Mathieu Poulicek

Methods for removing epiphytes from seagrasses: SEM observations on treated leaves

Abstract Several techniques commonly used to remove epiphytes from seagrasses were tested on the same Posidonia oceanica (L.) Delile material: shaking, scraping, sonicating and soaking in acid. The effects of each treatment on the epiphyte removal rate and on the seagrass itself were assessed by scanning electron microscopy. The advantages and disadvantages of each technique are discussed. It appears that the combination of an acid treatment with moderate scraping provides the best results as nearly all epiphytes are removed without seriously damaging the leaf substratum.

Food sources of two detritivore amphipods associated with the seagrass Posidonia oceanica leaf litter

Abstract This study focused on the ingestion and assimilation of Posidonia oceanica (L.) Delile litter by Gammarella fucicola Leach and Gammarus aequicauda Martynov, two dominant detritivore amphipods of the P. oceanica leaf litter. Scanning electron microscope observations indicated that leaf litter is highly colonized by diverse diatoms, bacteria and fungi, which may constitute a potential food source for the litter fauna. Gut content observations demonstrated that these species eat P. oceanica litter, and that this item is an important part of their ingested diet. Stable isotope analyses showed that the species do not experience the same gains from the ingested Posidonia. Gammarella fucicola displayed isotopic values, suggesting a major contribution of algal material (micro- and macro-epiphytes or drift macro-algae). On the other hand, the observed isotopic values of G. aequicauda indicated a more important contribution of P. oceanica carbon. The mixing model used agreed with this view, with a mean contribution of P. oceanica to approximately 50% (range 40–55%) of the assimilated biomass of G. aequicauda. This demonstrated that the two species, suspected to be detritus feeders, display in reality relatively different diets, showing that a certain degree of trophic diversity may exist among the detritivore community of the seagrass litter.

Chitin biodegradation in marine environments: An experimental approach

Abstract Chitin biomasses and production in marine environments are quite high. Planktonic biocenoses are the main producers and one should expect that sediments, mainlyorganoclastic 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 hydrolytic activities optimizes the recycling of most dietritic compounds including nearly 90% of the chitin produced. At sediment-water interface, the remaining material appears to be pulverized 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 t2 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).

Stable isotopic composition of chitin from arthropods recovered in archaeological contexts as palaeoenvironmental indicators

Abstract The effects of biodegradation and heating on the stable carbon, nitrogen, hydrogen and oxygen isotope ratios of chitin in arthropods were studied. Chitinous exoskeletons from seven aquatic arthropod species were subjected to anaerobic marine biodegradation in mud, to terrestrial biodegradation in soils, and to thermal degradation under anaerobic and aerobic conditions. The isotope ratios of chromatographically separated D-glucosamine hydrochloride and derivatives from treated and untreated specimens were then compared. Carbon, nitrogen, oxygen and hydrogen stable isotope ratios were all found to be conserved during partial degradation of chitin. Micromorphological comparative studies using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicated that no fungal chitin or other contaminants were present in the chitins that were chemically isolated from biodegraded substrates. Our results indicate that it will be possible to use stable isotope ratios of archaeological chitin samples for environmental and climatic reconstructions. An illustration of the utility of this approach comes from the observation that the stable isotope ratios of chitin from crustacean exoskeletons recovered from archaeological sites with ages up to 1400 years bp are in good agreement with measurements on modern crustaceans from similar environments.

Comparison of anaerobic and aerobic biodegradation of mineralized skeletal structures in marine and estuarine conditions

The knowledge of the biodegradation rates is essential to studies of the biogeochemistry and ecology of aquatic systems. It helps us to quantify the production and uptake rates of chemical components and their recycling, and to understand the mechanisms and rates of organic matter accumulation in sediments. Experimental studies of biodegradation processes in six types of mineralized skeletons were performed in shallow-marine waters of Calvi Bay, Corsica and in estuarine waters of Roscoff, Brittany. Three types of mollusk shells, sea urchin skeletal plates, crab cuticle and fish vertebrae were exposed to oxic and anoxic conditions over periods of 15 days to 30 months. After recovery of the substrates, protein assays, bacterial counts and organic carbon analyses were performed.Quantitative protein assays and bacterial counts indicate that biodegradation of mineralized skeletal structures occurs at a slower rate in anoxic conditions than in oxic conditions. Bacterial analysis showed that in anoxic environment, less than 0.5% of the consumed organic matter is converted into bacterial biomass. The aerobic biodegradation rate was positively correlated with the organic content of the skeletons.Anoxic biodegradation of skeletons occurred at much slower rates in estuarine sediments than in shallow marine sediments. Preservation of skeletal structures in estuarine conditions appears to be correlated with the abundance of dissolved organic matter rather than with high sedimentation rates.

Chitin in gastropod operculi

Abstract The operculi of 114 species of gastropods representing 38 families with wide ranging morphological, ecological and geographical differences have been examined for the presence of chitin using a quantitative enzymatic method along with qualitative tests. All the species having calcified operculi so far analysed (28 species) were relatively rich in chitin and neighbouring species had a similar composition. No chitin was found in the non-calcified, corneous operculi (86 species). The level of calcification was the unique character found to be correlated to the presence of chitin in the operculum; no other morphological parameters showed any reciprocity. Within the calcified operculi, chitin is restricted to the mineralized matrix. The tanned corneous sheet seems completely devoid of chitin. The possibility of a direct relation between calcification processes and the presence of chithin in the mineralizing matrix of some molluscan hard structures is discussed.

Morphology of the buccal apparatus and related structures in four Carapidae

The aims of this study were (1) to compare the morphology of the buccal apparatus, the suspensorium and the opercle in four species of Carapidae (Carapus acus, Encheliophis boraborensis, Encheliophis homei and Encheliophis gracilis) and (2) to investigate the relationships between their cranial anatomy, their carnivorous diet, and their well known ability to enter holothurians. The complex and strong dentition and the wide hyomandibular with thickenings that seem to suit the constraints of the adductor mandibulae muscles partly inserted on the neurocranium are signs of a carnivorous diet. C. acus, E. boraborensis and E. homei have extremely strong buccal pieces and can protrude their upper jaws. However, in E. gracilis, the jaws are more slender, and the insertions of the A1 along the entire length of the maxillary associated with the lack of mobility between the maxillary and the premaxillary prevent buccal protrusion. These differences could be related to the diet: C. acus, E. boraborensis and E. homei can feed on fishes and crustaceans, whereas E. gracilis feeds only on holothurian tissue. The cephalic morphology of the four species is not incompatible with entering the host. However, the neutralisation of the suboperculum spine by ecartilaginousi tissue could be considered to be a particular adaptation to this behaviour.

Chitin Degradation in Natural Environment (Mollusk Shells and Crab Carapaces)

Most marine sediments are organodetritic in nature, and many particles of the coarser fraction are constituted of more or less stable remains of biological origin (skeletal pieces of benthic or planctonic animals, plant remains,…)(1). In this respect, dead crustacean cuticles and mollusk shells introduce into the sediments the chitinoproteic compounds of their matrix, closely associated to calcified crystallites.

Chitin Production by Animals and Natural Communities in Marine Environment

Since the discovery of the remarkable properties of chitin and of its derivative, chitosan, offering a wide range of possible industrial and agricultural applications 1,2,3, the need of new and trusting sources of chitin for its isolation on an industrial scale was claimed.