Reconstructing historical patterns in bacterial communities from deep-sea sediments in the western Mediterranean: impacts of metal pollution and climatic variation.

Bacteria are a major component of deep-sea communities. Mesoscale changes were observed in the deep western Mediterranean by analysing deposited DNA in four sediment cores from SE Barcelona, at 1050 m (Bcn1050), and from insular open-sea areas of NW Mallorca (at 420 m and 1114 m) and a seamount summit at 1156 m. The dominant taxa were Gammaproteobacteria (e.g. Acinetobacter and Pseudomonas), Alphaproteobacteria, and Bacilli. A terrigenous-pelagic gradient explained spatial changes in bacterial composition and diversity, with Bcn1050 (near the mainland) showing lower species richness (S = 93.9) and diversity (H' = 3.8), and Isid1114, located in a coral (Isidella elongata) field, the highest (S = 181.7; H' = 4.3). A post-industrial increase in Hg and Pb in deep-sea sediments after the industrialisation (1940-1950) of Spain coincided with a decrease of evenness (J = 0.79-0.83) at Bcn1050, not at insular stations. After the 1960s a decrease in the abundance of dominant bacteria was positively correlated with Li and Al (terrigenous metals) and positive North Atlantic Oscillation (NAO) index values, confirming a drought period in the area. Among bacteria correlated with metals, only Staphylococcus exhibited higher relative abundance after the 1940s-1950s. Historical changes in deep-sea bacteria in the Balearic Basin over the last few centuries are caused both by natural and anthropogenic factors.