October 28, 2021

I, Science

The science magazine of Imperial College

Charting the small amount that's known about the depths of the ocean ...


This summer, Academy Award-winning director James Cameron will release his latest film about deep sea exploration. But this time, it’s not a fictional account, and the main star is Cameron himself.

In March 2012, Cameron descended nearly seven miles beneath the surface of the western Pacific Ocean for the deepest solo dive in history. At the southern end of the Mariana Trench, which lies between Japan and Papua New Guinea, the Challenger Deep is the deepest known point on Earth, and has only been visited once before: in 1960, by oceanographer Jacques Piccard and navy lieutenant Don Walsh.

While Cameron’s primary motivation was pure curiosity, the mission was also designed to be a scientific exploration of one of the least visited parts of the planet. The purpose-built submersible, Deepsea Challenger, collected data, samples and 3D imagery during the expedition and analysis of these results is still ongoing. Deepsea Challenger has since been transferred to the Woods Hole Oceanographic Institute in Massachusetts to help future deep sea exploration.

It’s estimated that only around 5% of the world’s oceans have been explored by humans to date, mainly due to deep sea exploration being expensive, difficult and dangerous. But as the number of manned submersibles increases, and with developments in robotics providing autonomous underwater vehicles, deep sea exploration is undoubtedly making progress.

Conditions at the bottom of the ocean are extreme. Natural light can’t penetrate the sea much further than 200 metres below the surface, resulting in total darkness. Barometric pressure increases by one atmosphere for every 10 metres you descend, causing crushing pressures. Deep ocean water typically fluctuates between 0°C and 3°C in temperature, yet hydrothermal vents on the seabed expel water at temperatures as high as 400°C.

Given such harsh conditions, scientists had previously assumed that life would be scarce, but once the tools were developed to explore this alien environment, the deep ocean turned out to contain an incredible biodiversity. Some scientists have estimated that as many as 10 million species live down there, making the levels of biodiversity in the ocean comparable to that of the world’s rainforests.

In order to survive in these extreme environments, deep sea creatures have had to adapt in unusual and unique ways. Photosynthesis is impossible so plants, the primary producers of nearly all of Earth’s ecosystems, can’t survive. Where do the organisms get their energy from?

Many depend on organic matter sinking from the more hospitable regions above, in what’s known as ‘marine snow’. But some creatures living around mineral-rich hydrothermal vents on the sea floor, use chemosynthesis — the process of using energy released by inorganic chemical reactions to produce food.

Giant tube worms rely on chemosynthetic bacteria living within them that oxidise hydrogen sulphide expelled from the vents using dissolved oxygen in the seawater. This provides the energy they need to produce organic molecules. Revealing chemosynthesis is considered to be one of the great scientific discoveries of the last century, and the process has been speculated to be behind the origins of life on Earth.

Deep sea chemistry is also responsible for bioluminescence, which is used by many deep sea organisms to produce light through chemical reactions. The light is used to see, lure prey, attract mates, or distract predators, in an otherwise pitch-black environment.

Female deep sea anglerfish use bioluminescent ‘baits’ on the end of their protruding dorsal spines to lure their prey. The ‘baits’ are bulbous growths full of bioluminescent bacteria, which enter the bait from the surrounding seawater through small pores, and stay there because they get access to nutrients provided by the host anglerfish in exchange for their bioluminescent properties.

Due to the rarity of encounters in the deep ocean, some anglerfish have also developed an unusual method of reproduction. When a male ceratioid anglerfish finds a female, he grips on with pincher-like tentacles and the two fuse together, eventually uniting their circulatory systems. The male’s body then atrophies and he becomes permanently dependent on the female for nutrients, in return providing sperm whenever she requires it. In some species, a single female host can be coupled to as many as eight males.

Given the range and novelty of species found in the deep sea, it’s not surprising that the search for valuable new commodities, which are in short supply on land, is now moving underwater.

As antibiotic resistance increases, researchers are going to further extremes than ever before in their efforts to discover new ones. The PharmaSea project, launched last year, is a collaboration between European scientists hoping to find new bioactive compounds from marine organisms.

Due to the remote nature of these hostile environments, life in each deep sea trench has evolved in very different ways, encouraging hopes of discovering a wide variety of novel microorganisms. By collecting mud and sediment samples from deep sea trenches, and culturing them back in the lab, researchers believe that the resulting deep sea bacteria could have valuable properties including possible new antibiotic agents.

And it’s not just drugs on offer either. Vast reserves of minerals, including rare-earth metals, are known to exist in the seabed. These are key components of our much-loved smart phones and burgeoning hybrid car industry and are currently in short supply. The deep sea is therefore predicted to be the next big target for mining, and 19 leases have already been issued for prospecting in international waters, covering a combined area the size of Mexico.

But because we know so little about the deep ocean, it’s hard to know what the effects of such invasive exploration could be. In February, scientists warned that we need to stop and think before moving ahead with this relatively new type of resource extraction. Given the harmful effects that have arisen from the large-scale destruction of the rainforests, there’s a pressing need for research to be carried out, and possible regulation put in place, before the world’s oceans are exploited.

As technological advancements make exploration of the deep sea easier, one of the last great unexplored regions on Earth is about to become a little more familiar. Let’s treat it with the respect it deserves.


IMAGE: Oisin Mulvihill