Bacterial diversity in deep Mediterranean sediments: relationship with the active bacterial fraction and substrate availability

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Abstract
We investigated vertical distribution and depth-related patterns (from 670 to 2,570 metres) of bacterial diversity in sediment samples collected along a transect in the warm deep Mediterranean sea. Analyses of bacterial diversity were compared with the abundance of benthic bacteria, their metabolically active fraction and the substrates potentially available for their growth. The number of active bacteria was dependent upon the availability of organic substrate in the sediment deriving from phytopigment inputs from the photic layer. The T-RFLP analysis revealed that the surface layers of all sediments analysed were dominated by the same ribotypes, but clear shifts in bacterial community structure were observed in deeper sediment layers. High values of bacterial diversity (expressed as D, H') and evenness (as J) were observed at all stations (a total of 61 ribotypes was identified), and as a result of the large fraction of rare ribotypes (c. 35%), the overall bacterial diversity in the deep sea region investigated was among the highest reported so far in literature. Biodiversity parameters did not display any relationship with water depth, but ribotype richness was related with the number and percentage of active bacteria, suggesting a coupling between organic inputs stimulating bacterial growth and deep-sea bacterial diversity.