Phorbin steryl ester formation by macrozooplankton in the Sargasso Sea

Plankton sampling during a recent cruise in the Sargasso Sea provided numerous gelatinous macrozooplankton (salps) whose large gut contents afforded an opportunity to investigate the formation of phorbin steryl esters (PSEs) during zooplankton grazing. Sufficient material was obtained from about 50 salps to allow detailed determination of distributions and carbon stable isotope compositions of free sterols and PSE-sterols. Phorbin steryl esters were abundant in the gut contents of the salps but were absent from a large volume (8000 1) sample of suspended particulate matter (SPM), the presumed dietary material of the salps. Free sterols in salp gut contents and in SPM had similar isotopic ratios suggesting that indeed the sterols in SPM might be a major source of sterols in the gut contents, but PSE-sterols were enriched in 13C by 1–2‰ compared to free sterols, indicating a trophic fractionation during PSE formation. These results further strengthen the role of zooplankton grazing on phytoplankton in PSE production.