Ocean’s deep dark zone – Benthos

Geetha Iyer

Fascination for exploring the oceans goes way back to Alexander the Great’s time. We know this through Aristotle’s work, Problemata. Writing about the siege of Tyre in 332 BC, he records that Alexander had asked his divers to destroy any submarine-like defenses that may have been built. While there are no records of submersible during his reign, legend has it that Alexander was lowered into the sea in a diving bell to watch the creatures living beneath the water.

From diving bells in the 4^th century to submarines, followed by bathysphere, the bathyscaphe Trieste, to the current robotic deep sea submersibles¹, ocean exploration has now conquered those abysmal depths of darkness to discover living organisms that challenge the very definition of life as we know it.

The images of tunas, whales, sharks, migrating turtles, swimming penguins, seals, congregating krills, etc., are, figuratively speaking, a drop in the ocean. We are familiar only with life in those zones where sunlight penetrates or from parts of the twilight zone, which extends to a depth of 1000meters, where sunlight does not penetrate. With the advent of technology, ocean exploration has now reached those pressure crunching depths, 5000meters and beyond to find the ocean teaming with lives humanity never imagined of. These zones are referred to as benthic zones that extend all the way to the sea floor.

So what kind of factors govern these zones? What is life like at such depths? What is biodiversity in these zones like? Twilight zone is 1000m deep. From here to the deepest part of the ocean 11,000m, the factors that living organisms have to face vary with the depths.

We have very little information from the ocean that makes up India’s coast. Deep sea mission has only recently been launched by India. The various missions from United States and countries such as Russia, Japan, etc., provide us with some very interesting information. What I describe below is drawn mostly from the various explorations conducted by NOAA² in the last 20 years. The data from the ocean sea floor as well as from various depths has been and continues to be collected using remotely operated vehicles (ROVs), unmanned robotic autonomous undersea vehicles (AUV) and more recently Alvin, the three manned American deep sea submersible, fitted with sophisticated equipment. NOAA announces live streaming of their underwater, deep sea expeditions for educators. Educators who have registered with them can watch the exploration in real time.

Deep sea ecosystems
Although our knowledge of the creatures³ residing in the benthic zone has increased, the deep sea ecosystems still remain poorly understood. The deep sea communities have to adapt to extreme conditions of pressure and temperature, lack of light, availability of oxygen and nutrients and to some extent the chemicals present in water.

Deep sea corals: Community based ecosystem

Coral communities^4,5 bring to mind those found in shallow tropical waters, where the temperatures are warm and sunlight is available. In contrast deep sea corals live in what scientists call, “barren underwater deserts”. Despite no light, in temperatures that are ice-cold and water chemistry that makes it difficult to construct skeletons, they are widely distributed. They feed on zooplanktons as well as other particles that are found in the water. They have a buffering mechanism that helps them manipulate the chemistry of the water to build their skeletons. As a result of the unfavourable conditions they inhabit, their growth is extremely slow in comparison to the shallow water coral communities. They are thus highly vulnerable to the slightest changes in their environment. Climate change is bringing about a change in the concentration of CO₂ in ocean waters which may pose challenges to their survival.

Hydrothermal vents: Gas-based ecosystems

These can be compared somewhat to hot springs, except that these underwater formations seen beyond the depth of 2700m are far more complex and from researchers’ remarks, have a beauty of their own. Like geysers, they spew out hot water that is mineral rich. They are discharged from vents that are heated by the earth’s magma. The metallic sulfides – the main minerals in hot water – are precipitated when they come in contact with the surrounding cold water. The precipitates accumulate to form tall towering chimneys that are both intricate and colourful in appearance. And around this vent flourishes an ecosystem, dark, with toxic sulphide gases as the predominant abiotic factors. Researchers are still discovering numerous new species in this ecosystem. One of the first to be discovered was the giant tube worm, 2m tall, called Riftia that completely subsisted on the hydrogen sulfide coming out of the vents. Even more remarkable was the discovery that they had no digestive system. Instead they had a large sac in which resided chemosynthetic bacteria. In this symbiotic association, the worm provided the raw materials and shelter and the bacteria produced, through chemosynthesis, sugars from Co2 using the energy from hydrogen sulphide. This ecosystem thus ran on earth’s geothermal energy.

Located along the mid-ocean ridges where the earth’s lava is close to the sea water, these were first discovered in 1976 near the Galapagos. Subsequently they have been found at depths of 4000metres and harbouring a wide variety of novel species of shrimps, giant clams, snails, mussels and other species of tube worms.

Cold Seeps: Natural gas based ecosystem
This deep-sea ecosystem occurs along continental margins. Here, energy is available abundantly in the form of methane, hydrogen sulfide and oil that seep out of the sediments of the ocean floor. At depths below 500m, methane forms a kind of ice – methane hydrate. It is estimated that there is more energy locked up in this methane hydrate ice than in all the fossil fuels combined. Here too animals with symbiotic bacteria in them are found – similar but different species of tube worms, clams, mussels. Some of the mussels harbour bacteria that use methane and not hydrogen sulfide for chaemosynthesis. The ecosystem here is powered by natural gas.

Brine pools
Brine pools are small lakes within oceans. They are formed when salt deposits under the ocean floor dissolve to form pools of water. This water is quite dense due to the high salt content and so separates out from the sea water. These pools have been discovered at the Gulf of Mexico and the Mediterranean Sea. Deep sea communities have been found to live at the edge of these pools. Mussels with symbiotic methane using bacteria have been discovered at these pools. Within the pools only microbes have been found to survive.

Hydrothermal seeps
A recent find in the deep sea, this is a system of vents and seeps coexisting to form a new hybrid ecosystem, as the name indicates.

Whale fall

This is another example of an ecosystem that comes into existence due to an opportunistic event. When whales die, they are either washed up on the shore or sink to the bottom of the ocean. Such a large source of nutrition is a rarity for the energy-poor deep sea, teeming with weird invertebrates and microbes. So when a whale falls to the sea bottom, an island of organic-cum-sulphide rich habitat is created. Feeding on the dead happens in series⁶ and goes on for years, for every part of the whale is eaten and by a variety of animals and microbes. Marine biologists from Monterey Bay Aquarium Research Institute found that one whale-fall community lasted for 50 years.

The first to arrive at the scene are scavengers such as the hagfish, sleeper sharks, rattail fish and amphipods which feed on the meaty part of the carcass – muscles, visceral organs. As they move out, hordes of polychaete worms, molluscs, unusual crustaceans and other invertebrates descend on the bony remains. Dr Craig Smith has found that a whale skeleton, depending on its age holds anywhere between 2-24 metric tonnes of oil in its bones. It has been recorded that upto 45000 worms per sq feet of the ocean floor blanket the sea floor a year after a whale has fallen. Marine biologists have found these organisms to be unique to whale fall. Of these – the bone-eating worm leads the show, while others feed on the sediments around the whale skeleton. The Osedax or the bone-eating worm has no gut, no eyes, no legs and the male is a tiny fella residing within the female. She has beautiful red plumes and root-like appendages, with symbiotic bacteria within to digest the lipids. These appendages bore through the bones to reach the marrow for the symbiotic bacteria to breakdown the lipids and release energy to the worms. This can go on for at least two years based on the size of the whale.

The holes drilled into the bone by the Osedax become the opening for the microbes to move in. The first to enter are the anaerobes followed by the chemosynthetic sulphophilic bacteria. The latter appear as a yellow mat covering the bony remains of the whales. The bacterial food web on the whale fall is unique; about 200 sulphophilic species of bacteria have been found on whale bones.

The existing belief is that life began in water. Evolutionary history tells us about the weird and strange creatures that once populated the earth. Molecular and paleoecological studies suggest that whale falls have served as hot spots of adaptive radiation for a specialized fauna; they have been the evolutionary stepping stones for vent and seep mussels and could have facilitated the formation of other new species in vents and seeps.

Conservation of oceans
These discoveries highlight the importance of taking care of oceans more than ever. The pollution of oceans through dumping waste, the destruction of its habitats through drilling and over exploitation for food are some threats that need to be seriously addressed.

As India begins to explore the deep sea, we must remain aware and alert to ensure that the exploration helps conserve the ocean and not overexploit its resources.

We have explored planets and stars, continued to search for extra-terrestrial life in space, while remaining ignorant of life in the deep sea, even at a depth of 4000m. Oceans are like the backyard of our earth, and yet the backyard had to, till recently, take a back seat to space exploration.

The benthic zone Distinguished as bathypelagic (1,000-4,000 meters), the abyssopelagic (4,000-6,000 meters) and the deepest of them all, the hadopelagic (6,000 to about 11,000 meters), these zones are without sunlight and any light seen here is that produced by the living organisms themselves. Extending from the Arctic to the Southern Ocean, they are dark, cold and pressure enormous. About 80% of volcanic activity on earth occurs on the deep sea floor of the global ocean. The deepest part is the Mariana Trench or Marianas Trench, 11,034m, located in the Western Pacific Ocean.

All images unless otherwise mentioned are from the NOAA website.

References

1. https://oceanexplorer.noaa.gov/technology/subs/subs.html – Different types of Observation Platforms: Submersibles
2. https://oceanexplorer.noaa.gov/explorations/explorations-by-topic.html – Explorations by NOAA
3. https://www.youtube.com/watch?v=80OG2BGrmyA – Ten weird animals from the deep sea
4. https://deepoceaneducation.org/resources/the-secret-world-of-deep-sea-corals/
5. https://deepseacoraldata.noaa.gov/gallery/gulf-of-mexico-deep-sea-corals
6. https://www.youtube.com/watch?v=URi8KccVkks – Video of whale fall showing feeders including Osedax sp.
7. https://oceanexplorer.noaa.gov/okeanos/edu/welcome.html – Ready to use educational materials on oceans and life in oceans
8. https://oceanexplorer.noaa.gov/image-gallery/welcome.htm – Images of deep sea organisms. Free resource

The author is a consultant for science and environment education. She can be reached at scopsowl@gmail.com.