Éditeur(s) : HAL CCSD Oldendorf/Luhe
Résumé : International audience
Large filamentous sulfur-oxidizing bacteria belonging to the Beggiatoacae family can cover large portions of shallow marine sediments surrounding mangroves in Guadeloupe (French West Indies). In order to assess the importance of Beggiatoa mats as an infaunal food source, observations were conducted of the area within mats and at increasing distances from mats. We used natural isotopic compositions and a C-13 enrichment study. Both revealed an ingestion of bacterial mats by associated meiofauna, dominated by rotifers and to a smaller extent by small polychaetes and nematodes. Compared to adjacent sites, sediment covered by bacterial mats presented a higher abundance of diatoms, whereas the total biomass of bacteria did not vary. This constant bacterial abundance suggests that the proportion of organic matter represented by sulfur bacteria is limited compared to the fraction of total bacteria. There was no significant difference in infaunal abundance in mats, suggesting that the availability of this chemosynthetic food resource had a limited local effect. Grazers presented a delta C-13 value increasing with distance from the mat. However, isotopic composition of phospholipid-derived fatty acids specific for diatoms and bacteria revealed that this change is related to modifications of delta C-13 dietary components rather than to changes in diet composition. These complementary methods revealed that the occurrence of sulfur-oxidizing bacterial mats does not necessarily affect grazer abundance and importance of bacteria in their diet. Despite its wide occurrence, Beggiatoa mats would consequently have a minor influence on the structure of the mangrove food web.
ISSN: 0171-8630

hal-01545912
https://hal.archives-ouvertes.fr/hal-01545912
DOI : 10.3354/meps11035

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
Haplognathia ruberrima is a cosmopolitan gnathostomulid species found in sulfur bacterial mats in mangroves in Guadeloupe (French West Indies). Haplognathia ruberrima presents a d13C value lower than all measured meiofaunal grazers and lower than the available measured food sources of this environment. This low d13C value can not be due to specific ingestion of 13C-depleted methanogenic bacteria because abundances of those bacteria are reduced in surficial and deep sediments as revealed by d13C of bacterial fatty acid. According to scanning electron microscope observations, no bacterial ectosymbionts were observed at the surface of the gnathostomulids, and transmission electronmicroscope views revealed the absence of bacterial endosymbionts. Energy-dispersive X-ray spectroscopy analysis detected low levels of sulfur (0.32%+-0.8) in biological tissues of H. ruberrima, confirming the absence of thioautotrophic bacterial symbionts in these animals. Consequently, the low d13C value of H. ruberrima can not be due to the presence of sulfur-oxidizing symbionts but more probably to the selective and exclusive consumption of free-living, sulfuroxidizing bacteria
ISSN: 0173-9565

hal-01253239
https://hal.archives-ouvertes.fr/hal-01253239
DOI : 10.1111/maec.12296

Éditeur(s) : HAL CCSD
Résumé : International audience
Filamentous sulfur bacteria of the genus Beggiatoa form large mats covering the sediment in the shallow waters of a Guadeloupean mangrove (French West Indies). The abundance of these bacteria varies over the year and their trophic role may, therefore, also vary. We investigated this variation by conducting a survey examining the stable isotopic compositions of four grazers and four food sources during nine sampling sessions in three different periods of the year. We analyzed bulk isotopic compositions for each component except for the bacterial and diatom communities, for which we carried out a compound-specific 13C analysis of phospholipid-derived fatty acids (PLFAs). Correlations between isotopic compositions revealed a predominance of diatoms in the diet of nematodes and the important role of detritus and bacteria in the diet of the polychaete Ceratocephale sp. None of the grazers had an isotopic composition correlated with that of Beggiatoa suggesting that sulfur bacteria were not a predominant part of the diet of any grazer. Beggiatoa has a large central vacuole, resulting in a very low carbon content-to-volume ratio, potentially accounting for its low level of attractiveness to grazers. Mangrove sediments are habitats rich in organic carbon, in which, the addition of a food source, such as Beggiatoa, would have a limited effect on the structure of the food web over the course of the year.
Food Webs

hal-01545906
https://hal.archives-ouvertes.fr/hal-01545906
DOI : 10.1016/j.fooweb.2016.04.001