In the year 1979, Corliss and a team of geologists, geochemists and geophysicists set out on their first expedition, near the Galapagos Rift, in search of hydrothermal vents. Nobody imagined the need for a biologist on board, as they were expecting barren land or a desert on the ocean floor. Surprisingly, they found a dense population of big white clams, many white crabs, purple octopuses, and a vent site with clusters of white stalked, red-tipped tube worms (Riftia pachyptila), which looked like a field of flowers, 1-2 meters tall, swaying in the wind(1). The very existence of life on the seafloor around seemingly inhospitable hydrothermal vents, actually permitting life to thrive, even without any sunlight, changed the view of the world towards life and Earth.
Hydrothermal vents are commonly found near volcanically active areas- often on mid-ocean ridges, where magma wells up to the surface or close beneath the seafloor. Through cracks or holes on the seafloor, ocean water percolates into the crust and gets heated up by the magma. The fluid gets hotter and leaches metals such as iron, zinc, magnesium and sulfur from nearby rocks. Through the openings on the seafloor, the heated fluid rises back into the sea through vents. Hydrothermal fluid temperatures can reach 400°C (750°F) or more. Clusters of tubeworms can be found attached to the rock surface of these vents. Tubeworms belong to the class Vestimentiferan of the phylum Pogonophora. The tube is made up of chitin and possesses a plume with tentacles that can be extended from the tubes. The plumes are bright red in colour due to the presence of haemoglobin molecules(2).
Tubeworms are gutless, with no mouth or digestive tract. So what do they eat? How do they survive in such extreme conditions? It is interesting to see that they can utilize the poisonous hydrogen sulfide gas coming out from black smoke chimneys of Hydrothermal vents. H2S with a filthy smell is toxic to any lifeform. But living beings in such extreme environments have found a way even to use such a poisonous gas as a source of nutrition. This becomes possible as a result of the mutualistic relationship between the bacteria and the worm which is the founding principle of survival under such conditions.
Tubeworms have a remarkable organ called the trophosome, which houses bacteria. The worm provides bacteria with shelter and in exchange, the bacteria fix carbon dioxide into organic carbon using reduced sulfur compounds through the process called chemosynthesis. It is fascinating to see such a relationship of mutual support deep down under the sea. The community of bacteria and tubeworms flourishes as long as the vent remains active.
But there are still many mysteries pertinent to this giant worm. Some interesting questions are- How did this highly obligate and a perfect mutualistic relationship between the worm and the bacteria evolve? How does the host particularly acquire its symbiont out of the diversity of bacteria present in the ocean? Furthermore, how do they populate the new vent sites and how do larvae migrate and settle? So many questions have remained unanswered yet. I am astounded to see the relationship of mutual support and symbiosis being displayed by tiny bacteria and worms with no mouth or gut. But why do humans, with their sophisticated organs and astute brains, fail to show altruism in times of crisis like the COVID-19 pandemic? Why are we resorting to irrational hoarding of food, medicines, masks and sanitizers, leaving behind those in genuine need ? It is time to self reflect and imbibe something from these unassuming creatures deep beneath the sea- if tubeworms and bacteria can, so can humans.
–Rashmi Kiran
References:
- J. B. Corliss et al., (1979) Submarine thermal springs on the Galapagos Rift. Science 203, 1073
- Felbeck H (1981) Chemoautotrophic potential of the hydrothermal tubeworm Riftia pachyptila Jones (Vestimentifera). Science 213: 336–338.
Manthan 