Hydrothermal Vents

Deep down under the sea where hydrothermal vents are found, the pressure is immense, the temperatures are freezing, and life thrives in its own unique way. Within pitch-darkness, hydrothermal vents are ecosystems that rely on microbes for primary production by using chemicals as an energy source through the process of chemosynthesis. 

These chemoautotrophs harvest the inorganic material that is released from hydrothermal vents and strips them of their electrons to create carbon molecules and nutrients. The metabolic processes of these microorganisms are diverse resulting in a wide range of electron sources, such as iron, magnesium, ammonia or even hydrogen. However, the most common pathways involve using sulfur compounds. Microbes have diversified themselves to survive and thrive in a variety of conditions that come with hydrothermal vents. Fluids around vents can range from being near freezing to being over 400°C. The water could be oxygen-rich with a pH of normal sea water or it could have no oxygen but be rich in sulfide and metals with pH as acidic as lemon juice. 

Larger vent animals rely on these microbes as the main food source for the food web either from grazing on microbial flats or through suspension feeding. Many vent animals form symbiotic relationships with chemoautotrophic microbes by having them occupy tissues of specialized organs or living on the animal’s body. In these symbiotic relationships, the microbes are given a stable habitat where they can interact with their electron donors and acceptors, and in return the microbes will give their host organic carbon as payment.

Imagine a worm-like creature as tall as a refrigerator living at the bottom of the sea. In the 1970s, geologists found masses of these living at hydrothermal vents in the eastern Pacific Ocean and they became known as tubeworms. Tubeworms are scientifically known as Riftia pachyptila and were one of the first animals observed when the vents were first discovered because of their size - they can grow up to 3 m (~10 feet) - and because of the bright red feather-like plumes at their tips which are their gills. 

This worm-shaped animal is housed in a hard tube-like shell made of chitin, which allows the tubeworm to live inside where it can retract in and out like a turtle to avoid predators. Tubeworms have adapted to handle the harsh conditions at hydrothermal vents: extreme pressure, complete darkness, and the extreme heat of the water that emerges from the vents. One of the most unique features of this species is the lack of a mouth which means they cannot eat or digest anything. Although they do not have a digestive track, they do have a specialized organ containing chemosynthetic bacteria. These bacteria use chemical energy from the vent fluids to convert inorganic material into nutrition used by the tubeworm. This process is known as chemosynthesis. The tubeworm’s bright red gills have a special form of hemoglobin, the protein in the worm’s blood that can carry oxygen, hydrogen sulfide, and carbon dioxide, which are essential for the chemosynthetic bacteria and, ultimately, the food for the Giant Tube worm.

Life for the Giant Tube worms is entirely dependent on the hydrothermal fluids emerging from the sea floor. When the hydrothermal vent stops venting it will cause the colonies of tubeworms to die.   

If you look in shallow waters near the coast, it’s pretty common to find clams and other shellfish. What if I told you there are clams that can survive in the deepest parts of the ocean? Hydrothermal vents are found on the seafloor where hot fluids and minerals come up out of the cracks. Calyptogena magnifica is a species of large white clams that live near these hydrothermal vents. Unlike clams that live in shallower waters that survive on filter feeding of algae, hydrothermal vent clams rely on chemosynthesis to feed themselves. Photosynthesis, where organisms use sunlight to create energy, can only be done in the surface waters of the ocean. In the deep sea where hydrothermal vents are found, no sunlight reaches, therefore organisms will turn towards chemosynthesis, which uses different chemical compounds to create energy. These clams require an endosymbiotic relationship with tiny organisms known as microbes that can be found in their gill tissue. Vent fluids contain a high amount of reduced sulfur compounds which clams take in, and the bacteria is used for carbon fixation, which then returns vital organic carbon back to the clam. 

Clams rely on these bacteria in their bodies that will take the chemical compounds released in the hydrothermal vent fluid and turn it into nutrients for the clam. This is known as a symbiotic relationship, where both the clam and the bacteria benefit from each other. Without this “friendship”, the clam nor the bacteria would be able to survive in the harsh environment of a deep sea hydrothermal vent. More research needs to be done on how the clams acquire these microbes, but it has been shown that the bacteria are passed on through the clams' eggs.

Pompeii worms are found exclusively in the Pacific Ocean. They live in self-fashioned tubes on the seafloor near hydrothermal vents, which spew out hot, black, chemically rich water. These tubes are built by a secretion of the worms that mineralizes over time. Together, the tubes form an area where a colony of Pompeii worms can exist. The worms then spend their lives with their tails inserted in the tubes, which contain very hot water, while exposing their heads to the cooler water on the outside.

Pompeii worms are the second most heat tolerant animals known to man (only bested by Tardigrades). They must withstand temperatures ranging from 27 degrees Celsius (or, 80 degrees Fahrenheit) to above boiling at times. A symbiotic relationship between the worms and bacteria, which rely on chemical energy rather than sunlight, makes this possible. The bacteria form a mat on the majority of the worm’s surface, and they also contain specialized enzymes that shield the worm from the extremely hot water. In turn, the bacteria benefit from the carbon respired by the worms. The Pompeii worm also feeds directly on bacteria, collecting them with tentacles or via absorption.

These worms are also specially adapted for the extreme chemical conditions surrounding the vents. For instance, there is a high amount of sulfides and metals present in the tubes they live in, which would seem to pose a threat to their metabolic processes. However, their bodies produce antioxidants to protect against these toxic chemicals. Pompeii worms have also evolved to have a low bodily pH. This enables them to transport oxygen through their blood using less energy. Thus, they are very efficient breathers since there is little oxygen available near the vents where they live.

There are many organisms found inhabiting deep-sea hydrothermal vent systems that rely on bacteria in their tissues to survive. Of these, mussels are among the most versatile. They can be found inhabiting vent systems all over the world and can attribute their versatility to not being as reliant on chemosynthesis by symbiotic bacteria as other vent organisms.

Bathymodiolus azoricus is a species of mussel that is the dominant organism living near hydrothermal vents in the Atlantic Ocean. They are approximately 9 cm (or, 3.5 inches) in length and have a yellow-orange colored shell. These mussels rely on two types of bacteria that exist in their gills for their nutrition.  These bacteria, called endosymbionts, convert sulfur or methane that come from the vents into usable food for the mussels. However, these mussels are mixotrophic meaning they do not solely rely on the bacterial chemosynthesis for food. They can also filter feed to gain nutrients from organic particles in the water and have been observed surviving for short periods without the bacteria in laboratory settings, although they do require the bacteria to survive long term.

While this species of mussel can respond to dynamic environments, human activity may still negatively affect this organism. Deep-sea mining activities have the potential to expose this species to high concentrations of copper by opening new paths for hydrothermal vent fluid to exit near mussel beds. While the mussels do have natural defenses against the harsh chemicals that come from the vents, increased exposure to these elements may be hazardous. A study from 2017 found that their gills could be severely damaged if exposed to high concentrations of copper for brief periods of time. However, it is currently unknown to what extent deep sea mining may impact these isolated organisms.

Imagine living in a place so dark that eyes may not be needed. Hydrothermal vents are a uniquely harsh environment for any marine organism that calls this habitat home. This ecosystem is most recognizable for its volcano-like black smoking vents that release hydrothermal fluids up to 370°C. Besides this, hydrothermal vent systems are defined by their lack of light, high pressure, acidic waters, potentially toxic chemicals, and constant water temperature fluxes varying anywhere from 4°C to 40°C. 

One particularly interesting organism found in this environment is the Vent Shrimp, a member of the Alvinocaridid family. The diet of the Vent Shrimp mainly consists of ectosymbiotic bacteria—meaning bacteria that live on their surface, not within the shrimp. These bacteria are found abundantly near active black smokers, where thousands of the Vent Shrimp crowd. As a primary consumer, the Vent Shrimp play the important role of attaining energy from bacteria to support the rest of the hydrothermal vent food chain. 

While they contain many of the same physical attributes as shallow water shrimp, these are specialized for their brutal environment. For instance, the Vent Shrimp’s eyes cannot see like normal shrimp do, but instead can detect extremely dim light sources, which many researchers believe is used for sensing the heat radiating from active black smokers. This is not only useful for locating ectosymbiotic bacteria, but also for avoiding scorching hot hydrothermal fluid. It has also been discovered that their antennae and nostrils have adapted to detect hydrothermal chemicals. While hydrothermal vents seem to be as distant as an alien world, they are not immune to human impacts. Drifting trash, deep sea mining, and climate change are all affecting daily life within the hydrothermal vent ecosystem.

To adapt to the harsh conditions of hydrothermal vents in the deep sea, one species of snails builds itself a suit of armor. This deep-sea scaly foot snail, called the Sea Pangolin, is a hydrothermal vent gastropod, or more commonly known as a “snail”. The Sea Pangolin uses iron sulfides to create its shell, giving it both an appearance and protection like a suit of armor. 

At hydrothermal vents, sea water has a high metal chemical composition which is different from the open ocean. These scaly foot snails use this unique characteristic for their own benefit. So far, this is the only living species of vent snail known to produce its own armor; however, this was a more common trait in gastropods over 540 million years ago. Sea Pangolins are found only at three vents in the Indian ocean, the only ocean where hydrothermal vent snails have been discovered. They are the first hydrothermal vent species to be classified as “endangered” and so could be at risk from deep sea mining. 

Unlike other hydrothermal vent snails that feed by predation or grazing, Sea Pangolins obtain their energy from endosymbiotic bacteria that fix carbon to provide the snail nutrients. This means they don’t really eat. Sea pangolins are unique in that they have a reduced digestive tract that is filled with these endosymbiotic bacteria. Other vent snails host epi-symbionts, that is bacteria on their gill surfaces. New discoveries are constantly being made about this species along with a push for conservation measures to be put in place to protect the future of Sea Pangolins. 

The greek word nema means thread. This is where the name nematode comes from, and, like the name suggests, these animals have a long narrow, worm-like body. The species of nematodes found at hydrothermal vents deep in the ocean are tiny -- ranging from about the size of an ant to a grain of rice. They have an exterior cuticle or skin that contains sensory neurons so the worms can identify their surroundings, but unlike other organisms, they have no separate membranes or cells and are just a unified mass of cellular material and nuclei. 

Like many other organisms on hydrothermal vents, their distribution is controlled by the characteristics of the vents, including chemical concentrations and temperature. For example, one species, Oncholaimus dyvae, is adapted to high sulfide concentrations and extreme temperatures, which explains why they are only found on active vents, never inactive. This nematode also contains chemosynthetic bacteria typical of other hydrothermal vent animals, which suggests it may also rely on these endosymbiotic bacteria for its nutrition. 

Deep sea mining for the rich mineral deposits at hydrothermal vents poses a threat to the nematodes that are adapted to live only in these extreme conditions. Pollution also harms nematode communities as shown in a long-term study that found an increase in nematode species diversity and abundance as organic waste pollution decreased. Increased temperatures related to climate change have also been found to lead to a decrease in nematode species diversity and abundance.

Although this worm looks like the stinging fireworms that are common in tropical reefs, Archinome rosacea is a species of polychaete that lives near hydrothermal vents. This worm was first described in the eastern Pacific Ocean but is now recognized at hydrothermal vent spreading centers in all oceans. 

Polychaetes are a diverse class of marine worms whose identifying feature is their segmented body, each segment of which has a pair of leg-like parapodia that are covered in bristle-like setae.  These setae are used for a variety of purposes depending on the species. 

Archinome rosacea is relatively large for a polychaete, coming in at ~2 in. (or, 5 cm) long. But what makes it truly special is that while most polychaetes in its habitat sustain themselves by partnering with symbiotic bacteria, this species is a carnivore that feeds on other polychaetes and crustaceans. It has adaptations that let it snatch food from the water above. These include large lips, antennae, and even primitive eyespots. Its digestive system is also split into multiple parts to digest its food more efficiently.

While deep-sea mining and the screening of the seafloor may not have a direct effect on this polychaete, it is theorized that the sounds and lights used by scientists and made by exploration vessels may temporarily disrupt the normal behavior of the invertebrates the worm feeds on, and underwater waves prompted by human activity may cause its prey to drift away.

The Pink Vent Fish, Thermarces cerberus, is tough enough to call the most hostile environment under the sea its home. Hydrothermal vents release toxic gases and heavy metals that most deep-sea creatures cannot tolerate. However, the Pink Vent Fish has many adaptations to make its stay at the vents more comfortable.

Hydrothermal vents fill the surrounding water with toxic hydrogen sulfide gas, making it even harder for deep-sea creatures to access the already limited supply of oxygen. The pink vent fish deals with this by having a much higher amount of hemoglobin in its blood. Hemoglobin is a molecule in the fish’s blood stream responsible for taking oxygen throughout the body. The higher hemoglobin concentration allows Pink Vent Fish to maximize their oxygen intake. 

Hydrothermal vent environments also have much higher concentrations of heavy metals than other areas of the ocean. The heavy metals found at hydrothermal vents include gold, manganese, and copper. In most fish, the presence of these metals in high amounts is fatal as they interfere with the fish’s biology. The Pink Vent Fish can tolerate the higher concentration of heavy metals by trapping the metals in a membrane inside its cells, making the metals unable to affect the fish’s biological processes.

The Pink Vent Fish has evolved over millions of years to become the most common fish found at hydrothermal vents, mainly due to its unique ability to tolerate the low oxygen, and highly toxic environment at the bottom of the ocean.

Hydrothermal vents can seem like an unforgiving, uninhabitable place. In the deep sea, temperatures are typically very cold, but at the vents, water emerges at scorching temperatures, with chemicals like sulfur incredibly common, and the water pressure alone could crush a human in seconds. However, this unique ecosystem is home to hundreds of different resilient species, such as the vent crabs.

Vent crabs are an important predator in this ecosystem, with the most abundant species being Bythograea thermydron. Their abundance makes them the most well-studied species of vent crab, which has helped researchers learn more about the vent crab family as a whole.

Like more recognizable coastal crabs, vent crabs have exoskeletons, jointed appendages, and claws. As larvae, vent crabs are thought to feed on phytoplankton, but as adults switch to tubeworms, mussels, or detritus, which is dead material, at the vents. Females brood their eggs externally on the undersides of their exoskeletons. However, vent crabs have thicker shells than coastal crabs, and they can withstand a temperature range from 2 to 30℃. Some vent crabs even have high concentrations of aluminum and sulfur in their exoskeletons. They also have completely white exoskeletons and reduced eyestalks, since eyesight isn’t entirely necessary in such a dark place. Vent crabs have the ability to detoxify sulfides like other inhabitants of hydrothermal vents, and they are incredibly dependent on the high pressure of their environment, to the point where even eggs hatched in regular atmospheric conditions will not develop any further.

Experiments done to try to study this species found that only conditions that mimicked the high-pressure habitat of hydrothermal vents managed to keep some of the larva alive for a week before they all died.

Some viruses have learned to beat the heat within the ocean and are known to live in or near  hydrothermal vents.

The abundance of viruses in the ocean was only realized in the 1990s. If you add up all the viruses in the world’s oceans, their weight would exceed the weight of all the whales in the ocean. Compared to surface sea water, where the number of viruses exceed 10 million per drop, the number of viruses at hydrothermal vents is largely unknown, but we do know they are diverse and infect many different bacterial hosts. Recently, from a study at three distinctly different hydrothermal vent sites in the Pacific and Caribbean Sea that represented a range in toxic metal concentrations and acid conditions, the viruses appeared to use different mechanisms for survival than their surface ocean counterparts. 

While most viruses keep the archaea and bacteria in check because they are a major source of mortality, at the vents this appears to be different. Most of the viruses at hydrothermal vents are known to be temperate rather than lytic. Temperate viruses lie dormant in the DNA of the host, while lytic viruses recycle their host’s DNA immediately to replicate themselves, killing the host quickly. The high abundance of temperate viruses suggests environmental factors play a large role in determining viral infection strategies.

Did you know that a single deep-sea mining event is estimated to have the same level of impact as a volcanic eruption? Human impacts on oceanic processes have been a grave concern to hydrothermal vent communities. These impacts can range from various commercial activities to scientific research expeditions. However, increased interest in seafloor mining of sulfide deposits, which are produced by the mixing of hot hydrothermal fluids, rich in dissolved metals and sulfur, with cold seawater as the fluids flow out of the seafloor from deep within the underlying crust, has created an issue big enough to threaten many hydrothermal vents and their dependent ecosystems. 

For example, a specific deep drilling event that occurred in coastal Japan led to a complete transformation of the hydrothermal field and colonization of bacteria and invasive species. Many of the marine organisms that live in vent ecosystems are not able to survive elsewhere, therefore species endangerment and extinction are also on the line. 

The long-term geological and biological consequences of mineral extraction are still uncertain, and the understanding of vent ecosystems is crucial to conservation efforts. However foreboding these impacts may seem, there are a few restoration efforts in place. A series of international environmental laws are being used to aid the recognition of active hydrothermal vents across the Earth. Specifically, the International Seabed Authority has created the Mining Code to regulate exploitation of marine minerals. Establishment of these protection laws should reduce the danger of habitat degradation and conserve hydrothermal vent ecosystems for the foreseeable future.