Just 6 kilometers (about 4 miles) of briny water separate the ocean's surface from the depths known as the hadal zone. Yet for as much as we know what lurks in its cold darkness, it might as well be another world.
The life that flourishes down there is also downright alien. A new study has recovered a plethora of never-before-seen microbes from areas of the ocean floor that include the Mariana Trench. Analyzing their alien methods for survival could give biologists a whole new bank of resources to use in everything from medicine to evolutionary research.
The hadal zone starts 6 kilometers (3.7 miles) deep, and goes down as far as 11 kilometers (6.8 miles). That lower limit is almost 30 Empire State Buildings, or around one-and-a-quarter Mount Everests. It's deep.
Researchers from institutions across China took 33 dives into the hadal zone with a manned submersible, collecting samples of sediment and seawater. Subsequent analysis identified 7,564 microbe species, of which nearly 90 percent were new to science.
"Our study focuses on a long-standing goal in microbial ecology: elucidating how environments shape microbial communities, especially in extreme conditions," write the researchers in their published paper.
Life is far from easy in the hadal zone. Temperatures are close to freezing, the water pressure is immense, and there's very little in the way of nutrients to snack on.
With that in mind, the diversity of species was surprising.
The microbes the researchers discovered generally applied one of two survival strategies. Some had smaller, simpler genomes, evolved for efficient living. These microbes showed evidence of enzymes designed to resist the stresses of living at such depths.
Other microbes were found to have larger genomes – not built for efficiency but for versatility. This makes them better able to adapt to environmental pressures, and to be able to survive off a wider range of matter for sustenance.
"Extraordinarily high novelty, diversity, and heterogeneity were observed in the hadal microbiome, especially among prokaryotes and viruses, which are impacted by both the broader context of extreme environmental conditions as well as the delicate topography in the hadal zone," the researchers note in an editorial accompanying their paper.
The microbes also tend to find suitable nooks and crannies in the depths of the ocean and stick to them: each of the sample sites visited by the researchers had their own particular mix of microbes, with little overlap between them.
At lower depths, cooperation seems to be more important for microbe survival, with these tiny organisms sharing nutrients and showing behaviors that benefit the community as a whole (including forming protective biofilms).
The research team has made its findings available online for other scientists to dig into, under the title of the Mariana Trench Environment and Ecology Research (MEER) project – adding to our understanding of how life survives in extreme conditions, and opening up new research opportunities in biotechnology.
"These findings indicated that environmental factors drive the high taxonomic novelty in the hadal zone, advancing our understanding of the ecological mechanisms governing microbial ecosystems in such an extreme oceanic environment," write the researchers.
The research has been published in Cell.