The global seabed is rich in polymetallic nodule resources. Because they are rich in a variety of strategic metals, they are considered to be the type of seafloor deposits with the greatest potential for development today. These resources are mainly found in the abyssal plains at water depths of 4,000 to 6,000 meters, generally far from land and with very low productivity. Over the past half century, scientific organizations and teams from many countries and regions have carried out impact surveys and experimental studies to monitor and assess the impact and recovery of benthic organisms, especially macrobenthic organisms, in response to the environmental damage that may be caused by deep-sea mining. In contrast, microorganisms inhabiting the environment of metal nodule deposits are challenged by extreme environmental conditions, such as heavy metals, oligotrophy, high pressure and low temperature, and few studies have been conducted on the mechanisms of microbial adaptation to the environment of metal nodule deposits, as well as on their diversity and metabolic capacity.
The environmental impact of deep-sea mining is a topic of great concern. Currently, the International Seabed Authority (ISA) is actively promoting Regional Environmental Management Plans (REMPs). The first REMPs area is the Clarion-Clipperton Fracture Zone area (CC Zone) in the eastern Pacific Ocean, which aims to protect biodiversity and ecosystem functions in the target area for deep-sea nodule mining in the Pacific Ocean. The Institute of Oceanography of the Chinese Academy of Sciences (IOCS), in collaboration with the Second Institute of Oceanography of the Ministry of Natural Resources (MNR) and Huazhong Agricultural University (HUAU), systematically studied the metabolism capacity of microorganisms in manganese nodule sediments in the CCZ. Recently, the relevant research results were published in Microbiome. The study reconstructed 179 high-quality genomes (MAGs) and categorized them into 21 bacterial phyla and one archaeal phylum by deep macrogenome sequencing of manganese nodule sediment samples. The study resolved the functional genes of the MAGs and presented evidence for the role of different microorganisms in the metal, carbon, nitrogen and sulfur cycles. The study can provide important scientific support for the International Seabed Authority's regional environmental management plan and national polymetallic nodule resource development and environmental remediation.
The study shows that heterotrophic and chemoenergetic autotrophic microorganisms have evolved mechanisms of resistance to heavy metals in these metal-rich sedimentary environments, mainly through enzyme-catalyzed metal redox (manganese, chromium, and mercury), membrane transporter protein-mediated metal transport (lead), and synergistic interactions of both of these (arsenic and copper). Iron and manganese are the two most abundant metals in sediment environments; iron may be used by microorganisms as an extracellular electron acceptor in the electron transport chain in the form of Fe(III), and manganese-oxidizing microorganisms oxidize manganese(II) mainly to manganese(III) or manganese(IV), with less transport of manganese ions. This highlights the importance of this oxidation reaction for microorganisms to maintain survival in energy-limited systems. Five chemoenergetic autotrophic microorganisms belonging to the phylum Thaumarchaeota or the phylum Nitrospirota were found to have potential manganese oxidizing capacity. And the discovery of a large number of metal oxidoreductase genes, including Mn(II) oxidase, Fe(III) reductase, Cr(IV) reductase, As(III) oxidase, and Hg(II) reductase, provides an important genetic resource for potential applications in heavy metal bioremediation.
It was found that in addition to oxygen and Fe(III), microorganisms mainly utilize nitrate as an electron acceptor to obtain energy through the oxidation of metal and sulfur compounds. Nitrate is mostly reduced to nitric oxide and discharged into seawater. In addition, microorganisms with diverse carbohydrases (CAZymes) did not show higher community abundance. Functional analysis of the dominant microorganisms in the study showed that they carried a higher proportion of functional genes related to metal, nitrogen and sulfur metabolism, while CAZymes were lower. Thus, the utilization of inorganic nutrients (rather than organic nutrient metabolism) for energy through redox reactions is the main adaptive strategy for microorganisms to maintain their survival in manganese nodule sediments. Based on the above study, the researchers proposed a model of microbial ecology in sediments of manganese nodule areas.
The research work was supported by the National Natural Science Foundation of China and the Strategic Pilot Science and Technology Program of the Chinese Academy of Sciences.
Metabolic functions of dominant microbial taxa in sediments from the deep-sea manganese nodule province
