Today's video is sponsored by Klima.
The deep sea can be a barren realm. With increasing depth, we find an exponential decline in biomass
which has driven creatures of the deep to adapt in weird and wonderful ways. Generally,
these organisms must rely on marine snow as a source of food. A trickle of fecal pellets
and dead organic material that drifts downwards from the surface waters where photosynthetic
primary productivity is possible. Below 200 metres, levels of ambient light from the sun
are too low for photosynthesis to occur. And at around 1,000 metres, aside from the infrequent
twinkling of bioluminescence, the ocean is drowned in pure darkness. Without photosynthesis,
the supply of scraps are all that remain to nourish any life. However, on the deep sea
floor, there are important regions where primary production is possible via a different mechanism
called chemosynthesis. These are the chemosynthetic oases of the deep sea, which represent some
of the only locations on Earth where the ultimate source of energy for life is not sunlight,
but the Earth itself. In this series of films, we’ll delve into their formation and ecology,
as well as the threats they face, and the importance of stewardship for these fascinating
environments.
The process of chemosynthesis is similar to photosynthesis. Both can be defined as the
creation of organic matter from the fixation of inorganic carbon using energy. But what
differs is the source of that energy. In parts of the deep sea, primary production is fuelled
by chemical energy, rather than energy from the sun. But this can only take place at certain
sea-floor environments where the required chemicals are released into the water. The
two main examples of such environments are hydrothermal vents, and cold seeps.
The former were only discovered in 1977 when scientists were exploring an oceanic spreading
ridge near the Galapagos Islands. What they discovered was a hidden world that revolutionised
our understanding of how and where life on Earth can exist. Since then, hundreds more
vent field have been discovered, often at depths of 2km or more, along Earth’s convergent
plate boundaries and at sea-floor spreading regions where the oceanic crust is moving
apart.
One major site of high vent abundance is the East Pacific Rise, where the fast spreading
rates have created vent fields dotted along the ridge, 10s of kilometres apart. In contrast,
the vent fields of the much slower spreading Mid-Atlantic Ridge may be 100s of kilometres
apart.
They form here because the rifting of tectonic plates creates fissures in the crust, and
allows hot magma from deep within the Earth to rise closer to the seabed. Upper parts
of the sea floor are very permeable. Cold seawater enters, and percolates down through
the crust where it becomes superheated and takes up minerals from the surrounding rocks.
This mineral-rich fluid then jets back into the ocean at extremely high velocities, and
temperatures exceeding 400°C. As the fluids mix with cold seawater, the dissolved minerals
precipitate out in smoke-like billows, and build towering chimney structures on the sea
floor.
There are a few varieties of hydrothermal vents, characterised by the specific mineral
content of the vent fluid. Black smokers emit the hottest, darkest plumes, forming chimneys
over 50 meters tall (180 feet) with high levels of sulphides 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.
Although the throat of vent chimneys can reach around 400°C, there is a very sharp temperature
gradient between the fluid and the surrounding seawater. Across a distance of around 10cm,
temperatures can drop from over 300°C to just 2°C.
Most vent animals live at far cooler temperatures. But prokaryotic microbes, including forms
of both archaea and bacteria, are able to tolerate fluids as hot as 122°C. Here, they
carry out chemosynthesis via a number of different pathways that depend on the specific conditions
of their micro-environment and the chemicals that are present. Typically, they use energy
stored in the chemical bonds of hydrogen sulfide and methane to create glucose from
water and dissolved carbon dioxide.
The result of this chemosynthetic primary productivity is the presence of vast assemblages
of animal life, concentrated at these regions. The supply of nutrients forms the basis of
a food web for a diverse community of specialised organisms. An oasis of life in the deep.
To understand just how significant these communities are, you only have to compare the life of
hydrothermal vents with non-chemosynthetic deep sea environments. Out on the abyssal
plain, life is present but scattered. Animals must spread out in order to stand a chance
of gaining enough nutrients from marine snow to sustain themselves. But at vent systems,
the chimneys are encased with dense colonies of rust-coloured snails, swarms of deep-sea
shrimp, or expansive aggregations of ghostly white crabs competing for space on the rocks.
Remarkably, these varied and abundant species are all sharing a single resource. They all
rely on the chemosynthetic microbes as a source of food, meaning vents are sites of significant
interspecific competition. That is, competition between members of different species. Often,
interspecific competition can lead to the extinction of one or more of the species competing.
The organism that is less suitably adapted may lose out on the resources is requires,
and become out-competed.
The idea that in a stable ecosystem, no two species can have exactly the same niche and
stably co-exist, is known as the competitive exclusion principle. But when this doesn’t
lead to extinction, interspecific competition instead causes specialisation of the different
animals. A phenomenon called resource partitioning occurs, where species with overlapping fundamental
niches evolve different adaptations. It helps the species coexist because there is less
direct competition between them. This is what occurred at hydrothermal vents to make them
so stable. The competing crabs, worms and shrimps may all be in pursuit of the same
resources, but they have developed very different ways of acquiring them.
Squat lobsters and limpets graze the microbial matts that surround many of the chimneys.
We also find suspension feeders, like deep-sea mussels, feeding on free-living microbes that
are suspended in the water.
Yeti crabs farm the bacteria in filamentous hair-like colonies on their bodies, reducing
the pressure on the crabs to compete for space with other species like shrimps. The crabs
are able to move around and take the bacteria with them, with the microbes acting as epibionts
inhabiting their surface. Contrastingly, giant tube worms are sessile, meaning they are fixed
in one place and cannot move. Their competitive advantage arises from their ability to form
an endosymbiotic relationship with the microbes. They store them **within** their tubes, effectively
holding them captive and benefitting from all of the nutrients they produce. 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 predatory grazers like limpets and crabs.
Another denizen of deep sea vents, the Pompeii worm, farms bacterial colonies in a similar
fashion to yeti crabs, but its higher thermal tolerance allows it to inhabit locations on
the vent structures that are far hotter than those that the crabs can endure. Here, they
dwell within U-shaped tubes which can reach temperatures up to 80°C. Thus, much like
creatures of the rocky intertidal zone, there is zonation between different animal species,
which occurs due to the presence of a temperature gradient and varying abundances of different
microbe varieties. In a way, the worms and crabs have become geographically isolated
from one another within the same vent system.
All the creatures we’ve discussed so far can be classed as primary consumers, but organisms
from higher trophic levels are also present. The octopus, for example, is one of the top
predators of deep-sea vents, along with white zoarcid fish which feed on the tube worms
and shrimps. Some deep-sea skates, which tend to dwell along the continental slope, visit
hydrothermal vents to feed, but also to lay their eggs. They do so in order to use the
volcanic heat to accelerate egg development and reduce the usually years-long incubation
time.
The zonation of life at vents leads to a higher abundance of filter feeders and predators
in the periphery, further from the chimneys. Animals like stalked barnacles and predatory
anemones are less tolerant of the chemical-rich, low-oxygen conditions found closer to the
fluids, but they can still make a living here. Beyond that, we find non-vent deep sea fauna
existing on the abyssal plain near the vent systems at higher abundances than they’re
typically found. This is because, even hundreds of metres away from the vent itself, animals
can still make use of some of the exported organic matter produced as a result of the
chemosynthetic primary production.
~~On an oceanic scale, we find that the geographic isolation of separate vent fields leads to
distinct communities of endemic organisms. For example, we tend to find giant tube worms
in great abundance along the Pacific Mid-Ocean Ridge, while they appear altogether absent
in parts of the Mid-Atlantic Ridge where it’s the shrimps that dominate instead.~~
In all, over 590 animal species have been identified living at hydrothermal vents. And
a surprising majority of these organisms are unique to this environment, having become
specialised in such a way that means they rely entirely on the chemosynthetic conditions
of the vents.
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
led to scientists speculating whether these vents could be where life on Earth originated.
Although unproven, there is substantial evidence to suggest this may be the case. 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 chemical
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.
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 they are
not the only major chemosynthetic oases in the deep sea. In part 2, we’ll explore the
life of cold seeps or cold vents. These are regions where cold, hydrocarbon-rich water
escapes from the ocean floor, hosting their own distinct assemblages of life, and forming
peculiar landscapes at the bottom
of the ocean.