The bulk of sunlight penetrates the ocean to ~200 meters deep, and only the most venturous rays reach a depth of 1,000 meters. So we can infer that light is absent in most parts of the ocean since its average depth is 3,700 meters. The greatest depths are found in the underexplored "oceanic trenches" that are elongated and narrow seabed depressions below 6,000 meters. More than 30 oceanic trenches on Earth are distributed mainly along the Pacific Ring of Fire. The deepest site is located in the Mariana Trench at 10,984 meters (Figure 1).

Oceanic trenches are extreme environments where darkness is absolute, temperatures are low (-1 - 4 ºC), and the high hydrostatic pressure exceeds the equivalent of 600 Earth atmospheres. Although these conditions may seem adverse to life, trenches exhibit favorable characteristics as their steep slopes act as a funnel that collects and accumulates organic matter, increasing food availability for organisms in the deep. Diverse organisms, such as fish and crustaceans, have developed various adaptations to live in these places. Despite this, its presence is scarce, but microbial life is a dominant component. Microorganisms, such as bacteria and archaea, are ubiquitous and abundant in all kinds of aquatic environments and trenches are no exception. 

These tiny, invisible living beings play a key role in the cycles of elements fundamental for climate regulation and the development of life, such as carbon, nitrogen, and sulfur. Despite the importance of microorganisms in all ecosystems being superlative, we only know the identity of the bacteria and archaea from the waters of four oceanic trenches: Mariana, Kermadec, Japan, and Puerto Rico. The Millennium Institute of Oceanography, established in the University of Concepcion in Chile, has attempted to characterize the microbial communities of the Atacama Trench located off the coast of Peru-Chile, a poorly explored ecosystem with a maximum depth of 8,081 meters (Figure 2).

Studying microbial communities is difficult because it is impossible to select and identify each tiny individual directly from the environment. Their study and identification are mainly based on community DNA sequencing. Preliminary findings show that the predominant bacteria of the Atacama Trench are typically heterotrophic (i.e., they feed on the organic matter that is so abundant in this trench), and the dominant archaea are typically autotrophic (i.e., they produce their own organic matter for food through a process called chemosynthesis). For now, these discoveries represent more questions than answers. We still do not know what functions these microorganisms carry out in the deep ocean, how they impact the cycling of elements and climate change, and what adaptations they have developed to thrive in this ecosystem. But there is one thing we know for sure: When it comes to living under pressure, microorganisms know how to do it.