The hadal biosphere: Recent insights and new directions

Abstract

The hadal zone is the deep ocean with depths exceeding 6000m. With an area approximately equal to the size of Australia, but constituting the deepest 45% of the vertical depth of the global ocean, it should come as no surprise that the hadal zone is the last great frontier of the ocean science. Historically, the hadal trenches are believed to have limited activities of life, microscopic or macroscopic, due to the extreme environmental conditions such as high hydrostatic pressure, lacking of sunlight and food supply, as well as frequent subduction-zone earthquakes. However, seminal discoveries have been made recently by scientists working at hadal trenches on microbial and faunal community structure, abundance, diversity, and mechanisms for endemism, speciation and adaptation, likely regulated by a myriad of specific geological, physical, and chemical factors of the hadal zone, forming the so-called “hadal biosphere”. These discoveries ignited a new wave of hypotheses and theories on organismal metabolism, energy acquisition, and the origins of life on Earth. Here, we review the recent development in understanding the hadal biosphere and discuss future directions on hadal biosphere research.