Temporal fluctuations in the trophic role of large benthic sulfur bacteria in mangrove sediment

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.