorophyll a, which decreased in concentration with increasing irradiance. On the basts of biochemical composition, an irradiance of 100 μE·m−1·s−1 (12:12 h L:D cycle)for the culture of Isochrysis sp. (clone T‐ISO) may provide optimal nutritional value for maricultured animals, although feeding trials are now necessary to substantiate this.

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