Chitin biodegradation in marine environments : an experimental approach

Chitin biomasses and production in marine environments are quite high. Planktonic biocenoses arte the main producers and one should expect that sediments, mainly organoclastic ones, will constitute some kind of reserve compartment for the biogeochemical cycle of this polymer. In fact, this is not the case. The low chitin biomass in most marine sediments can only be explained if chitin is weathered at the same rate as it is produced. In order to test this hypothesis, we developed an experimental approach to chitin biodegradation in marine environments. In open water conditions, zooplanktonic remains are first degraded by autolytic processes making most organic compounds readily susceptible for further hydrolysis by extrinsic decomposers. Different populations (with high densities and various hydrolytic potentials) follow each other. The sequence of hydrolyc activities optimizes the recycling of most detritic compounds including nearly 90% of the chitin produced. At sediment-water interface, the remaining material appears to be pulvirized and incorporated into the aerobic sedimentary layers while the decomposer community changes once again. Sediment chitinoclasts are opportunistic and densities react quickly to chitin input. In sediments, oxic and anoxic, chitin appears essentially present in the form of chitinoproteic matrices inside mineralized skeletons. A rich population of microborers develops on these matrices by secreting extracellular hydrolases. Densities of microborers of 250-450 * 10 3 cm-2 are currently encountered. Anaerobic decomposers are more adapted to refractory compounds than aerobic ones. This leads to a nearly complete mineralization of the chitinoproteic matrices embedded in the biotic sedimentary layers (more than 90% of the chitin weathered within less than two years).