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 the
n, 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 becaus
e 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 cha
racterised
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 specialis
ed
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, o
r 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
ma
y 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 coexi
st 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 t
he 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 sto
re 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 de
ep 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 abu
ndances 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 sl
ope, 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 li
ving 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. In all, over 590 animal species have been
identified living at hydrothermal vents. And a surprising majority of these organisms are
unique to th
is 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 o
f 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 vents are not the only sea-floor regions where primary
productivity can occur. Along continental margins and down in oceanic
trenches, we find cold seeps. Regions where cool, hydrocarbon-rich water escapes from
the ocean floor and p
rovides the necessary conditions for chemosynthetic assemblages
to form. These mirror vent systems in a number of ways, but they give rise to an alternative
set of conditions for life to adapt to and exploit. Let’s dive in and explore the biology
that thrives here, the geology that anchors it, and the chemistry that fuels it. The geological origins of cold seeps differ
from hydrothermal vents. While vents form from volcanic activity at sea-floor spreading
regions, cold seeps instead arise at the
other end of oceanic plates, where they are subducted
at the continental margin. Their formation begins with the burial of organic material
under sediments on the sea-floor. These organic compounds degrade over time, producing methane.
Over time, geological processes such as the tectonic compression of sediments at subduction
zones forces the methane from deep reservoirs up through the overlying sediments. Anaerobic
microbes dwelling below the sediment surface oxidise this methane using sulphat
e, producing
hydrogen sulphide and bicarbonate ions as a byproduct. This hydrogen sulphide, along
with any residual methane, then serves as a vital energy source for **chemosynthetic**
microbes. Thus, it is a consortium of two distinct sets of microbes that makes primary
productivity possible at cold seeps and lay the foundations of food webs here. The result is an environment remarkably similar
to hydrothermal vents. We have a flux of sulphide and methane at the sea-floor, chemosynthetic
microb
es using these compounds, and an abundance of life exploiting this primary productivity,
fulfilling similar ecological niches and forming biodiversity hotspots in the deep sea. 70% of the Earth’s surface is abyssal plain.
A great, flat sediment-covered region of the deep sea-floor characterised by low biodiversity
due to the absence of primary productivity. But upon finding a cold seep, you’re met
with a sea-floor region that’s booming with life. Beds of bathymodiolus mussels, endemic
to chemosy
nthetic oases, encase the landscape. These animals seem to dominate chemosynthetic
communities due to their ability to form an endosymbiotic association with the microbes.
They host the bacteria within their gills and benefit directly from the food and energy
they produce. The mussels also support other species. Grazers like gastropods and orbiniid
polychaetes, as well as scavengers like shrimp and squat lobsters picking at organic detritus
produced by the mussels. Siboglinid tube-worms, belongi
ng to the same family as the tube-worms
of hydrothermal vents, can be found here too. In a similar fashion to vent tube-worms, they
allow microbes to inhabit their internal trophosome. An organ specially adapted for housing chemosynthetic
microbes. But one key difference between tube-worms at vents and seeps arises from seep environments
being soft-sedimented rather than hard rock. Hydrothermal vents are located at mid-ocean
ridges where sediment hasn’t had time to accumulate on the newly formed
oceanic crust,
so the worms must obtain the sulphides from the water via their plume. In contrast, cold
seep tube-worms are able to burrow **down** **into** sediments and take up sulphide via
their **roots**, rather than just the plumes. They are even able to establish this extensive
root system in the hard, carbonate rocks that form at cold seeps. The continuous supply
of sulphides sustains them for very long periods of time, allowing them be slow-growing. It’s
estimated that they may be able
to live for over 200 years, forming tube-worm ‘bushes’
which can contain hundreds of individual worms. Just like the mussel beds, these bushes create
a complex 3D environment that supports a range of other organisms. Nestled among the cracks and crevices created
by the mussel beds, we find another cold-seep denizen you might recognise from hydrothermal
vents. Although these yeti crabs are a different species to the ones at vents, they exhibit
a similar trophic adaptation, and farm colonies of ba
cteria on their bodies as epibionts inhabiting
their surface. These crabs have developed long hairy claw-bearing arms, or chelae, which
they wave directly in the sulphide-rich seepage. Microbes living among the hairs provide food
for the crabs. Predatory organisms such as octopuses, fish
and larger crabs are attracted to this vibrant community to complete the food chain. Incredibly,
the abundant life of cold seeps significantly reduces the flux of methane from the seafloor
to the water column, a
nd organisms here represent the only biological methane sink in the ocean.
This is called the benthic filter. At cold seeps with bacterial matts, it’s thought
that between 50 and 90% of released methane is consumed and taken out of the system. Since
methane is a potent greenhouse gas which contributes to a warming climate, this consumption of
methane by seafloor life has a major impact on regulating the global climate. Variations in the underlying geological processes
can give rise to different
types of cold seep environments, often with distinct communities
adapted to the unique abiotic challenges. Types of cold seep include mud volcanoes,
gas hydrate beds, asphault seeps, and brine pools. Mud volcanoes form when methane gas
and warm fluidised mud rise from over a kilometre beneath the sea-floor, exploding out of the
sediment in pulses. Over time, this builds up mud in the shape of a cone that protrudes
from the sea-floor and may be many kilometres wide. These are perhaps the most cha
llenging
cold seep variety for life to inhabit, as the soft flowing mud makes it almost impossible
for animals to settle. But along the borders of mud volcanoes beyond the region prone to
eruption, we occasionally find bacterial mats and meadows of tube-worms, thriving on the
methane. Not all sites of methane seepage form mud
volcanoes. Methane hydrate, sometimes called ‘methane ice’, is a frozen form of methane
combined with water that forms in high-pressure, low-temperature conditions, meaning
it is
stable here at the deep sea-floor where it forms mounds. During their 2017 expedition
in the Gulf of Mexico, NOAA’s remotely operated vehicle encountered these gas hydrate mounds
at depths of around 1,050 metres. On the outcrops themselves, the only macrofauna identified
as living directly on the methane hydrate was a species of polychaete called ice worms,
*Hesiocaeca methanicola*. At only 2-4cm long, these worms burrow into the hydrate, creating
small depressions, and survive by grazing
on chemosynthetic bacteria that grow on the
hydrate surface. Around the edges, streams of methane bubbles could be seen rising up
towards the surface, sometimes encased in clear tubes of hydrate. The bubbles seemed
to attract assemblages of fish, darting in and out of the streams of bubbling methane,
as well as fields of anemones and sea stars. This footage, captured by MBARI scientists
in 2011, shows a deep-sea crab mistaking the movement of a stream of methane for food;
as it came in contact
with the bubbles, the gas solidified, coating the crab’s claws
and freezing its mouthparts together, forming a ‘milk moustache’ of solid hydrate. At around 3,000 metres below sea level, also
in the Gulf of Mexico, scientists discovered that vast areas of the sea-floor were covered
with swathes of asphault. These asphault seeps formed when petroleum deposits deep beneath
the sea-floor leaked out into the water. Lighter components rose up towards the surface, and
left behind denser hydrocarbons th
at solidified on the seabed. The result is an underwater
landscape that resembles a cooled lava field. But despite seeming inhospitable, a unique
community of organisms have been found living on and around the asphault, exhibiting a new
kind of cold-seep habitat that remains poorly understood. In 2014, a sonar showed a cluster of structures
at a depth of 1,900 metres in the Gulf of Mexico, with a large size indicative of a
sunken wreck. When explorers from NOAA approached the site with their ROV
, they stumbled upon
an enormous formation splayed out over the seabed like a flower. Later dubbed a ‘tar
lily’, this was discovered to be an extrusion of asphalt. The thick, gooey hydrocarbon mass
had been squeezed out from deep reservoirs in rope-like formations that bent and flowed
into petal shapes, becoming hard and brittle over time. More volatile hydrocarbons dissolved,
so the petals shrunk. Cracks formed, and the lily provided a hard surface that bacteria
could colonise, using the oil fo
r primary production and transforming this extrusion
into a localised chemosynthetic paradise. Anemones and corals anchor themselves on the
ledges, while tube-worms wedge themselves among the many fractures. Another process, salt diapirism, occurs when
salt flats are buried beneath other types of sediment. The salt layer is more buoyant,
so it pushes its way to the surface, forming diapirs or vertical intrustions through the
overlying sediment like salt domes and pillars. Some of the salt dissol
ves and flows into
the diapirs to form a lake of dense, anoxic, hyper-saline water called a brine pool. This
can be 5 times saltier than normal seawater, and it’s the presence of dissolved methane
in the brine that means these surreal pools often coincide with cold seep activity**,** allowing
for chemosynthetic life to thrive. But what makes brine pools unique among cold seep environments
is the zonation of the life found here. Due to the toxic nature of the brine itself, animals
are limited to
inhabiting just the shores of these unusual lakes. The mussel beds form
a 5-metre wide ring that encircles the lake. Dwelling any closer to the brine will result
in toxic shock, while living any further away will starve the endosymbiotic bacteria living
in their gills of the necessary compounds for chemosynthesis. The insular nature of cold seeps is similar
to hydrothermal vents. They can be considered islands of abundance on the sea-floor, isolated
from one another in patches dotted along the c
ontinental margins. But they are also transitory,
existing only briefly, with the seepage of any one site not lasting forever. To understand
how the activity of cold seeps ends, we need to take another look at the process of methane
oxidation within the sediments. As a byproduct of the reaction that forms hydrogen sulphide
for chemosynthesis, bicarbonate ions are released. The bicarbonate ions react with calcium in
seawater to create calcium carbonate in the form of biogenic rock. This explains
the origins
of carbonate reefs, towering rock chimneys, and spires that protrude from the sea-floor
at cold-seep environments. Structures that grow over time, supporting assemblages of
life and microbial reefs for a brief period, before eventually blocking the seepage altogether.
The size of carbonate reefs in some regions of the Atlantic Ocean indicates that persistent
seepage may have been taking place for an estimated 15,000 years. This self-destructive process may give rise
to a pattern of e
cological succession at certain cold seeps, beginning with the arrival of
bacteria which aggregate into bacterial matts. During this initial stage, colonies of mussels
form near the seep, along with their associated fauna. Carbonate outcrops begin to form, attracting
tube-worms which settle and live alongside the mussels. Once seepage stops, the mussel
beds are starved, but tube-worm meadows live on a while longer due to their ability to
burrow down and tap into the sulphide flux directly. When
the carbonate cap grows too
large for even tube-worms to access the sulphides, all that remains is a lifeless rocky outcrop.
But there may be another stage to this succession, as the die-off of tube-worms clears the way
for colonisation by stony corals. The corals, along with other sessile animals, make use
of these outcrops as vital attachment points amidst the soft muds of the abyssal plain.
The deep-sea coral gardens shown here are formed around structures that may well have
once been sites o
f chemosynthetic activity in the form of cold seeps, centuries prior. Overall, cold seeps are an unusual collection
of deep-sea habitats that show off the ability of life to make the most of an energy source
in the most challenging of environments. From carpets of asphalt seeping from ancient hydrocarbon
deposits, to explosive mud volcanoes, and toxic lakes of brine, these regions push the
limits of adaptation in the depths. But they also bring to light the importance of some
deep-sea communitie
s in helping to regulate our climate through providing a benthic filter
for potent greenhouse gases like methane. We’ve taken a look at the two main examples
of chemosynthetic oases - hydrothermal vents and cold seeps - to explore how life has adapted
to survive here. But on the sea-floor, food-fall events have also been known to create temporary
sites of partial chemosynthesis. In particular, the carcasses of whales or the remains of
sunken wood host uniquely specialised assemblages of life. Le
t’s take a closer look. Sometimes, the degradation of large food-falls
at the bottom of the ocean can create partially chemosynthetic environments. In the case of
sunken whale carcasses, the supply of organic material supports an ecological succession
of communities over time. During the first stage, vibrant assemblages of mobile scavengers
gather out of the depths and strip the bones of all flesh in a matter of months. The enrichment
opportunist stage follows, during which invertebrates colonis
e the bones and make the most of any
remaining organic material that enriches the surrounding sediments. But it’s the third
stage, called the sulfophilic stage, when the carcass begins to support chemosynthetic
life. This is due to whale bones being incredibly rich in lipids. After bone-eating osedax worms
begin to colonise and burrow down into the skeleton, anaerobic bacteria are able to get
into the core to exploit the lipids. As they break these down, they produce sulphide as
a byproduct, whi
ch seeps out and can support bacterial matts of chemosynthetic microbes
that carpet the whale bones. The result is a temporary, localised chemosynthetic environment,
able to support a community of animals surviving not on flesh, but on the energy released by
the microbes. We find grazers like amphipods and suspension feeders like tube-worms, blooming
out of the surrounding sediment. But of the organisms that participate in this
stage of succession, very few species are shared with hydrothermal v
ents and cold seeps.
Because these chemosynthetic oases are relatively short-lived, animals here may represent ancestral
species belonging to the lineages of specialised vent and seep inhabitants. Thus, whale-falls
may serve as ecological stepping-stones that once drove the evolution of life to such conditions.
With around 1600 new whale carcasses settling on the sea-floor each year, and with each
one lasting several decades, these islands of abundance are relatively common. The distance
between
carcasses may only be 10s of kilometres, so it’s conceivable that whale-falls may
provide an evolutionary interface between non-chemosynthetic sea-floor scavengers, and
the highly specialised animals that appear well-suited to life at vents and seeps. This is not true, however, of other types
of food-falls. While the high bone-lipid content of whales makes their carcasses unique, most
other carcasses are too small to support such complex communities. These mackerel, attached
to an ROV and sent
to the sea-floor, were devoured in less than a day. This jellyfish,
consisting of soft, gelatinous flesh, was consumed entirely in a matter of hours. Other
large vertebrates like this devil ray observed at 1.2 km deep off the coast of Angola, appear
to be less appetising. This was the first time a natural ray food-fall had ever been
documented, and yet despite being covered with more than 50 eelpouts, a considerable
amount of flesh endures than what you’d expect of a carcass this large. The cart
ilage
that makes up the skeleton is too thin for bone-worms and bacteria to colonise. The sandpaper-like
skin is too tough for the jaws of many scavengers, and the high levels of ammonia found within
the flesh of sharks and rays makes them an unpalatable meal. Even the carrion of larger
elasmobranchs, like this whale shark, are too much of a challenge for most animals.
It’s for this reason that whales are considered keystone species, providing vital ecosystem
services in both life and death. But
perhaps less well-documented than the
scavengers at whale-falls, are the creatures that appear to rely entirely on wood instead. Deep beneath the ocean surface, familiar but
unexpected features can be found resting against the sea-floor. When trees become uprooted
by storms or ships capsize at sea, losing their buoyancy as the pressure of the ocean
forces out any air trapped within, bits of wood sink to the ocean floor where they create
fleeting oases of life. The scarcity of food in parts of t
he deep ocean creates an environment
where very little goes to waste. The animal and microbial life that dwells down here has
become incredibly resourceful, able to make the most of even unexpected resources. So
it comes as no surprise that the deep ocean hosts complex biological communities that
are adapted to thrive on this sunken wood. They’re called wood-falls, and they’re
able to support these biological hotspots by providing both a concentrated source of
nutrients, and a solid surface that
sessile animals can anchor to. Colonies of peculiar bivalves encrust the
surface of the wood. Belonging to the genus *Xylophaga* and growing no longer than an
inch, these bivalves are wood-fall specialists. They are endemic to these regions, not found
in the surrounding sediments or indeed at any other site of chemosynthetic primary production
on the sea-floor. This is because, in order to consume the solid, fibrous wood, they have
had to specialise, evolving a number of traits that not many sp
ecies possess. A ridged, beak-like
shell is used to bore into the wood, while a large muscular foot extends from the shell
and is used for leverage. They behave like living chisels. The bivalves ingest the fragments
they break off, but even now the wood is tricky to digest without help. The polymers cellulose
and lignin which stiffen wood are indigestible to all but some fungi and bacteria. Thus,
*Xylophaga* host endosymbiotic bacteria within their gills that do the hard work of digesting
the wo
od for them. This way of life is similar to that adopted
by another species of bivalve - the inaccurately named Giant Shipworm. Unlike *Xylophaga,*
these can reach a metre in length, but like their smaller cousins they use the two valves
of their shell to bore through wood. While the *Xylophaga* bore through the inner
sapwood and heartwood, squat lobsters of the species *Munidopsis andamanica* instead opt
for the outer layers. Rather than boring, the squat lobsters have developed spoon-like
claw
s that tear away at the wood surface. They too then employ the services of bacteria
to help digest the wood. The bivalves and squat lobsters represent the astounding ability
of life to make the most of any available resource, for these are deep-sea animals,
adapted to thrive on land plants. In many ways, these ecosystems mirror the
detrital communities that form on terrestrial deadwood, where worms and beetles break down
the debris and cycle the nutrients back into the ecosystem and enrich surro
unding sediments.
In fact, both are dominated by crustaceans - on land, woodlice burrow down into felled
stumps and branches, while in the depths it’s their cousins the squat lobsters that take
advantage of this resource. Another similarity is that woodlice, termites and other terrestrial
detritovores must also rely on endosymbiotic bacteria to help digest the wood. But one key difference between deadwood habitats
on land and out at sea can be explained by taking a closer look at the **microbial
**
life of deep-sea wood-falls. The aforementioned bacteria that are able to break down the wood
do so anaerobically, forming sulfide as a byproduct. This sulfide can then be used by
chemosynthetic bacteria, making wood-falls sites of partial chemosynthetic primary production,
similar to whale-falls, and providing an alternative source of nutrition that other animals, not
specially adapted to feed on wood, can exploit. This footage shows wood debris on the sea-floor
being converted into a sulphi
dic hotspot. A microbial sulphur biofilm rapidly develops
on its surface, setting up the foundations of a vibrant food web that relies on these
chemosynthetic microbes. Bathymodiolus mussels are one example of an animal that relies not
on the wood itself, but on the chemosynthetic microbes, exploiting the nutrients they produce.
Unlike the wood-fall specialists, these mussels are not endemic. In fact, they belong to the
same genus that colonise other varieties of chemosynthetic oases. The toweri
ng chimneys
of deep-sea hydrothermal vents. And the carbonate reefs and mounds of methane ice that form
at cold-seeps. Their presence here at wood-falls indicates just how important these isolated
food sources are to deep-sea biodiversity. Evidence suggests that they serve as wooden
stairs for certain animals - ecological stepping stones that allow the mussels to disperse
between vents and seeps, in a similar manner to whale-falls. Both environments may form
corridors on the sea-floor that allow
chemosynthetic communities to spread to new chemically similar
habitats. And they do so by producing thousands of larvae, released into the ocean where the
first stage of their life cycle sees them waterborne. Currents disperse the larvae far
and wide, inevitably bringing some to suitable sites of chemosynthesis where they can settle
and begin to grow. The rapid utilisation and exploitation of
wood by deep-sea organisms means wood-falls are often short-lived.
In the case of sunken vessels, a w
ooden hull may last lass than 100 years before it is
fully decayed, meaning wrecks like this 207 year old whaling ship in the Gulf of Mexico
endure as nothing more than a patch of scattered artefacts. Seen here is part of the ship’s
tryworks, large iron pots that were for used to boil down blubber. But in March of 2022, the wreck of Ernest
Shackleton’s ship Endurance was found with all its wood intact, 107 years after it was
lost in 1915. Situated 10,000 feet Below the Ocean's Surface in Antarct
ica, it appears
suspended in time. This is because wood-falls here are rare, and animals in the Southern
Ocean have not adapted to use wood as a food source. With the nearest land mass, Antarctica,
being devoid of any trees, there has been no pressure to do so. In addition, animals
that are capable of breaking down wood are unable to migrate this far South. The sharp
temperature gradient at the Antarctic Polar Front served as a barrier to these animals. But despite hosting no fauna that is able
to digest the wood itself, Endurance is still an oasis of life for deep-sea creatures that
use it as a solid surface to attach to. The colonisers of Endurance are predominantly
filter feeders, making use of specialised appendages to pull organic particles out of
the ocean - dead animals and excrement, sinking from above. Sea lilies, a type of echinoderm
closely related to starfish, use hair-like structures to adopt this way of life, living
alongside sea squirts which instead trap food by cycling
water in and out through tube-like
siphons. A Brisingid sea star also decorated the helm, waving its arms in the water to
catch debris. Anemones the size of dinner plated adorn the
ship, representing a fascinating phenomenon called polar gigantism. This is the tendency
of polar marine life to grow much larger than tropical counterparts and is likely due to
a combination of cold-driven low metabolic rates and high oxygen availability in the
polar oceans. One unexpected hitch-hiker on the Enduran
ce was a small white crab - the
first time a crab has been observed in the Weddell Sea. Similar communities to those observed at Endurance
tend to dominate metal shipwrecks in other parts of the ocean, including many that endure
from the Second World War. The lack of wood rules out any possibility of partial chemosynthesis.
But nonetheless their rusting hills are utilised by corals, anemones and goose barnacles, along
with other filter feeders. Fish and octopuses may use the wrecks for shelter,
while some
creatures may use them for another reason altogether. Close to the sea-floor, currents
are slowed by friction, preventing the oxygen and food from being transported here. So not
only do shipwrecks provide vital attachment points amidst the soft muds of the abyssal
plain, but they also allow sea-floor dwellers to reach faster, more nutrient and oxygen
rich currents. This is true of the wreck of the Titanic, dotted with long white corals
reaching up into the flow of water. In conclusion
, food-falls highlight the ways
in which seemingly isolated deep-sea ecosystems are more interconnected then they seem at
first look. Whale-falls and wood-falls are vital stepping stones that allow opportunistic
deep-sea creatures to disperse and colonise new regions and adapt to new conditions and
ways of life. But they are also important islands on the sea-floor for non-chemosynthetic
life, with man-made wrecks creating unexpected foundations for corals, anemones, and the
innumerable species t
hat rely on them for food and shelter.
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