April 14, 2013 | 9
The deepest, darkest, scariest place on the maps I loved pondering as a child was a crescent-shaped canyon in the western Pacific Ocean.
It was called the Mariana Trench, and at the very, very bottom was the lowest point on Earth’s surface, the Challenger Deep. Its floor was seven terrifying miles down.
What was down there? It was fun to imagine. I didn’t know, and it didn’t seem likely anyone would anytime soon.
In 1989, James Cameron had fun imagining what might be at the bottom of a similar canyon when he made the “The Abyss”, which imagined quite a lot at the bottom of an unspecified Caribbean trench. Eleven-year-old me loved it.
Then, last year, he answered the question for himself.
In February and March, he descended to the bottom of both the New Britain and Mariana Trenches, lights and 3-D Hi-Def cameras blazing, in a slender, lime-green sub called the Deepsea Challenger. He also dropped several autonomous landers built on similar design.
He wasn’t the first observer, of course. The manned Bathyscaphe Trieste touched down in 1960 and several remotely operated vehicles have been there since. But the Trieste was unable to observe much, and for whatever reason, little seemed to reach the public about what has been found since. As I wrote back in February, all footage of the Challenger dives and landers are being withheld by National Geographic until their can release their own 3-D feature film. Perfectly understandable, considering they helped bankroll the probably exorbitant operation.
The result, though, is that after all these visits to Challenger Deep, the view of the bottom remains largely and frustratingly shrouded to the rest of us.
When Someone Offers to Tell you What’s at the Bottom of the Deepest Spot on Earth, You Say Yes
A month ago I wrote about about an important presentation at an obscure meeting in New Orleans by a grad student who worked with Cameron’s data. Her name is Natalya Gallo, and she is a first-year graduate student in biological oceanography at the Scripps Institute of Oceanography at UCSD.
Under the supervision of her adviser Lisa Levin, she spent at least 80 hours, and one might reasonably infer considerably more, examining the 80 hours of footage brought back by the landers and sub that James Cameron piloted to the bottom of the New Britain — located off the coast of Papua New Guinea — and Mariana Trenches in February and March 2012. She was looking for megafauna — big animals and other big stuff — and her job was to identify and count what she could.
After I wrote last month I hoped someone was in New Orleans to report on her findings (as it turned out, no one was), she graciously contacted me and offered to tell me about what she found herself. There are days that make this job awesome. The day I got to ask anything I wanted of a person who was one of the few people in the world who knows what’s at the bottom of Challenger Deep was one of those days.
Then, while writing up what she told me, I discovered the American Geophysical Union had posted a recording of the December 2012 presentation in which Cameron, microbiologist Douglas Bartlett, astrobiologist Kevin Hardy, and others presented their findings. It had received only light media coverage at the time. It was a gold-mine. Most of the imagery was hidden by copyright protective coverings. But the audio was not. With these two sources in hand, I’ll try to describe what this team knows about what’s down there.
Are You Going to Eat That?
Sadly, there weren’t any kraken lurking in Challenger Deep — at least not in plain view. But there were other interesting things. The large life forms that Gallo identified fall into three categories: gigantic, nearly invisible, and really weird.
In the first category are amphipods, which are shrimp-lke crustaceans. They’re not very glamorous, but they are very abundant. Where water is found, so too are amphipods. In the Challenger Deep, there seem to be several white or pale pink species. You can see a few starting at 1:01:52 in the AGU video.
What makes the Challenger Deep amphipods special is their size. At the presentation, Bartlett noted that most ocean-going amphipods are the size of the final segment of your thumb. The ones that fit in the team’s traps maxed out at 17 cm. The ones that couldn’t reached 30. That’s one foot long.
Gallo also noticed on video review what looked at first like sticks buried in sand and carefully organized in strange patterns. It was only after watching the tape repeatedly that she realized she was looking at sea cucumbers — sea cucumbers so craftily hidden that Cameron had not even seen them during his dive. Their shade of beige exactly matched the sandy substrate.
Sea cucumbers are echinoderms like sea stars, and they specialize in roving the abyssal plains of the world, harvesting food from the sediment with their feeding appendages. Others site themselves to intercept ocean currents, and catch things that drift by on their feeding tentacles.
The cucumbers of the Challenger Deep were of the second sort. Sea cucumbers like them were dredged from the trench by Soviet trawlers at least as far back as the 1950s, Bartlett said, but the Soviets’ specimens were smaller.
And Soviet trawls were unable to observe their peculiar bottom behavior. Within the same group, the sea cucumbers were all pointed in exactly the same direction, a strategy seemingly calculated to maximize the harvest of the few nutrient-bearing ocean currents that reach the bottom.
The cucumbers also appeared to be frozen in place. The only thing that ever seemed to move, Gallo said, were their feeding appendages.
If Life Gives You Sand, Make a Sand Castle
The last large creature that Cameron’s cameras observed in the Challenger Deep is probably not one you would recognize as being alive at all. Yet they were the most abundant residents of the Deep, reaching concentrations of more than 20 per two minute sample of film.
Gallo described them as “crushed sand castles” jutting from an otherwise featureless plain. The year before Cameron made his dive, Scripps and National Geographic released a drop cam into the Sirena Deep, another part of the Mariana Trench. The resulting video should give you a somewhat fuzzy idea of their appearances. (It also shows there’s at least one other large resident of the Challenger Deep: a jellyfish.)
Somewhere inside those unstable, irregular piles of sand, a giant, filamentous protist called a foraminiferan lives.
Foraminifera — often called forams — are amoeba-like protists. Most forams sprout filamentous, sticky branched pseudopods with which they snag food. They often grow calcium carbonate shells (technically, “tests”) — often of great complexity and beauty — through which they poke said pseudopods. They crawl through the sea sediments of the world snaring prey and generally raising hell.
At the bottom of Challenger Deep, calcium carbonate shells are not an option because the intense pressure — over 1,000 times sea-level — dissolves the mineral. Not a problem. The microscopic forams down there make soft shells, presumably of protein or other organic polymers. The Japanese rover KAIKO discovered over 400 species of soft-shelled forams living in the sediment of Challenger Deep in 2005.
But there’s another option: glass. Sand grains — usually made of silicon dioxide, the main constituent of glass* — withstand Mariana-class pressures. The shells of deceased diatoms and some radiolarian skeletons are also made of silicon dioxide. Ocean sediment has sand grains, cast-off shells, and microbial skeletons to spare. If you can glue those puppies together — perhaps along with some of your own handy fecal pellets — you’ve got yourself the makings of a pressure-proof shell. Often, not much of a “shell”. But a shell nonetheless.
And that is exactly what the xenophyophores at the bottom of the Mariana do. Unfortunately, their sand-shell shanties, though pressure proof, are often not particularly sturdy, thwarting most collection attempts thus far. As a result, we know a lot less about their biology than we would like. We do know they live all over the world at great depth, and seemingly, the deeper and more hostile the environment, the better.
Xenophyophores trap and eat tiny particles by engulfment, as true “amoebas” do. They were not distributed evenly in the trench — some areas have more and some less. The giant protists, Gallo says, are probably siting their homes — as the sea cucumbers do — to maximize current-borne food extraction. As the currents encounter their lumpy dwellings, the water slows and particles drop out.
Inside the sand shanty is a branched collection of tubular filaments that has many nuclei and no cell partitions, leading some to describe them as among the largest “single-celled” organisms on Earth. Plasmodial slime molds — landlubbing crawling bags of cytoplasm of which I am enormously fond — are also contenders for that title.
But according to Christopher Taylor at the Catalogue of Organisms, the idea that xenophyophores are giant amoebas or the world’s largest cells is a bit misleading. Describing them as giant amoebas is not really accurate if they are more of a tubular network. They are only giant cells if you also consider filamentous fungi like zygomycetes that lack cell-wall partitions (the bio-term is “coenocytic“) to be giant cells also. As with so much in biology, Taylor has pointed out, the boundary between single-celled and multi-cellular is not so apparent as you might think.
In my own judgment, xenophyophores seem to be claiming a marine niche similar to the terrestrial niche occupied by lichens. That is, they specialize in taking spots that are so harsh that they have almost no competition for their home. Interestingly, both lichens and xenophyophores also have filamentous bodies and shield themselves with chemicals or foreign objects (there are some lichens that actually live *inside* rock and only pop out out to reproduce), although this may be pure coincidence.
Gallo estimates there could be 50-100 species of xenophyophores in the Challenger Deep.
The Real Challenger Deep Party Scene is Microbial
Bacteria and archaea were not the focus of Gallo’s study, but I’d be remiss not to mention the recent discovery that bacteria thrive in the sediments of Challenger Deep — and are found there in even greater abundance than on the surrounding abyssal plain. Bacteria aren’t the only microbes, and I earlier noted the hundreds of microscopic foraminifera species found there. The authors of this study — Ronnie Glud et al. at the University of Southern Denmark — hypothesize that the canyon walls act as a nutrient funnel that concentrates bacteria chow in the trench. Why that nutrient enrichment doesn’t seem to extend to the top of the food chain, I do not know.
The Cameron team also discovered there are bacteria in the Mariana Trench that blur the boundary between micro- and macroscopic. At the AGU meeting, astrobiologist Kevin Hand described what he called “an astonishingly bizarre microbial ecosystem” on talus blocks in the Sirena Deep (where the drop-cam video of the xenophyophores was taken). There, one of the remote landers filmed dark brown, shag carpet-like bacterial mats sprouting from rocks on the floor. You can see images of them at 1:26:55 and 1:27:12 in the AGU presentation.
These bacteria seem to be living off of the products of a spontaneous, ambient-temperature chemical reaction between rock and seawater. The mats are composed primarily of bacteria called Paracoccus denitificans, which seem to be the primary producers in the system, feeding on hydrogen and methane released by seawater-induced serpentinization of the rocks on which they’re growing. The other bacteria in the mat seem to be the beneficiaries of trickle-down economics from the Paracoccus system.
So much for the Challenger Deep. Given that our bottom time in hours is probably still in the double digits down there, I am sure there are still surprises waiting. But knowing this much is still enormously exciting.
It’s Raining Food
The New Britain Trench — which Cameron visited as a test-run for Mariana — which at 8,200 meters deep is only 2,700 meters shallower than Challenger Deep. But life there was very different. The trench bottom was home to an array of sea cucumbers,sea anemones, jellyfish, comb jellyfish, and giant amphipods. On the walls of the trench were hundreds of white, slender stalked anemones.
Unlike the sea cucumbers of the Mariana, the sea cucumbers of New Britain appeared to be crawling in all directions, sifting the sediment for food, and they dominated the community. The cameras even saw sea pigs, a kinds of leggy pink sea cucumber sporting tentacles and a frilly Liberace-style feeding apparatus. Sea pigs travel in herds across the abyssal plains of the world, and do, in fact, resemble a pig.
Acorn worms were also abundant at the bottom of the New Britain. These peculiar worms are of special significance to us. You may know that echinoderms — like sea stars, sea cucumbers, and sea pigs — are the group of animals most closely related to our own group, the chordates, animals with a dorsal nerve cord.
Acorn worm larvae strongly resemble echinoderm larvae. And adult acorn worms — together only with chordates — have gill-like pharyngeal slits for breathing and feeding. They also have a hollow nerve cord not unlike our own. Thus, it looks like acorn worms, echinoderms, and chordates all shared a common ancestor. The informal name for acorn worms — hemichordates — hints as much.
There’s Something Special about 3.7 Kilometers
Though this post is about trench bottoms, trench walls can also be intriguing. Halfway down the New Britain Trench the cameras encountered a strange community not observed above or below. At the top and bottom of the trench, the large animals lived on the surface of the sediment. But for some reason, around 3.7 km deep, most animals lived in the sediment. This community had been previously observed off the coast of Chile at a similar depth, Gallo said, so there seems to be something special about the depth.
Here, the sand was covered in hundreds, if not thousands, of strange circular patterns in many sizes and shapes. Gallo described them as “rosettes” or a like a spoked wheel without a rim; Cameron called them “starbursts”. Some were perfectly spherical. Some were slanted in one direction. Some overlapped. She counted over 100 in a 2-minute film sample.But what are they?
They did not manage to catch the animal that made them on film. But scientists know from other marine environments that the strange sand art is the product of animals called spoon worms.
Spoon worms belong to the group of segmented worms called Annelids. Earthworms are the most familiar member of the group. The sand pattern size differences seem to indicate a variety of ages and species are present.
Spoon worms have added to the basic annelid body plan a proboscis, which Gallo described as a “tongue”.
The proboscis seems to lick or comb sediments for food, perhaps while their bodies remain comfortably ensconced in the safety of their burrow in the center of the diggings. The worms may have good reason for caution. Two-and-a-half foot long lizard fish — large for the depth — were seen here also, and may keep an eye out for spoon worm snacks.
Together Gallo and I tried to find any image of these rosettes from other ocean sites on the web to share with you here. We failed. “All I can see at these dive sites is thousands of these rosettes,” she said, “and there’s not even a single image.” It is, perhaps, worth pondering this in light of the cornucopia of images available of the moons and planets of our solar system.
What they did not see in either trench were any trilobites, fascinating armored and calcite-lensed creatures that dominated Paleozoic oceans, and have long been thought extinct. Sorry, Brian Switek (and me too, for that matter).
A Tale of Two Trenches
Gallo and her team reached several conclusions from her hours of trench gazing. One, she said, was that biodiversity decreases with depth. This is unsurprising, since food follows the same curve.
The team’s most important finding was this: clearly, there is no one “trench-bottom community”. But why were the communities of the two trenches so different? Based on what scientists know so far, the tale of these two trenches is a story of food.
The New Britain Trench is near Papua New Guinea, and a lot of crap from land ends up down there. Gallo saw palm fronds, leaves, sticks, and even coconuts at the bottom.
But one ecosystem’s trash is another ecosystem’s treasure, and nowhere is that truer than in a trench. Many of the sea anemones, soft corals, and stalked echinoderms called crinoids (also called feather stars or sea lillies) in the New Britain Trench used the debris that fell into the depths as a source of food and base of operations. In turn, they attracted crustaceans and fish to the mounds.
Far out in the western Pacific, there is much less to eat. Land is far away, and the waters above the Mariana Trench are not particularly productive, Gallo said. The macroorganisms that live on the bottom must be masters at surviving on scraps. Only a few large life forms can capitalize on the conditions. But they do, and judging by their abundance, life’s not too bad, pesky Japanese rovers and National Geographic explorers notwithstanding.
This insight makes the prospect of further exploration incredibly exciting. The evidence so far suggests that each trench may have a personality. It suggests that unexplored trenches, due to local constellations of temperature, nutrients, geology, salinity, chemistry, etc., may contain species and ecosystems we have yet to imagine.
In other words, each unexplored ocean trench is a surprise package waiting to be unwrapped. And somewhere, somehow, trilobites — or something even more fantastic — may still be awaiting the pop of flashbulbs and champagne bottles. Let’s go.
*4/15 Clarification: Though sand’s silica is often in the form of crystalline quartz, rather than amorphous glass, as per David Bressan’s comment, below.
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