Phylogenetic characterisation of bacterial assemblages and the role of sulphur-cycle bacteria in an Arenicola marina bioturbated mesocosm

A culture-independent analysis of the bacterial assemblages in the sediments of a lugworm Arenicola marina L. bioturbated mesocosm was carried out using a molecular RNA-gene based approach. Clone libraries were constructed for 4 spatial locations in the mesocosm: deep yeast layer (DYL), mid sand (MS), new casts (NC) and old casts (OC) after 29 d of bioturbation. In total, 495 clones were analysed and grouped into 159 unique phylotypes. All libraries were dominated by Epsilonproteobacteria, with Alpha-, Delta-and Gammaproteobacteria, Verrucomicrobia and members of the Cytophaga-Flavobacteria-Bacteroides group also prevalent. Phyologenetic comparison of the sites with respect to evolutionary distance showed that the libraries fell into 2 groups: (1) DYL and MS and (2) NC and OC. There were significant differences in species composition between but not within the 2 groupings. Bacteria involved with sulphur cycling are most likely to be affected by lugworm bioturbation. An Epsilonproteobacterium phylotype with >95% 16S similarity to Sulfurimonas denitrificans was abundant in the DYL sand layer but was not present in the egested sand (NC, OC). Promotion of the growth of sulphide-oxidising bacteria and their subsequent digestion would be consistent with the 'gardening' hypothesis and suggests that chemoautotrophic processes in shallow-water anaerobic sediments may be enhanced by the activities of the lugworm.