[en] Hydrothermal vent environments, particularly those associated with the vestimentiferan Riftia pachyptila, are believed to be among the highest chitin-producing systems. In order to elucidate the chitin cycle in these environments, we estimate the in situ chitin degradation rates of tube-worm exoskeletons. Our in situ experiments show that the tubes of Riftia are highly stable structures. Comparative measurements of the degradation rates of Riftia tubes and crab shells immersed at deep-sea vents show that the tubes would be degraded within 2.5 years, whereas the time for the total degradation of the vent crab (Bythograea thermydron) carapaces would not exceed 36 days. The importance of the microbial participation in this degradation was estimated for Riftia tubes. Based on previous work, we calculated chitin production by a population of Riftia tubes of about 750 g m(-2) year(-1) (763). From our in situ experiments, we estimated a microbial chitinolysis rate of about 500 g m(-2) year(-1) (496) (65% of the chitin produced). Exoskeletons containing beta-chitin appear more stable in natural environments than those containing alpha-chitin and would thus be less available as carbon and nitrogen sources. In contrast, isolated beta-chitin was hydrolysed faster than alpha-chitin during in vitro degradation experiments; for instance, Riftia beta-chitin was degraded about 3- to 4-fold faster than Bythograea alpha-chitin. A stabilization process by disulfide bonds of the proteins-chitin link, rather than the crystalline form of the chitin (alpha/beta), accounts for the resistance of Riftia tubes to enzymatic attacks.