Metabolomic analysis of symbiosis between chemoautotrophic bacteria and marine invertebrates

Olavius algarvensis is an oligochaete characterized by the absence of digestive and excretory systems. It harbors a thick layer of extracellular symbionts below its cuticle. It is assumed that the worm gains most of its nutrition though those bacteria. The study of its metabolomic pro le re- vealed another of its characteristics. Olavius algarvensis has -sitosterol as main sterol which is unusual for an animal , most of them having cholesterol as main sterol. This nding arises the question of the origin of this phytosterol in the gutless worm. Due to its lack of digestive sys- tem and to the absence of sterols in bacteria, Olavius algarvensis can not gain phytosterol through its diet. Furthermore only plants are able of -sitosterol synthesis. It was hypothezised that Olavius algarvensis takes up -sitosterol from its environment.Olavius algarvensis lives in the vicin- ity of the seagrassPosidonia oceanica which happens to have -sitosterol as main sterol. It it not the only metabolites that can be nd in both systems, sucrose and myo-inositol are sugars found in high abundance in both organisms. To test this hypothesis, the sterols content of Olavius al- garvensis habitat was investigated. Sterols were extracted from sea water, pore water and sediment sampled along a transect going from a seagrass meadow to plain sand, away from direct plant in uence. Two sterols were detected in Olavius algarvensis environement, cholesterol and -sitosterol. The highest concentration were measured inside the seagrass meadows. Pore water was richer in sterols than the overlaying seawater by a factor 1000. Concentration in the pore water were ranging from 1 to 17 M. The next question to arise was is Olavius algarvensis able to take up sterol from its environement. To answer this question, worms were incubated with di erent sources of sterol for up to 9 weeks, time points being collected every 2 weeks. The worms were incubated with pure cholesterol, labeled deuterium -sitosterol-d7 or seagrass leaves. Their metablomic pro les were then analysed. The worms did not take up the labeled -sitosterol- d7 in detectable amounts. The ratio between cholesterol and -sitosterol was analyzed as well. Some trends were visible but should be interpreted with caution due the high variability within an incubation and the rela- tively low replication (n=3). Metabolomic analyis of gutless oligochaetes and seagrasses from Belize revealed that Olavius algarvensis from Elba are not the only gutless oligochaetes having -sitosterol as main sterol. Literature research revealed that other organisms as phyto-parasitic ne- matodes and some insects also have a phytosterol as main sterol. This suggest that this observation is more widespread and therefore of possible importance