Most of us have a fixed mental picture of the ocean: it’s bright, full of people, and lit by the sun. Reefs made of coral. A lot of fish are near the surface, catching light. It looks like the ocean in documentaries and vacation pictures. However, that picture of the sea only shows a very small part of what’s down there. Around 95% of the ocean’s volume is always cold and dark, and most things that try to stay alive think that it is always hostile.
However. There is life down there, and it comes in forms that continue to make us feel bad about what we think biology needs.
One type is deep-sea sponges. They don’t move. They don’t hunt. They live in large groups on the ocean floor, which scientists sometimes call “gardens.” These “gardens” can cover thousands of square kilometers and are anchored in the dark, deep below the surface where sunlight can’t reach. Every day, each sponge filters thousands of liters of water through its body. They give off nutrients that help fish, brittle stars, and other living things that don’t have any other food sources nearby. These are not small parts of the ocean’s ecosystem. They’re building blocks.
It’s not as clear how they handle all of this. The curtain has been pulled back by a new study in the journal Microbiome, and what’s inside is really strange.
Scientists looked at Calyx sponges that they found 830 meters below the surface. They found not just one way for the sponge’s microbes to stay alive, but two that worked at the same time. About sixteen percent of the microorganisms that live inside the sponge use chemosynthesis. This is a process in which ammonia, which is a type of biological waste made by the sponge itself, is used as energy to build biomass from carbon dioxide that is dissolved in the water. It’s kind of like what plants do when they get sunlight. Fuel that is very different for the same job.
The other 84% are doing something that is harder to explain. These microbes are heterotrophs, which means they get their energy from eating organic matter. This is how most animals, including humans, do it. The issue is that there isn’t much organic matter at 830 meters. Whatever falls from the surface, like dead plankton and pieces of algae, is broken up by bacteria and small crustaceans as it goes down. When things get to the seafloor, they are usually stripped of anything that would make them easy to digest.
This is the strange part of the discovery. The heterotrophic microbes inside Calyx sponges seem to have an incredibly large collection of enzymes that can break down complex, hard compounds. In particular, they can break down xylan and pectin, which are found in the tough cell walls of algae. These microbes can use the dead shells of algae that other living things can’t use as food. They break it down, get energy from it, and change the molecules into nutrients that the sponge can use. It’s like the sponge has a processing plant inside its own body.
There’s a bigger point here that should be looked at more closely than it is now. These groups of sponges are involved in the ocean’s carbon cycle. They move organic matter around in ways that keep whole food webs going in places that would be mostly empty otherwise. Deep-sea trawling, which breaks up sponge gardens, or the growing push toward deep-sea mining for rare metals used in electronics and batteries could change this role. It could take hundreds of years to see the effects and even longer to understand them.
The UN has said that deep-sea sponge gardens are in danger as marine ecosystems. That’s worth some money. But it’s still possible that we’re moving faster to get what we want from these places than to figure out what we’d be giving up. That doesn’t make me feel good. At this point, it’s also probably a true one.
