By: Naomi Grace-Decker
Deep in the dark water, at a pressure high enough to easily implode any terrestrial body, life still seems to find a way to thrive. The deep sea is a cold dark scary place in our minds, but life has amazing ways of surviving, and within the deep a whole alien world lives. The deep sea has been a stable, thriving home for so many millions of years, deep sea species such as mollusks, jellyfish, corals, and sharks have survived all five of planet Earth’s mass extinctions.
Let’s take a moment to step outside and look at the abundance of gifts our planet has provided us. The plants appreciate the rain, the bugs appreciate the plants, the birds appreciate the bugs, and so forth. Even in the frigid expanse of polar deserts, a polar bear finds solace in the hunt for a seal. A common thread runs through all these food chains: energy. Most of our terrestrial (and aquatic) ecosystems rely on the sun as a main source of energy, of course, the energy dwindles as the trophic levels go up, but the energy was there to begin with. It’s challenging to envision an ecosystem devoid of the sun yet at just 1,000 meters below the ocean’s surface, sunlight fades into darkness. Now we have to ask the question: How does life sustain itself in the seemingly uninhabitable place on all of Earth?
Nutrients and Energy in the Deep
When there is a will there is a way, and in the deep sea, there is a will. Organisms in the deep sea have many unique ways of capturing energy since it is so sparse. These organisms sustain themselves by consuming excrement and organic matter that descends from the upper layers of the ocean. Creatures of the deep have specifically adapted to gather this material. One name for this material is called “marine snow”.
Marine snow is a mixture of well, feces, and dead organic matter. Marine detritivores thrive off of this. One of these creatures is the vampire squid (Vampyroteuthis infernalis). The small burgundy cephalopods open their tentacles like an upside-down umbrella, gathering as much marine snow as they can (Hoving & Robison, 2012). While these creatures have evolved physically to optimize their foraging, other deep-sea feeding strategies rely solely on chance.
A phenomenon that is unsurprisingly called “whale fall” is just that, a whale carcass falls and sinks to the bottom of the ocean. One whale can feed an ecosystem for well over a year. The 2019 Nautilus Expedition captured one of these events live, showing tiny octopuses, fish, and crustaceans feeding on a whale calf (EVNautilus, 2019).
While all of these ways in which energy is cycled into the deep sea food chain are good and well and ultimately work for many species, these are not renewable. Eventually, the whale carcass will be completely stripped of all its nutrients. One day the currents or environments could sway, throwing off the supply of marine snow. So how do ecosystems in these extreme depths secure a plentiful supply of consistent renewable energy?
Offerings from the Earth: Hydrothermal Vents, Cold Seeps, and Mud Volcanoes
Any 12-year-old should be able to relay the basic information on the geological functions of the Earth. We can start with the layers, we have the inner core, outer core, mantle, and crust. The rock plates that make up the crust shift and move atop the gelatinous mantle. At specific locations in the deep sea, where the crust is thinnest, tectonic plates collide, forming what scientists refer to as subduction zones (Palin & Santosh, 2021). Here, hot magma from the mantle comes into contact with the almost freezing ocean water, creating a chemical reaction in which large vents spring up.
Hydrothermal vents are phenomenal geological formations. Many elements make up a hydrothermal vent system. These vents are typically located on mid-ocean ridges where near-freezing seawater seeps into the cracks in the crust. This water, now in the crust, heats up becoming far less dense and rises out of the crust. This hot mineral-rich water then mixes with the cold water of the ocean floor where the minerals precipitate out, creating the “chimney” of the hydrothermal vent (Georgieva & Smith, 2021).
These vents send out plumes and plumes of hot, toxic, mineral-rich water. With a basic understanding of biology, you would think this area of the ocean would be desolate, with no signs of life, but you would be wrong. Here at these hydrothermal vents, life like none other exists and thrives.
Cold seeps, in contrast, lack the rapid pace and excitement associated with hydrothermal vents. Cold seeps are prevalent in nearly every ocean basin, typically occurring where continental crust transitions to thinner oceanic crust. These locations release similar chemicals and gases to hydrothermal vents, just at a much colder temperature. Cold seeps often harbor fauna with significantly longer lifespans compared to their counterparts inhabiting hydrothermal vents. Currently, over 100 locations of cold seeps are under observation, each characterized by sediments enriched with minerals surrounding the site (Suess, 2014). Fluids emerging from cold seeps originate deep within the Earth’s crust. When the pressure from these fluids and gases increases, they are forced out of weaker, thinner fissures in the crust (Joseph, 2017).
Related to cold seeps, mud volcanoes are also a common culprit of nutrient-rich sediments and water on the ocean floor. Mud volcanoes are a bit more violent than the gentle cold seep. This geological phenomenon occurs in both marine and terrestrial environments. During a mud volcano event, pressurized fluids and gases from the mantle mix with sediments, often resulting in a violent expulsion of hot, mineral-infused mud. Terrestrial mud volcanoes can have significant consequences, as evidenced by the 2006 Java, Indonesia incident, where a drilling team inadvertently triggered a catastrophic event by disturbing a pocket of natural gas in the Earth’s crust. This volcano killed dozens, and displaced an estimated 30,000 individuals, as all of their homes were instantly filled with toxic hot mud that quickly hardened into a cement-like structure (Whitelaw & Sanders, 2008). Aquatic mud volcanoes tend to be much less disastrous. They offer the advantage of heat exchange with the surrounding water, facilitating rapid cooling of the expelled mud. As the mud cools, it transforms into new sediment for the ocean floor, thereby enriching the ecosystem in which it is situated (Foucher et al., 2009). Unlike cold seeps, mud volcanoes do not consistently emit chemical enrichment, experiencing periods of both activity and inactivity. Both cold seeps and mud volcanoes do not need access to a subduction zone of plate boundaries at all to create plumes of minerals, they can happen just wherever natural gas exists within the crust.
The Worm that Started It All
The discovery of Riftia pachyptila was a full-circle moment for biology. First observed in The Galapagos Rift and the East Pacific Rise in the eastern Pacific Ocean, the same area where Charles Darwin conducted much of his work. These worms stirred up much controversy, as they did not have a standard gut or mouth, leading to confusion. Many believed these worms would be filter feeders as many deep sea creatures are, but scientists soon proved otherwise (Stewart & Cavanaugh, 2006).
Sitting in the trophosome of the giant tube worm is an abundance of crystallized sulfur. This is quite odd considering we know that sulfur is usually toxic to life forms on earth. But within the trophosome of the tube worm, there are also bacteria. This bacteria’s primary food source is just that: sulfides. During chemosynthesis, bacteria convert the sulfides breathed in by tube worms into sulfur and energy, enabling their host organisms to thrive.
These vent-dwelling worms have also gained some amazingly specialized adaptations. Worms of the same species often develop different phenotypes depending on the levels of minerals within the environment, lovingly categorized as “long and skinny’ or “short and fat” (Carney et al., 2007). While phenotypic differences are common in all species, Carney et al. also determined that there are genotypical differences in many tube worms of the same species, that are of the same phenotype. The complex physiology of this species can be somewhat altered just based on slight environmental differences.
One of the truly most amazing adaptations of this worm is the adaptation to the worm's hemoglobin. In a typical organism, hemoglobin serves as a transporter of oxygen to tissues throughout the body. In the tube worms hemoglobin, oxygen is bound but so is sulfides. Zinc ions within the worms' hemoglobin effectively bind and transport sulfides throughout the body, ensuring that the excess sulfur does not harm the worm’s tissues. (Hourdez et al., 2002). The zinc ions form a reversible bond with sulfides, allowing the hemoglobin to carry both essential elements.
Within this relationship, bacteria benefit from having a protective host that provides them directly with their food, and the host (tube worms) gets provided with energy they otherwise would not be able to extract.
These two organisms live in harmony and have coevolved together in ways that are hard to even imagine. Tube worms start their life as small larvae, swimming freely in the ocean currents. Only in this stage of life can these tube worms acquire the chemosynthesizing bacteria needed for the rest of their lives (Sato & Sasaki, 2021). Once the larvae have the bacteria, they are set and then can continue down their life path and mature into stationary adult tube worms. The bacteria acquired as larvae remain the tube worms’ lifelong companions, unable to reenter their host, which can live over 200 years, and eventually perish (Gibson et al. 2010).
Examining the fundamental life cycles of these two organisms raises a question: How do they maintain equilibrium and avoid extinction? Tube worms and bacteria have seemingly worked out a deal with each other, one in which the tube worm has decided to grow to a humongous size, very, very slowly, and the bacteria agreed to stay at a sustainable carrying capacity within their host at any size the tube worm may be throughout its life cycle (Sato & Sasaki, 2021). This agreement also brings up the question: How do larvae tube worms get any bacteria if it is all living within their giant parent? Luckily for baby tube worms, when an adult dies, a massive amount of bacteria seeps from its body, allowing this beautiful song and dance of symbiosis to carry on for the rest of each species' respective existences.
Chemosynthetic Bacteria
Many of us have an understanding of how plants make food. Plants harness water and sunlight to generate energy, sustaining themselves in the process. Plant cells do this with the help of an organelle called chloroplast. Animals have a somewhat similar organelle in their cells, this organelle is probably the most famous of them all, the “powerhouse” of the cell, the mitochondria. Mitochondria utilize energy from food consumed by animals to produce ATP, which in turn fuels cellular activities.
In extreme circumstances, there is also another way cells can produce energy for themselves. Through chemosynthesis, bacteria can convert inorganic compounds into energy independently of sunlight. All these bacteria need is CO₂, O₂, and H₂S to create sugar.
Chemosynthesis has evolved multiple times, demonstrating its viability as a sustainable life strategy. As said before, chemosynthesis does not utilize energy from the sun to create energy for cells. Chemosynthesis utilizes chemical energy stored in inorganic compounds and carbon fixation (Marchetti, Lockwood, & Hoopes, 2023). A common chemical used in chemosynthesis is hydrogen sulfide and in that case, the chemical equation is:
CO2 + 4H2S + O2 → CH2O + 4S + 3H2O
This is opposed to the other ways of cellular metabolism such as photosynthesis, respiration, and fermentation.
When going back to chloroplast and mitochondria, there is a biological theory suggesting that these two organelles were once independent, free-living species that subsequently evolved to inhabit the cells of other organisms. This theory is called endosymbiosis (Sato, 2020). The same can be said for these chemosynthetic bacteria and creatures of the deep sea. When looking at tube worms, the whole species would not exist without the help of chemosynthetic symbionts. These bacteria have functionally become a part of the worms' physiology.
Some speculate that life itself may have originated from this very biochemical process (Fitzpatrick, 2013). At the very early stages of the earth, the atmosphere and seas were made of a majority of the chemicals responsible for chemosynthesis (Wald, 1964). We understand that life originated in the oceans, possibly through the random assembly of amino acids aided by entropy, leading to the emergence of life. The origins of life will always be unknown, but the hypothesis that chemosynthetic bacteria are to blame is rather exciting.
Other Species that Have Symbiotic Relationships with Chemosynthetic Bacteria
While the interactions between tube worms and their bacteria are remarkably intricate and specialized, numerous other organisms also rely on chemosynthetic bacteria for assistance.
A fascinating example of symbiosis involving these bacteria is the relationship they share with the yeti crab (Kiwa tyleri). These arctic crabs seem to swarm around hydrothermal vents, relishing in the warm water they provide. These crabs do not need melanin due to their low-light habitat, taking on a white appearance. They derive their fitting name from the setae, or hairs, that cover their entire body. These hair-like features that grow on the crab increase their surface area and are also the perfect breeding ground for bacteria. We would commonly think that having a breeding ground for bacteria directly attached to your body would be disadvantageous, but for the yeti crab, it is a blessing. Yeti crabs have been observed waving their arms near hydrothermal vents, effectively feeding the chemosynthetic bacteria inhabiting the area. With more nutrients, these bacteria can grow their population, and ultimately the crab can eat the bacteria right off their claws. These crabs essentially cultivate their food, while the bacteria benefit from being transported to a food source, creating an ideal situation for both species (Thatje et al., 2015).
While the relationship between yeti crabs and bacteria is captivating, numerous other species also utilize chemosynthetic bacteria in similar ways. Alviniconcha is a species of spikey little snail that resides by hydrothermal vents. They functionally use the bacteria the same way as tube worms, relying on the bacteria to process sulfides and provide nutrients to the snail. There are quite a few bivalves that also utilize the help of bacteria, vesicomyidae, solemyidae, lucinidae, and bathymodiolus are just a few species that do.
How This All Affects the Ecosystem
When looking at your local ecosystem, imagine if suddenly there were no plants. The impact would be ultimately catastrophic. There would be no way to convert the energy coming from the sun into anything consumable within the food chain. The same could be said for ridding an aquatic ecosystem of symbiotic chemosynthesizing bacteria.
While obviously, the hosts of the symbionts benefit tremendously from their interactions, so do many of the other residents of the ocean floor. Predators are attracted to these areas, as large colonies of bivalves, crustaceans, and snails gather in these spots. Detritivores also greatly benefit from organic waste produced by the abundance of organisms congregating in these locations. This symbiotic relationship is functionally equivalent to the primary producers of the terrestrial world.
This relationship has been the basis of so many ecological niches. While many organisms gain their nutrients directly from the surrounding environment, these chemosynthetic bacteria enable entire ecosystems to thrive in environments where traditional energy sources are scarce. Without them, the delicate balance of life in these ecosystems would be disrupted, leading to cascading effects throughout the food web.
Furthermore, the significance of symbiotic relationships extends beyond just nutrient assets. They contribute to the biodiversity and resilience of ecosystems, providing stability in the face of the environmental crisis we are currently in. As we continue to explore and understand these intricate connections, it becomes increasingly clear that preserving these relationships is paramount for the health of our planet.
Symbiotic relationships, such as those between chemosynthetic bacteria and their hosts, play a crucial role in maintaining the balance of ecosystems. By acknowledging and protecting these relationships, we can better safeguard the biodiversity and sustainability of our planet for future generations.
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