The effect of selenium on the lipids of two unicellular marine algae

Abstract

The incorporation of selenium into the lipids of two unicellular marine algae has been investigated. Axenic cultures of the green algae Dunaliella primolecta and the red algae Porphyridium crueutum were grown in the presence of sublethal quantities of selenium (10ppm) as selenite. Both algae were found to contain selenium bound to all purified lipids, except for saturated hydrocarbons. Of the lipids which contain selenium, carotenoid pigments show the greatest selenium concentration ((beta)-carotene: 1.3(mu)gSe/mg lipid; zeaxanthin: 1.1 (mu)gSe/mg lipid) in both algae. P. cruentum contains about ten times as much lipid-associated selenium as D. primolecta, even though the lipids of both algae were very similar. This selenium has been shown to be incorporated non-metabolically into the lipid molecule. The lipid-associated selenium is probably non-covalently bound to the lipid molecule and may interact with double bonds. Selenite does not affect the lipid composition of D. primolecta, as compared with algae grown in the absence of added selenium. A selenium-induced 40% decrease in the cell content of eicosapentaenoic acid (20:5(omega)3) and 20% decrease in arachidonic acid (20:4(omega)6) in polar lipids (glycolipids plus phospholipids) was observed in P. cruentum. A 25% decrease in the chlorophyll (alpha) content of this red algae also occurred. The cell content of other fatty acids, phospholipids and glycolipids was unaltered by selenium. These results are consistent with a selenite-induced oxidation of P. cruentum lipids. The presence of glutathione peroxidase was not demonstrated in either algae. However, a non-enzymatic, glutathione-dependent activity capable of reducing H(,2)O(,2) and t-butyl hydroperoxide was present in both algae. This activity was attributable to endogenous algal antioxidants such as ascorbate. Selenite induces a doubling of this activity. This increase is probably due to the presence of selenium compounds which are also antioxidants. Selenium is therefore able to increase the antioxidant potential of algal cells. However, no in vivo selenium-induced protection of algal lipids from oxidation was apparent.