Bacterial symbionts in East African mangrove crabs

Mangrove communities are highly productive ecosystems releasing large amounts of organic matter to adjacent coastal waters. Organisms in the mangrove ecosystems have to cope with different stresses like high salinity, periodic tidal flows, anoxic niches in soil and fluctuating thermal regimes. Moreover, mangrove soil and sediments present high contents of lignin-derived tannins and polyphenols that plants produce to preserve the tissues from microbial pathogens. Despite the harsh conditions a large animal biomass characterizes the mangrove ecosystem mainly represented by the true burrowing crabs sesarmids and ocipodids (Crustacea, Brachyura) that play an important role in the ecosystem functioning as ecosystem engineers (Kristensen and Alongi 2006). How the mangrove crabs cope with the harsh conditions of the ecosystem, especially how they manage the high C/N ratio of their diet are intriguing questions. It has been hypothesized that mangrove crabs can take advantage of an associated symbiotic microbiome (Thongtham and Kristensen 2005) that may complement nutritional deficiencies like occurs in other arthorpods (Moran and Dale 2006). The aim of this study was to evaluate the diversity of microbial symbionts associated to two representative mangrove crab species, Uca urvillei (ocipodids) and Perisesarma guttatum (sesarmids), collected from two different populations in a Kenyan and in a South African mangrove forests. The bacterial community associated to different crab organs and tissues such as gill, hindgut, gonads and eggs was characterized by sequencing PCR-amplified 16S rRNA genes following separation by denaturing gradient gel electrophoresis. Rather highly diverse bacterial communities have been associated to all the organs examined. A range of novel taxa showing low sequence identity with previously identified bacteria have been found associated to all crabs organs. rRNA gene sequences related to sulfur- oxidizing bacteria have been identified in the gills suggesting the chemolithoautotrophy may play a role in the maintenance of the crab metacommunity. In hindgut tissues sequences related to nitrifying and denitrifying bacteria were found supporting the hypothesis of a complex nitrogen cycle occurring in the gut.