Microbial Transformation of Fecal Pellet Lipids During Sedimentation

Abstract Changes in the composition and distribution of fecal pellet lipids during sedimentation were investigated in laboratory experiments. Fecal pellets were collected from the marine copepod Calanus helgolandicus fed on the dinoflagellate Scrippsiella trochoidea , split into fractions, and treated in one of three ways (antibiotics, poisoned or untreated controls). Pellets were incubated in the dark at 10°C using revolving cylinders to allow continual sinking, and incubated for up to 16 days. Bacterial numbers were monitored in fecal pellets and surrounding water by epifluorescence microscopy and fecal pellet lipids were extracted and analyzed by capillary gas chromatography and gas chromatography-mass spectrometry. Significant differences in lipid composition and distribution were observed among all three treatments compared with freshly collected pellets, with the largest changes occurring in untreated pellets. Fatty acids showed the greatest shift, with substantial variations in odd and branched chain fatty acids. In pellets “aged” for five days, small amounts of hop-17(20)-ene were present which increased in concentration among pellets aged for 16 days. The distribution and concentration of algal sterols in fecal pellets showed only slight changes during the incubation period for all three treatments, suggesting that sterol distributions in fecal pellets are not substantially altered by microbial activity in upper oceanic waters over relatively short (16 days) time periods.