Distribution and stability of steryl chlorin esters in copepod faecal pellets from diatom grazing

Steryl esters of the chlorophyll a transformation product pyrophaeophorbide a (SCEs), were formed when the copepod Calanus helgolandicus was allowed to graze on the marine diatom Thalassiosira weissflogii, confirming an earlier preliminary experiment. All of the animal and algal sterol components were found in the SCEs although there were distributional changes attributed to alteration prior to esterification to pyrophaeophorbide a, which resulted from utilisation by the copepod to produce cholesterol. This indicates that where there is insufficient cholesterol in the zooplankton diet in the natural environment, sedimentary SCE sterol distributions may not accurately reflect the original phytoplanktonic free sterol distribution. Although the concentration of SCEs dropped markedly as a result of pellet ageing, their abundance relative to the other major chlorins increased. This relative stability provides an explanation for a previous observation that SCEs in surface sediments are present in higher abundance than in the corresponding water column particulates and accounts for their significant abundance in the solvent extractable chlorins in sediments. Two novel products, tentatively proposed as the phytyl ester and the 24-methylcholesta-5,24(28)-dien-3β-yl ester of 132-oxopyrophaeophorbide a were also observed in the pellets, although the widely occurring rearranged sedimentary chlorin 132-hydroxychlorophyllone a (chlorophyllone) was only detected in aged pellets.