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Needs must…

You may have ridden out the pandemic in compact living quarters without, say, much natural light or air conditioning. Perhaps you lived with roommates or family in an atmosphere that, as time wore on, grew increasingly toxic. 

Things could be worse! You could be a member of the Alviniconcha species—specifically, a small, spike-studded snail who thrives in an environment inhospitable to most aquatic life; mere meters from deep-sea hydrothermal vents that constantly spew toxic chemicals into the water. Think you have limited natural light? Try living nearly 10,000 feet below the surface of the ocean, where complete darkness envelops you 24 hours a day, under pressure so intense all the air pockets in your body would instantly collapse. 

And forget Seamless. Forget food—at least the kind you ingest with your mouth. Your survival hinges on bacteria living in your gills (you have gills!) in a symbiotic relationship that provides you with energy, via a process called chemosynthesis. It’s like photosynthesis, but chemosynthesis is driven by chemical reactions instead of light. As there’s no sunlight and minimal oxygen present, the bacteria that dwell within Alviniconcha use hydrogen and sulfur molecules to produce sugars and other macronutrients that the animals then use as food. “There’s very little food so deep in the ocean,” says Dr. Corinna Breusing, postdoctoral researcher at the University of Rhode Island and co-author of a recent paper on the snails and their symbionts. “Having your own food-producing machine is much better than waiting for it to fall to you.” While chemosynthesis is common around hydrothermal vents, it can occur in places outside of vents, such as in cold seeps and whale falls and even salt marshes: anyplace the proper mélange of inorganic compounds is brewing. 

The researchers studied Alviniconcha living at the bottom of the Lau Basin, in the southwestern Pacific Ocean, and found that the type of bacterial symbiont determined where their particular host species could live. “The symbionts have different metabolic capacities and adaptations, so we think that the symbionts influence the distribution of the animal,” Breusing says, adding that snails with Campylobacteria dominated at vents with higher concentrations of sulfide and hydrogen, while those with Gammaproteobacteria were able to thrive at sites with lower concentrations of sulfide and hydrogen. Meaning: your chef-roommate, who happens to live in your respiratory system, also decides where you hang your hat (so to speak).

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Most hydrothermal vent-dwelling animals, such the aforementioned snails and deep-sea anemones, as well as some species of mussels and tube worms, depend on bacteria that they pick up from the environment, but there is a species of deep-sea clam that passes their symbiont down from mother to offspring, like a fancy set of dinner plates. (This is rare in the marine world, Breusing says.) In the case of the deep-sea clams, where the symbiont is inherited, the symbiont cannot thrive outside the host and dies with it. But if a symbiont is taken up from the environment, it can be released back into the environment after its host dies, ready to help feed a brand-new host.

Alviniconcha might not pack the same visual punch as much marine life does much closer to the surface, but their very existence points to the origins of life on Earth. Before oxygen was free and plentiful, microbial life had to work with inorganic compounds like methane and ammonia, which over millennia dissolved into the seas. Much is still murky about how these little snails co-evolved with the bacteria that enable them to survive, but these fascinating ecosystems indicate that our education about life at the margins is just getting started…

“Life at the Edge of Impossible“: ten thousand feet under the sea, these snails thrive with a little help from their friends; from Adrienne Day (@adrienneday).

* Dr. Seuss

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As we examine extremes, we might send redefining birthday greetings to Carl Woese; he was born on this date in 1928. A microbiologist and biophysicist, he made many contributions to biology; but he is best remembered for defining the Archaea (a new domain of life).

For much of the 20th century, prokaryotes were regarded as a single group of organisms and classified based on their biochemistry, morphology and metabolism. In a highly influential 1962 paper, Roger Stanier and C. B. van Niel first established the division of cellular organization into prokaryotes and eukaryotes, defining prokaryotes as those organisms lacking a cell nucleus. It became generally assumed that all life shared a common prokaryotic (implied by the Greek root πρό [pro-], before, in front of) ancestor.

But in 1977 Woese (and his colleague George E. Fox) experimentally disproved this universally held hypothesis. They discovered a kind of microbial life which they called the “archaebacteria” (Archaea), “a third kingdom” of life as distinct from bacteria as plants are from animals, Having defined Archaea as a new “urkingdom” (later domain) which were neither bacteria nor eukaryotes, Woese redrew the taxonomic tree. His three-domain system, based on phylogenetic relationships rather than obvious morphological similarities, divided life into 23 main divisions, incorporated within three domains: Bacteria, Archaea, and Eucarya.

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