Biogeochemical Mn Cycle in the Deep-seabed Sedimentary Environments of Mariana Trench

The significance of microbial role in the formation of ferromanganese deposits, one of the most important metal sinks in the ocean, has a great influence on global ocean metal chemistry. Nevertheless, the mechanism about how microbes contribute to the Mn cycle for nodule formation in the deep ocean especially the trench seabed environments remains enigmatic. In this study, nodule and around sediments were examined by geochemical and microbial methods to reveal these biogeochemical processes at a site of the southern Marianna trench with a water depth of ~5,525 m. Bulk geochemistry indicated that trench nodules were generally formed under diagenetic conditions with typical high Mn/Fe ratios (6.88-39.86) and a high growth rate (83.9-4651.3 mm/Myr). High-resolution in situ analysis divided the microlayers of individual nodule into three types based on the different geochemical characteristics: Type I. its interior part with the diagenetic origin and extremely high Mn/Fe ratio (>20); Type II. laminae from the exterior part with hydrogenetic origin and lower Mn/Fe ratio (0.01-2.5); and Type III. laminae from the exterior part with the diagenetic origin and moderately high Mn/Fe ratio (2.5-15). The 16S rRNA gene sequences analysis showed that microbial communities of nodule bearing sediments were distinct from with reference sediments. The co-occurrence of potential Mn-reducing bacteria Shewanella and Colwellia might be the key factors that provide sufficient Mn(II) for nodule formation from the dispersed Mn oxides in trench sediments. Moreover, the metagenomic analysis discovered the occurrences of functional genes involved in Mn transport, regulatory, and manganese oxidation in the sediment samples, further suggesting an active Mn cycle in the trench seabed environments. Potential Mn-oxidizing bacteria with functional gene cotA encoding multicopper oxidases might play a significant role in the process of Mn(II) oxidation. By integrating geochemical and microbial data, we proposed a potential biogeochemical model of Mn cycle and nodule formation in the trench seabed environments.