Use of lipids to study the trophic ecology of deep-sea xenophyophores

Xenophyophores, widespread giant deep-sea protists, dominate the megafaunal organisms visible on the seafloor in some areas. Owing to the extreme fragility of their tests (external part), little is known about them, and their diet is largely a matter of speculation. The extensive accumulation of fecal pellets (stercomes) within their tests attests to the degree to which xenophyophores feed on particulate matter. During the EUMELI 2 cruise, xenophyophores were recovered intact at the surface of box-cores from an abyssal mesotrophic site in the northeastern Atlantic. They were identified as Syringammina corbicula. In our study, we compare the lipid composition of xenophyophores collected at the top of cores with sediment from the same mesotrophic environment devoid of these protists. The distribution of lipid classes was obtained by the use of the latroscan (TLC-FID) technique, and the molecular composition of several lipid classes containing fatty acids and sterols was investigated. Of special interest are the triacylglycerols of xenophyophores, energy storage lipids in animals, resulting in part from fatty acids found in their diet. They contained a higher level of bacterial fatty acids than those in the reference sediment. It would seem that, in addition to particle trapping, xenophyophores also feed on microbes. Our study points to the occurrence of a specialized bacterial flora, probably growing on the stercomes, which might serve as a secondary food source different from the microflora found in the control sediment.