Abstract:

The oceans, with an average depth of 3,800 meters and an average pressure about 38 MPa, cover about 70% of the surface of the Earth. Geological structures under the seawater, such as marine sediments, oceanic crust, hydrothermal vents, and the cold seeps, vary significantly with regard to physical and chemical properties. In combination, these diverse environments contain the largest microbial ecosystem in the world. In deep seawater, the major microorganism groups are Alpha-& Gammaproteobacteria, and Marine Group I. In deep-sea sediments, the abundance of microbes is related to the content of organic matter and distance from land. Methane Oxidizing Archaea (ANME) and sulfate reducing bacteria (Deltaproteobacteria) are common in deep-sea cold seep environments; while in hydrothermal vents, the richness and dynamics of chemical substances have led to highly diversified archaeal and bacterial groups. In contrast, the oceanic crust is mainly composed of basic and ultrabasic rocks rich in minerals, and as a result houses microorganisms that are mainly autotrophic, utilizing iron, manganese and sulfur. Because more than 99% of deep-sea microorganisms cannot be cultured, an understanding of their diversity, physiological features, and biogeochemical roles remains to be fully achieved. In this article, we review and summarize what is known about the distribution and diversity of deep-sea microorganisms in diverse habitats. It is emphasized that there is much to learn about these microbes.

Key words: microbiology, biodiversity, deep subsurface biosphere, microbial ecology