Cold. Still. Dark. When describing
the largest of all the world’s biomes, these three words would perhaps be the furthest
from your mind. And it’s true that here we do not find an abundance of life, but the sheer scale of
the volume of the oceans means that a significant proportion of all life on earth is found in
this alien place. Millions of years of the same, stable conditions under hundreds of atmospheres
of pressure have evolved specialised life-forms that bear little resemblance to anything
found in the shallow seas or indeed the land. With no light, life must rely upon the discarded
waste from above, and yet this realm yielded a remarkable secret kept hidden until just a
generation ago. This strange, dark and vast world is the final biome we’ll look at in this
series. It is the world of the deep oceans. When making this video, I had to throw out
everything I’d learned in researching the previous dozen chapters in this series.
Because in all the other biomes of the world, the basis of each ecosystem was a
miraculous process known as photosynthesis, the creation of organic matter from CO2, water and
light. The deep oceans have little to no light, and so for life to survive here, it must rely
upon debris falling from the sunlit seas above… or something entirely different. But before
we can understand what life survives here, we must understand a bit more about the
characteristics of the oceans themselves. In the last episode we touched on the origins of
the oceans, and why it is that they take up 71% of the Earth’s surface. But the emphasis in that
video was upon the shallow seas adjoining the great continental land masses. These shallow
waters make up only a tiny fraction of the total volume of water in the oceans, which is
estimated to be 1.3 billion cubic kilometers, or 1.3 billion, billion cubic meters. That’s around
97% of all the water on the surface of Earth. The depth of the open ocean varies considerably
but the majority of it falls from the continental shelf down to depths of between 3,000 and 6000
meters, in what are known as abyssal plains. These are extensive flat areas of thin ocean
crust that in total cover about half of the surface of the earth. Ocean ridges, where most of
the new crust of the Earth’s surface is formed, lie at relatively shallower depths. The graveyards
of these oceanic tectonic plates, however, lie in deep trenches, when they are forced
below continental or other oceanic plates. The deepest of these trenches is the
Marianas Trench, east of the Philippines, with the Challenger Deep being a further
depression within this trench, with its lowest point being 10,920m below sea level, the lowest
piece of the Earth’s crust to be found anywhere. With all that weight of water sitting upon it,
its no wonder that pressures are extremely high at such depths. Water pressure increases by 1
atmosphere, or 1 bar, with every 10m of depth, and so at a typical abyssal plain depth of 4,000m,
the pressure is at around 400 bar, or almost 6,000 psi, while in the Challenger Deep, the
pressure reaches almost 1,100 bar, or 16,000 psi. Because the oceans are so vast, and,
naturally, so flat at the surface, winds are free to develop as part of large weather
systems, be they tropical, temperate or polar in origin. The nature of climate dictates that
weather systems will form in certain typical ways in certain areas, and produce a prevailing
wind direction on a given patch of ocean. When the wind blows in the same direction in
this way, friction between the air and water surface will produce ripples, and waves which in
turn move the water in the ocean as a whole mass. This is the genesis of ocean surface currents
which carry warm tropical water up to temperate latitudes, or cool temperate waters down to the
tropics. Some currents, such as the Gulf Stream off the Eastern Seaboard of North America are so
powerful that a water velocity of about 1m/s is achieved, and the effect of this current, which
originates in the warm seas of the Caribbean, is to warm the continent of Europe by a significant
fraction beyond what it should otherwise be. Cold ocean currents returning to the
tropics, such as the Peru current result in cold air that can hold little moisture,
and contribute to the driest desert on Earth. So the effect of these ocean currents
on global climate cannot be overstated. Such currents only operate in about the first
100m of ocean depth, and so the water below this is generally still. However, there is a more
complex system of currents that run far down into the depths and span the entire globe. The
water of the oceans is saline, meaning salty, in that it is composed of about 3.5% of dissolved
salts, the majority of which is sodium chloride, or common table salt. This salt fraction is highly
uniform throughout all the oceans, but there are places where the salinity will increase, such
as when intense winds evaporate water molecules, but leave the salt behind in the water, and
when sea ice forms, also leaving salt behind. When this occurs in cold surface water in the
polar regions, this more saline water, being more dense, falls into the depths below and, having
to go somewhere, is forced toward the equator and eventually around the globe, resurfacing in
other areas in a gigantic conveyer belt of water. This system is known as the Thermohaline
Circulation, since it is a combination of water temperature and salinity that acts as its
motor. Incidentally, where great upwellings occur, when deep ocean water comes to the surface,
rich mineral sediments are brought up with it, and these act as a fertiliser for phytoplankton
at the surface, which then rapidly reproduce into huge algal blooms. Being the basis of the food
chain, such abundance of plankton has a knock-on effect in all other species up this food chain,
and such areas, off the coast of Newfoundland, Peru and the Eastern Cape of South Africa,
are known for their rich fishing grounds. Such movement of water between the layers of the
ocean is the exception, however, and not the norm. This is because in general, the layers of
ocean water sit stably on top of each other, since surface water is heated by the
sun, while the water below is not. As cold water is more dense than warm water,
it will therefore stably sit below the warmer surface waters. This effect is most pronounced in
the tropics, while at the poles, the temperature of seawater is more or less the same from
the surface down into the abyssal depths. Through wave action, surface water
gets mixed down to a depth of about 500 metres or so, and so the temperature
falls only slightly down to this point. Below 500m however, and waves no longer have an
effect, and with so little sunlight reaching this depth, the water suddenly becomes much colder,
until at 1000m in depth, the water has cooled to below 7°C everywhere on the planet.
This sudden change in water temperature is known as the Thermocline, and the resulting
change in water density is so pronounced that submariners will pick it up as a “false bottom”
since sonar will reflect off this layer. Below the thermocline, the water
continues to cool as we go deeper, but at a much slower rate, with most of the
ocean’s water being at around a chilly 4°C. Ok so that tells us about the environment
of the deep oceans. So what kind of life can survive in such a regime of intense pressure, cold
temperatures and, most importantly, the absence of light? To begin, we have to look at what is
going on above, in the sunlit zones of the ocean. Here, phytoplankton build their tiny bodies
from photosynthesis, and in the open ocean these in turn are fed upon by a whole food chain
of zooplankton, krill, fish, sharks and whales. In the twilight zone of the middle depths, fish
can swim up to the shallower depths at night to feed upon plankton or krill, and be safe from
predators that would otherwise see them during the day. But below this zone, creatures are
wholly dependent upon what falls down to them. All this energy and biomass in the sunlit
zone has to go somewhere, be it in the form of organism death or excreta, and… the only way
is down, a long way down to the deep ocean floor. It is estimated that a piece of organic debris
descends the ocean at about 100m per day. This allows for plenty of time for creatures at various
depths to help themselves to these falling meals. The technical term for this is marine snow,
since it mostly comprises microscopic or particulate sized debris, since most of
upper reaches are indeed plankton or krill. However, large animals also eventually die,
and especially as in the case of the death of a large whale, such a carcass, when it eventually
reaches the ocean bottom, will act as a meal for thousands of creatures over many months.
The dark, high pressure abyssal reaches of the ocean produce probably the most alien
looking lifeforms out of any found on our world. The most immediately noticeable adaptation to the
lack of light is bioluminescence. Many creatures use light generated within their bodies to attract
a mate, or even prey, or to camouflage themselves against being a silhouette from predators below by
faintly lighting the undersides of their bodies. Siphonophores are perhaps the most spectacular
in this regard, and amount to a rare beauty in a world otherwise populated by, let’s be
honest, some pretty horrific creations. Many of us are familiar with pictures of the
anglerfish family, which uses a bioluminescent lure in front of its face to attract prey. Other
forms appear to have come from a different planet, being unlike anything we find in other biomes.
With no light available, there are no plants, and so most of the biomass is in the form of animals
either free swimming, or fixed to the ocean floor. Corals can be found here, but lack the
photosynthetic symbiotes of their tropical sunlit cousins. Sponges are common, and can grow to over
a metre in diameter, as are starfish that scavenge upon the bottom for any marine snow that might
have made it through all the ocean layers above. Most of the forms that are here are believed to
be very ancient in origin, having changed little over hundreds of millions of years, since the
depths of the ocean, as mentioned, are extremely stable in terms of temperature, and so are
isolated from the direct effects of any shifts in global climate on the surface. These adaptations
must cope with intensely high water pressures, which can affect even the formation of
proteins at the most basic level within cells, and so any species attempting to move
into the depths from above has found it very difficult to cope with competition
from the ancient species already there. Another adaptation appears to
be in the form of gigantism. Species found in the depths that are related to
more shallower species are usually much larger, such as giant spider crabs, and most famously, the
giant squid, which can grow to a length of 12-13m, and possessing eyes the size of dinner
plates – the largest in the animal kingdom. The giant squid is the favourite prey of the
sperm whale, which will dive to depths as far as 7,000m in search of it. The sperm whale is
able to survive such crushing depths through the evolution of a rib cage made of flexible cartilage
instead of bone, which allows the collapse of the lungs to a minute size on its long way down.
It is said that we know more about the surface of the moon than we do of the ocean bottom, and
one of the reasons why is because specimens of creatures caught in the depths do not survive
the journey back to the surface intact, since their bodies are wholly adapted to
the intense pressures found only down there. Also, the development of submersibles that can
survive such pressures are usually very expensive one-off creations, such as the Trieste that
first ventured to the Challenger Deep in 1951. And it’s because of such inaccessibility that
perhaps the greatest secret of the depths was only revealed within my own lifetime. In 1977,
a submersible exploring the hydrothermal vents in the geologically active mid-ocean ridge off
the Galapagos islands discovered, in the total absence of light, a whole ecosystem thriving, with
the basis of the food chain not being marine snow from above, but bacteria that were building their
bodies through chemosynthesis of hydrogen sulphide found in the hot, chemical rich waters of the
vents. Subsequent visits to hydrothermal vents around the world by other teams have revealed
more communities living in such fashion. For thousands of years we have been aware that
plants have formed the basis of all food chains, which we ourselves ultimately depend upon. So it
is fitting that at the very end of this series that has explored all these myriad biomes
that depend ultimately upon photosynthesis, that we reveal the one exception to this, the
secret so recently uncovered, of chemosynthetic bacteria forming the basis of a completely unique
biome altogether independent of the life-giving star that is our sun. What this discovery implies
for the potential for life on other planets, such as the ocean of Jupiter’s moon, Europa that
lies in the darkness beneath kilometres of ice, or perhaps the chemically rich surface of
Saturn’s moon Titan, is anyone’s guess. Just as we believe we have understood
life, something new comes along to shake our understanding. The effect
of life upon our world is complex, ever-changing, and in so many places, stunning
in its beauty. And without it, we, as conscious self-aware beings who depend upon the biomes
of the world for our own food and sustenance, would not exist to enjoy the privilege of
gazing upon these living landscapes of Earth.