Animals living in one of Earth’s most extreme environments—beneath the bottom of the sea—are not just making the best of the situation, according to new research. The animals are actually shaping their situation, wriggling through the sediment and paving the way for other forms of life.
The team sampled sediment cores from the Pacific Ocean’s Japan Trench off the coast of Honshu. The group studied the hadal zone—the deepest part of the ocean—at about 4.66 miles (7.5 kilometers) beneath the surface. Even there, life finds a way. The team’s research was published today in Nature Communications.
Bioturbation is an engineering process by which animals or plants oxygenate and irrigate sediment. Many creatures do this by burrowing, and those under the seafloor are no exception. Bioturbation affects the way that nutrients cycle through sediments in an ecosystem, and, in turn, how the ecosystem functions.
“Deep-sea benthic communities have often been seen as low in diversity and biomass due to harsh conditions like limited food and energy,” said study lead author Jussi Hovikoski, a sedimentologist at the Geological Survey of Finland, in an email to Gizmodo. “However, recent sampling in hadal trenches has revealed a surprising variety of life, including holothurians, polychaetes, bivalves, isopods, actinians, amphipods, gastropods, and bottom-dwelling fish.”
Last year, a team of researchers discovered macroscopic life beneath the deep ocean’s seafloor, complicating our understanding of the living things in one of Earth’s most extreme environments, while also offering a hint at what life could look like beyond Earth—perhaps in the subsurface oceans of moons in our solar system. The recent study adds to that saga, by showing how similar deep sea environments turn over, refreshing the local environment with nutrients and oxygen.
In their study, the researchers analyzed 20 sediment cores from the Japan Trench. The group scanned the cores with X-rays to understand their structure—how the sediment was deposited and the sequence of inhabitants that dwelled within it.
“The data show that as the turbidity current slows down at the bottom of the trench, its sediment particle cloud condenses, locally suffocating the benthic fauna,” Hovikoski said. “This is followed by intense colonization, where opportunistic benthic species return to exploit the new nutrient-rich and oxygenated bottom sediment.”
The sediment was laid down by gravity flows, the researchers said, meaning that silt and other matter from higher up in the trench drifted down and deposited onto the sample site. Coming from higher up in the water column, the newly arrived sediment is comparatively nutrient-rich and oxygenated—a welcome treat for the most submarine of the bottom feeders.
In the video below, you can see a 3D scan of the trace fossil Pilichnus: burrows branching downward through the deep ocean sediment, likely created by bivalves—the final arrivals to the scene, according to the researchers.
“Over time, as organic matter decomposes, the bottom sediment becomes anoxic, and the metabolism of the microbial community changes,” Hovikoski added. “This final phase of colonization is represented by invertebrate species that utilize the microbial communities. Based on burrow morphologies, these likely include certain bivalve species.”
The bottom of the sea is a lively place. In lieu of sunlight filtering from above, creatures glow with their own bioluminescence, and some rocks even produce oxygen. We’re getting better at understanding these deep sea quirks; earlier this year, the famous Alvin submersible was certified to dive more than 21,000 feet (6,500 meters) under the sea, expanding humanity’s ability to visit the ocean’s depths.
Taken in tandem with astronomical analyses of nearby moons like Europa and Enceladus, the findings from the waters off Japan have us itching to send a probe to these faraway moons. As far as scientists are aware, life as we know it needs water to eke out existence. The latest findings offer another suggestion at what that alien life could look like—but we won’t know whether it’s out there until we look.