It’s time for another installment in the Ecosystem Ed. series! In each of these posts, I focus on a different type of ecosystem around the world and break it down to the basics: what, where, why, how, and who, along with some fun facts. First, just a quick reminder of what an ecosystem is: a community of living things and nonliving natural components that exist together and interact with each other. An ecosystem can be as big as a giant coral reef, or as small as a single rotting log on a forest floor. There are several categories of ecosystems on Earth, ranging from rainforests to deserts to grasslands. This week, we’re focusing on the DEEP OCEAN, one of Earth’s least-understood and inaccessible ecosystems. (Heads up – this post is kind of a long one. I’m just really excited about the deep sea, okay?)

What is the deep ocean?

At first glance, this may seem like a pretty silly question. Obviously, the deep ocean means the parts of the oceans that are deepest, right? Yes, but it’s more complicated than that. The ocean is divided into 5 zones based on depth and how much sunlight can reach that depth.

Technically, the “deep ocean” includes anywhere deeper than 1800 m, meaning that the deep ocean begins in the bathypelagic, or “midnight zone”. At such depths, there is little or no light and the pressure from the water above is immense. And yet, life thrives down here – the deep ocean is the largest habitat on the planet. And we humans have only seen tiny little pieces of it! It’s even been said that we know more about the moon than the bottom of the sea.

Where are the deepest parts of the ocean?

The seafloor is as varied as the surface of dry land, with mountains, plains, valleys, and, occasionally, canyons. These features can be much larger in scale than the ones on land though, with the valleys much deeper and the mountains much taller (notably, the world’s tallest mountain is not in fact Mt. Everest, but Mauna Kea in Hawaii at over 10,000 m, measured from its base on the ocean floor).

The deepest known part of the ocean is the Challenger Deep (11,034 m down), a region of the Mariana Trench in the western Pacific, a few hundred km southwest of Guam. Other notable deep ocean valleys include the Horizon Deep in the Tonga Trench (10,882 m down), and the Galathea Depth in the Philippine/Mindanao Trench (10,540 m down). Trenches like these tend to form as a result of tectonic plates colliding and pushing each other down – many of the deepest trenches are found around the infamous “Ring of Fire”, the circle of volcanic activity that surrounds the Pacific Ocean and Pacific Plate.

How does anything survive there?

Living at the very bottom of the ocean presents some tough challenges! First of all, sunlight doesn’t reach the deep ocean, which means no plant life, no photosynthesis, and no plant-based energy production (the main form of energy production in just about every other ecosystem on the planet). Creatures living down there have to get energy from other sources instead. Many survive off of a sort of detritus known as “marine snow”, a nice name for a not-so-nice material. Marine snow is made up of organic material (including dead creatures and waste) that drifts down from the shallower parts of the ocean where sunlight does penetrate (also known as the “photic zone”). Along the seafloor, energy can also come from volcanic vents that produce lots of nutrients (a major factor in those super-deep trenches formed by plate collisions).

The other main challenge that deep sea critters have to deal with is extreme pressure. About every 10 meters below the surface, the pressure increases by 1 atm. So if you live 10,000 m down in the Mariana Trench, you’re experiencing 1,000x the amount of pressure we feel at sea level from the weight of the atmosphere above us! But deep sea dwellers are specially adapted to deal with it, and they have just as much trouble dealing with low pressure as we would have with such high pressures. Until the recent invention of traps incorporating pressure chambers, most of the specimens we brought up from the deep ocean to study did not survive the trip to the surface.

Who lives there?

Because it’s so difficult for us to access the deep ocean, we know very little about what lives there, especially compared to how much we know about ecosystems on land. Scientists estimate that the vast majority of unknown species live in the ocean. But here are a few we do know about:

• Ambush predators: Many fish species that live in deep ocean are ambush predators that use their big sensitive eyes and bioluminescence to catch prey. These include things like angler fish, gulper eels, and deep-water sharks (the sorts of creatures that make people say “nope” when it comes to the deep ocean). Many squids and other cephalopods also fall into this category.

• Chemotrophs: In the very deepest areas like the trenches, the main source of energy is from hydrothermal vents, columns of water heated by the Earth’s molten mantle. Hydrothermal vents spew out tons of nutrients along with the hot water, which makes an ideal environment for chemotrophs, or organisms that survive off of chemical soup. These include extremophile bacteria, which are a food source for other micro-organisms, which then become a resource for small invertebrates, which in turn feed larger species like fish, crabs, and tube worms. Around some hydrothermal vents, tube worms have reached sizes of over 2 m long!

• Scavengers: These are the literal bottom-feeders, the creatures that eat detritus and scraps that fall to the sea floor. Marine snow is a popular food source for these guys, as are “whale falls”, which are pretty much exactly what they sound like. When a whale or other large sea creature dies and sinks to the bottom, it becomes a feast for crabs, hagfish, bivalves (e.g. clams and mussels), worms, snails, and much more for months or sometimes years, depending on the size of the creature.

Why are we interested in deep sea life?

Why do we bother trying to explore such inhospitable environments? What does anything that happens at the bottom of the ocean have to do with us? As it turns out, we can look to the deep ocean to see both the distant past and (hopefully) the future.

For the past, those hydrothermal vents are a pretty good candidate for the origins of life on Earth. The world’s oldest fossils are of microorganisms in the precipitate (solid fallout) from hydrothermal vents, from about 4.28 billion years ago (for reference, the Earth itself formed about 4.54 billion years ago). These tiny, single-cell organisms were the dominant form of life on Earth for almost 2 billion years. During that time, fundamental processes like photosynthesis evolved, and the atmosphere gained oxygen. Multicellular life first appeared in the oceans about 2.1 billion years ago, followed by the Cambrian explosion about 541 million years ago. During the Cambrian explosion, most of the major animal groups we know today appeared, and life forms became much more complex.

For the future, scientists are looking at deep ocean ecosystems as potential indicators of life we might find elsewhere in the universe! Two of the most likely candidates for extraterrestrial life are Jupiter’s moon Europa, and Saturn’s moon Enceladus, both of which consist of a layer of thick ice covering a vast liquid ocean. Water volcanoes have been observed on Enceladus, while the ice on Europa appears to shift in the same way that tectonic plates do on Earth, which suggests that the subsurface oceans have warm currents running through them. Europa and Enceladus are also likely to have similar hydrothermal vents that could provide nutrients for some form of life. NASA is preparing for missions to explore Europa, and eventually, hopefully, dive below the surface.

Fun Facts:

• The oceans cover over 70% of the world’s surface, and hold about 1.35 billion cubic kilometers of water.

• Because of the abundance of life in the deep oceans, it’s thought that bioluminescence is the most common form of communication on the planet! Check out “Nature’s Night Lights” for more on bioluminescence.

• One effect of immense pressure in the depths is that the density of water increases (density refers to the amount of stuff that fits into a given space). In the Challenger Deep, water’s density is almost 5% higher than it is at the surface!

• The deepest diving animals that aren’t fish are: emperor penguins (535 m), leatherback turtles (1,280 m), elephant seals (2,388 m), sperm whales (2,250 m), and Cuvier’s beaked whales (2,992 m).

• The deepest a human has ever dived successfully is 332 m (over 1000 feet), by Ahmed Gabr in 2014, with top-of-the-line scuba gear and a team of experts. The record for deepest free dive (no breathing equipment) goes to Herbert Nitsch – 253 m in 2012 – and he nearly died in the attempt.

• The deepest a human has ever been – in a deep-sea exploration vessel – is into the Challenger Deep itself, by none other than film director James Cameron! Cameron has been fascinated with ocean exploration for a long time, and some speculate that he only made the movie Titanic as an excuse to dive down to the wreck of the actual RMS Titanic, which is about 3,800 m below the surface of the north Atlantic Ocean. Before Cameron took the plunge alone in 2012, Jacques Piccard and Don Walsh made the first descent into the Mariana Trench in 1960.

Heads up that there will be no blog post next week. Happy Thanksgiving!