The deep ocean floor is a place that appears entirely devoid of life.
Food is scarce, and with very little energy making its way down here
to where the sunlight does not reach, survival in the depths is a challenge for any creature.
Every now and then, islands of life appear around sunken carrion, providing a temporary surge of
nutrients, but eventually these fade away, and the deep sea becomes lifeless again. However,
there are some locations in the deep that life has clung to for millions of years,
and where food and energy is ever abundant.
Here, at depths of 2000 metres or more, life gathers around hydrothermal vents in great
numbers. Hot, mineral-rich fluids supply chemicals and nutrients, which form the basis of a food web
for a diverse community of specialised organisms. An oasis of life in the deep.
Since their discovery in 1977 near the Galapagos Rift, hydrothermal vents have redefined our
understanding of the requirements for life to survive. Up to that point, we had thought all
life depended on sunlight to survive, but this extreme ecosystem represents the only place on
Earth where the ultimate source of energy for life is not sunlight, but the Earth itself.
Over 300 animal species have been identified living at these vents. 95% of these organisms
are unique to the vent ecosystem, and they were new discoveries, previously unknown to science.
Many don’t appear to be closely related to any other creatures on the planet,
suggesting that they likely belong to a divergent evolutionary path
that splintered off in the distant past. Or perhaps,
they represent to the primordial community of life, from which all life on Earth evolved.
Unlike whale-fall ecosystems that are fleeting and support life for only a number of years,
these vent communities are ancient and well-established. But just how
are these organisms able to survive in these conditions, so far from the reach of the sun?
Though they are toxic to humans, the chemicals that are expelled within the superheated water
contain nutrients that bacteria are able to convert to energy, much like plants
convert sunlight during photosynthesis. This process of converting chemicals to
energy became known as chemosynthesis, and it redefined our understanding of life.
Like plants and algae on land and in the shallows, the chemical-harvesting microbes are the primary
producers in the food web here, and are eaten by larger animals. Bottom feeders like limpets graze
on the mats of microbes that coat the chimneys, while suspension feeders like mussels feed on
bacteria that drift in the surrounding water. But there are also more complex creatures,
like the Yeti crab, which encourages the growth of microbes on its claws and shell, and using
highly specialised hairy mouth appendages, it scrapes the bacteria into their mouths.
The creatures here have remained unchanged for millions of years, such as these tube worms,
living in a symbiotic association with the bacteria that live inside them. The worms
absorb hydrogen sulphide and other chemicals from the vent fluids in order to feed the bacteria.
In return, the bacteria provide the carbon that the tube worms require in order to live.
It is also thought that the bacteria benefit by being sheltered within the tube worms,
and are therefore protected from predators like the limpets and crabs.
Some deep-sea skates, which are related to sharks and rays, have become highly evolved to live in
this environment. They lay their eggs near the heat-producing vents, utilising the volcanic heat
to accelerate egg development and reduce the usually years-long incubation time of their eggs.
Hydrothermal vents form when water seeps into the earth’s crust
through cracks and fissures in volcanically active regions, such as the mid-ocean ridge.
The water becomes superheated with the hot magma that is found within the Earth’s layers.
The water boils up to be released from the ocean floor, and picks
up dissolved minerals along its journey. This forms the towering chimney structures,
as the dissolved minerals solidify when the heated water mixes with the cool seawater above.
There are many varieties of hydrothermal vents, characterised by the specific mineral
content that is brought up by the heated water. Black smokers emit the hottest, darkest plumes,
forming chimneys over 50 meters tall (180 feet) with high levels of sulfides
that precipitate on contact with the cold ocean to form the black smoke. In
contrast, white smokers contain barium, calcium and silicon.
Other vents are characterised by the shimmering streams of water that vary with temperature,
some being up to 400 degrees Celsius (750 F), while some vents simply emit carbon dioxide gas
in a constant stream of bubbles.
The discovery of deep sea hydrothermal vents was groundbreaking for another reason.
Their unique conditions of immense energy and the abundant nutrients of these chemical gardens
got scientists speculating whether these vents could be where life on Earth originated. It
remains to this day to be only a theory, yet it is one that is supported by an abundance of evidence.
Firstly, some of the thermophilic, or heat-loving,
vent microbes are among the most primitive organisms known on Earth.
Evidence is also given by the fact that many of the building blocks of life
are found at the vents, suggesting that the precursors of life harnessed carbon dioxide and
hydrogen available in those primitive conditions to create these complex organic molecules such as
amino acids and nucleotides. This eventually drove the creation of cellular membranes,
complicated proteins and ribonucleic acid (RNA), a molecule similar to DNA.
In conclusion, hydrothermal vents support unique ecosystems and
their communities of both highly-specialised, as well as simple organisms in the deep ocean. The
islands of abundance they create in the otherwise barren depths are sites of outstanding scientific
interest, providing a new insight into what is truly necessary in order for life to survive.
But with these vents only having been discovered as recently as the 1970s,
who knows what else lies in the depths, in wait of discovery?