Differences in the sterol composition of dominant antarctic zooplankton

Abstract

The composition of free sterols was determined in Antarctic zooplankton species with various feeding behaviors. In the Southern Ocean, the dominant calanoid copepods Calanoides acutus, Calanus propinquus, Metridia gerlachei, and Euchaeta antarctica were investigated during different seasons and compared with the euphausiids Euphausia superba, E. crystallorophias, and Thysanoessa macrura. In addition, the Arctic copepods Calanus hyperboreus, C. glacialis, and C. finmarchicus were studied for comparison. Analyses were performed using gas chromatography and mass spectrometry. The zooplankton species exhibited a simple sterol content of up to six sterols. In the copepods, cholest-5-en-3beta-ol (22.1 to 60.5%, range of sample means), cholesta-5,24-dien-3beta-ol (22.3 to 45.2%), and cholesta-5,22E-dien-3beta-ol (4.3 to 33.4%) contributed most, while in euphausiids the sterol composition was less complex with cholest-5-en-3beta-ol always accounting for more than 75% of the total. Although sterols are membrane constituents and are expected not to vary considerably, differences in the abundance of sterols were observed between the species and the seasons. In herbivorous copepods, cholesta-5,24-dien-3beta-ol increased by a factor of 1.5 to about 45% during the main feeding period in summer; this sterol is a metabolic precursor of cholest-5-en-3beta-ol in the process of the dealkylation of dietary C-24 alkylated phytosterols. Cholest-5-en-3beta-ol decreased by the same proportion. Omnivorous and carnivorous copepods showed average levels of cholesta-5,24-dien-3beta-ol below 25%. These changes in sterol composition between copepod species seem to reflect their different feeding modes.

Influence of food web structure on the biochemical composition of seston, zooplankton and recently deposited sediment in experimental freshwater mesocosms

“…C 27 Diunsaturated sterols, particularly cholesta-5,22-dienol, were found in high relative abundance in zooplankton from fish and fishless enclosures. Cholesta-5,22-dienol has already been reported as an important constituent of the sterol fraction of zooplankton (Prahl et al 1984(Prahl et al , 1985Harvey et al 1987;Mühlebach et al 1999). This sterol was not observed in seston from both fish and fishless enclosures.…”

“…Surprisingly, cholesta-5,24-dienol was either present in low proportion (in fishless enclosures) or absent (in fish enclosures), while it has been reported as the second most important sterol in many crustaceans (Prahl et al 1984(Prahl et al , 1985Harvey et al 1987;Mühlebach et al 1999). It is well established that crustaceans have no capacity for de novo sterol biosynthesis (Goad 1981).…”

“…Another possible explanation could be that the conversion process yielding to cholesterol takes place early after ingestion of phytosterols, so that the concentrations of the intermediates involved in the biochemical transformation remain low. This scenario could occur either if phytoplanktonic food is scarce (Serrazanetti et al 1994;Mühlebach et al 1999) or if the majority of the available phytosterols are not suitable precursors for the biosynthesis of cholesterol (Prahl et al 1984;Klein Breteler et al 1999;Martin-Creuzburg and Von Elert 2004). Of course, these two possibilities are not exclusive.…”

Lipid composition of tropical and subtropical copepod species of the genus Rhincalanus (Copepoda: Eucalanidae): a novel fatty acid and alcohol signature

“…Sterol profiles are not well studied in copepods, but one major difference did occur between profiles from our study and previous work. Desmosterol (cholesta-5, 24-dien-3β-ol) was notably absent in the copepods in the present study, a compound which was often the second most abundant sterol found in zooplankton (Harvey et al 1987, Serrazanetti et al 1992, Mühlebach et al 1999. Desmosterol is usually thought to occur because it is an intermediate in the conversion of dietary phytosterols to cholesterol (Goad 1978).…”

Sterol composition of steryl chlorin esters (SCEs) formed through grazing of algae by freshwater crustaceans: relevance to the composition of sedimentary SCEs

“…Most species within a class (Crustacea) or even within a phylum (Arthropoda) are considered to display similar biochemical capabilities for sterol metabolism (Goad 1981;Rees 1989). Mühlebach et al (1999) examined the sterol content in the guts of Antarctic copepods and euphausiids and found that the cholesterol content in the guts of the calanoid copepods was 22.1-60.5% and that of euphausiids was more than 75% of the total sterols, although no indication of the dietary phytoplankton was given. Thus, if sterols in the gut are incorporated in SCEs, cholesterol should form a significant portion of sterols in SCEs, although the contribution of sterol esters was estimated to be less than 1% of total sterols in the guts of copepods (Mühlebach et al 1999).…”