Investigation of Microbial Communities in Deep-sea Polymetallic Nodules from the Pacific Ocean and Indian Ocean

Deep-sea polymetallic nodules are valuable for their high abundance of metals, such as manganese, nickel, cobalt and copper. Microorganisms were known involved in the formation of nodules. Yet it is poorly understood what kinds of microorganisms are involved in the geochemical formation of nodules. To better understand the relationships between metals and microbial communities in deep-sea polymetallic nodules and surrounding sediments, we investigated samples of 20 deep-sea polymetallic nodules and 9 surrounding sediments from the Pacific Ocean and Indian Ocean. The results of metal composition showed that manganese, cobalt, copper, and nickel are much more abundant in the nodules than those in the surrounding sediments. Some metals have significant high correlation, such as 0.96 (p-value<0.001) between copper and nickel. Relative abundance of taxa in the microbial communities was investigated by using culture-independent high-throughput sequencing, followed by classification and statistic analysis. Principal component analysis showed the microbial communities in the nodules are significantly different from those in surrounding sediments. Correlation-based network analysis was performed, which helped us obtain a comprehensive understanding of relationships between metals and microorganisms. The order Rhizobiales, to which some Mn(II)-oxidizing microorganisms belong, shows high correlation (r>0.6, p-value<0.001) with manganese, nickel and copper. The order Solirubrobacterales shows high correlation (r>0.6, p-value<0.001) with manganese and cobalt. The results suggest that the microorganisms belonging to these two orders may play important roles in the geochemical formation of nodules. This study shed light on further studies concerning the formation of the deep-sea polymetallic nodules and isolation of the uncultured microorganisms involved in this geochemical process.