Influence of iron on fatty acid and sterol composition of marine phytoplankton and copepod consumers

We conducted laboratory experiments to determine the influence of iron on the fatty acid ( FA) and sterol composition of the diatom Thalassiosira oceanica, the cryptophyte Rhodomonas salina, the prymnesiophyte Isochrysis galbana, and the copepod Acartia tonsa that fed on these algae. Algal cells were cultured with no added Fe or with Fe added at 100 nmol L(-1). Fe-deficient culture medium resulted in lower growth rates, cell volumes, and photosynthetic efficiencies for all algal species. The total mass of FA per cell was 2.2-3.1 times as high in Fe-replete cells as in Fe-deficient cells; on a carbon-normalized basis, the FA concentrations of Fe-replete algal cells were 1.4-1.5 times as high. Differences were noted between Fe treatments for specific saturated, monounsaturated, and polyunsaturated FAs within each algal species. Total C-normalized 20:5 omega 3 + 22:6 omega 3 concentrations declined 18%, 25%, and 35% in Fe-deficient T. oceanica, R. salina, and I. galbana, respectively. Across all species, another consistent difference in FA composition between Fe treatments was for 18:4 omega 3, which had a C-normalized concentration that was similar to twofold higher in Fe-replete algae. No consistent pattern for all algal species was evident for Fe effects on algal sterol concentration and composition, and in response to Fe limitation sterols were conserved more than C and FAs in these cells. Copepods feeding on Fe-replete and Fe-deficient algae displayed smaller differences in FA and sterol concentrations than in their algal diets. Differences in lipid concentration and composition between Fe-replete and Fe-deficient algae may partly explain the lower egg production rate of copepods fed Fe-deficient algae.