Photo courtesy of Dr. Monika Bright, Department of Marine Biology, University of Vienna, Austria
By Mohd Amirul Faisal
The Earth’s ocean is already proven as one of the most uncharted areas on the planet. This is given by the fact that we humans only explored less than 10% of these entire bodies of water that also had origins from outer space. Our ocean harbors a wide variety of marine biomes from lush and eye-catching coral reefs to shallow coastal regions that are often visited by a plethora of land-dwelling organisms.
However, the deeper parts have still remained a mystery even with our latest technologies and endeavors. It is filled with all sorts of bizarre marine organisms but what mesmerized the scientific community was the discovery of hydrothermal vents, underwater fissures that are geothermally heated by flowing magma underneath the planet’s crust. It was remarkable that what we thought as barren wastelands void of a single life form turned out to be inhabited by millions of multicellular organisms. One of them, as you might assume are giant tube worms, a common inhabitant that lives near the hydrothermal vents.
Hydrothermal vents are home to an assortment of creatures from crabs to giant tube worms and octopuses.
You might be wondering how did these worms actually thrive and survive in a region that is continuously fueled by extreme scorching temperatures where most marine organisms will perish. Not to mention an entire environment that does not receive any penetration of sunlight which renders the process of photosynthesis useless. The answer lies with another organism or should I say,micro-organism that inhabits within the internal parts of the worms: bacteria.
A pitch black volcanic region
Perhaps it is better if we try to familiarize ourselves with the characteristics of an underwater ecosystem that is as rich as that of a rainforest occupied by numerous hydrothermal vents. To put it simply, these are chambers that can be found near active volcanoes where seawater unceasingly seeps through the vents, circulates within the earth’s crust just above the mantle, and finally escape out of the surface as superheated vent fluid that can reach temperatures up to 400°C (752°F).
What causes the surrounding temperature to be extremely high? As aforementioned earlier, chains of chemical reactions result in the formation of hydrogen sulfide from the reduction process of sulfate through geothermal activities. It turns out that these chemicals provide an essential food source for the bacteria which is something that we are going to explore later.
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Back in the early 90s when technology was still in infancy, scientists thought that the deepest parts of the ocean could not support any lifeforms due to the extreme conditions. These entire very hard-to-reach regions are akin to that of a lifeless underworld, untouched by the sun or as they initially thought.
Not until they decided to disembark on an underwater exploration by using one of the early designs of a submersible that can only fit a few people. What they found sent shockwaves throughout the scientific community. As you expect, a thriving ecosystem inhabited by a plethora of strange marine organisms from clams to crustaceans such as shrimps and crabs. One of the most peculiar would be the giant tube worms, large polychaete worms with their given scientific name,Riftia pachyptila. Believe it or not, they are the first-ever organisms to be described undergoing a symbiosis relationship between an animal and you guessed it, the bacteria symbionts that instead of carrying out photosynthesis like most microbes, sustain themselves through chemosynthesis.
The power of symbiosis
In the natural world, it is all about survival of the fittest and in this case, these giant tube worms had evolved with a solution by colonizing their internal parts with millions of symbiotic bacteria. Many research suggests that the bacteria might have colonized the worms when they are in their larva stage as you can see in the diagram below. Obviously, when it comes to symbiosis, it is a double edge sword as not all of them benefit each other but for the deep-sea giant tube worms, it can be considered as some of the most evolutionary prospects ever documented in nature.
The giant red tube worm is bigger than the majority of species of worms that exist in Earth’s history (can reach a maximum length up to 3 meters long) but even so, it does not have a mouth nor intestines to promote digestion so what source of food that it depends on to survive? The answer has already been mentioned in the previous section and that is sulfur that originates from the heated fissures. You see, for a completely dark environment void of any rays of sunlight penetration, any organism that tries to carry out photosynthesis would be completely impossible. However, chemosynthesis ultimately solves the problem by generating your own food by utilizing chemical energy instead of relying on sunlight energy.
The entire process can be immensely intricate. Nonetheless, for the host and in this case, the giant tube worms to obtain food, it must deliver the essential nutrients they need to ensure the survival of the bacteria symbionts with their provided scientific name; Cand. Endoriftia persephone or Endoriftia for short. To do this, within the internal parts of the worm lies an elongated organ called the trophosome and it is where hemoglobin, a protein molecule mingled in red blood cells that carries oxygen throughout the body. The process here is slightly different though. Instead of exclusively carry oxygen, the hemoglobin protein binds itself with hydrogen sulfide that derives from the hydrothermal vents and oxygen independently.
It is a special type of hemoglobin however because it actually intercepts both the oxygen and sulfur from reacting, hence allows the host to transport these elements directly to the residing bacteria without causing them to create distinct chemical products. What makes these colonies of bacteria special is their remarkable ability to oxidize hydrogen sulfide into pure sulfur which explains the presence of yellow-colored crystals that are formed within the trophosome.
The last stage involves carbon fixation. In a general sense, carbon dioxide that is emitted from the violent reactions occurring near the vents is reduced to pure carbon through the reduction process. At the same time, they also assist in synthesizing specialized molecules by employing both hydrogen sulfide and oxygen into the equation. This organic carbon is then consumed either by digesting the bacteria directly or in the form of soluble organic molecules. Without any fierce competition in such a niche environment, the tube worms are able to flourish in huge populations.
Other deep sea creatures that benefit from such unique microbiome
These giant tube worms are not the only marine organisms that cooperate with their inner microbes. As mentioned before, other groups of deep sea-dwelling worms, clams, and even shrimps also harbored millions of microbes that also carry the same function to ensure one’s survival in a hazardous environment.
If you ever heard of the yeti crab, you might be surprised that the bacteria can be found inhabiting its bristly claws. It is also the same region where it commonly obtains its source of food most of the time just like the denizens of the vents. It is almost like another completely different kind of ecosystem here except that the inhabitants are all 100% microbes. The process of chemosynthesis may explain why some of these animals do not have any guts nor a feasible digestive system unlike most terrestrial animals. They are totally dependent on the microbes which provides them everything that they need to survive.
Our world is made of one humongous reservoir that stores billions of microbes, each group with their own specifications and functions depending on how the host organism would react to its surrounding dynamic environment. As if we initially believed that there would be no life down beneath the depths of the ocean but the natural world keeps on surprising and challenge our minds at the same time.
For these creatures, life at the darkest depths of the ocean can be perilous not if they are equipped with the superheroes that would sustain them for a very long time. In fact, life itself originated from the oceans billions of years ago. The ancestors of bacteria had evolved to complex lifeforms that begin their first steps on land. However, in a merciless natural world, it would be better if one can be filled with microbes that can provide you a lifetime supply of food and nutrients without having to superfluously compete with other organisms.
References:-
- Bright, M. and Lallier, F. (2010). The biology of vestimentiferan tubeworms. Oceanogr. Mar. Biol. 48, 213–266
- Bright, M. Eichinger, I., and von Salvini-Plawen, L. (2012). The metatrochophore of a deep-sea hydrothermal vent vestimentiferans (Polychaeta: Siboglinidae). Org. Divers. Evol. http://dx.doi.org/10.1007/s13127-012-0117
- Bright,M.,Klose,J. and Nussbaumer,A.D. (n.d). Giant tube worms.Current Biology Vol 23 No 6 R224. Retrieved from https://core.ac.uk/download/pdf/82746028.pdf.
- Klose, J., Aistleitner, K., Horn, M., Krenn, L., Dirsch, V., Zehl, M. and Bright, M., 2016. Trophosome of the Deep-Sea Tubeworm Riftia pachyptila Inhibits Bacterial Growth. PLOS ONE, 11(1), p.e0146446.
- Stewart, FJ, Cavanaugh, CM. “Symbiosis of Thioautotrophic Bacteria with Riftia pachyptila.” Prog Mol Subcell Biol., 2006, 41:197-225
- Weber RE, Vinogradov SN. “Nonvertebrate hemoglobins: functions and molecular adaptations.” Physiol Rev, 2001, 81:569–628
- Yong, E., 2017. I Contain Multitudes. 1st ed. Vintage, 20 Vauxhall Bridge Road,London SW1V 2SA: Penguin Random House UK, pp.170-174.
Et illuminare 