SCIENCE NOTES  
Summer 1998
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U N D E R S E A      F L U R R I E S

Miniature habitats of ocean life
fall like snow and
feed the deep sea.

By Mary Beckman

Near the surface of the open sea, a worm-like creature releases the delicate cage it had built around itself. Out of its own mucus, the 3-centimeter-long creature--known as a giant larvacean--had crafted an elaborate house containing chambers within chambers. The sticky, soap-bubble-like chambers captured plankton, protozoa, bacteria and other tidbits from the diverse life swimming in the twilight sea. But the chambers have become clogged with more food than their tenant can eat. The giant larvacean drops its cage and begins to craft another--for the third time today.  
  DUST falls through the oceans and settles on the sea floor in the same way that it settles out of the air and accumulates on furniture. But the dust in the ocean teems with life. Ocean dust, known by oceanographers and scuba divers as marine snow, helps clean the surface layer of the ocean. Scientists believe it may also bring important nutrients into the deep sea, carrying with it diverse populations of microorganisms. Mary Silver, a professor of ocean sciences at the University of California at Santa Cruz, calls an ocean dust particle a"buzzing micropolis of life."
     In the movie Titanic, marine snow is obvious. In scenes where the small submarine explores the sunken ship, floating white particles cloud the water.
     "Most people think, 'Ah, that's pollution,' " explains Silver, one of the first to study marine snow. But the marine dustballs are bits of dead animals and plants and, as Silver describes it, "anything that's out there, all tumbled together." On these dustballs, tiny organisms live, eat and sometimes reproduce. The snow also serves as food for bigger creatures, such as fish.
     On the wall outside of Silver's laboratory hangs a magnified photo of a snow particle. Near it is a photo of ordinary dust, a bit of fluff scrounged from under a file cabinet or lab bench to make a comparison. The two dustballs look like siblings.
The giant larvacean's unwanted cage is destined to become a particle of marine snow, its contribution to the recycling of ocean nutrients. Tumbling end over end, the discarded house slowly floats downward. In the photic zone of the open sea--the upper layer of ocean, in which sunlight penetrates enough to allow plants to grow--the turbulent water slams unicellular algae onto the sticky walls. A free-falling shell fragment from a dead crustacean whirls around the abandoned house and lodges in a mucousy crevice.  
  SILVER wants to know what role snow plays in transferring nutrients into the deep sea. The abundant life below the surface requires sustinence, she says, but "we really don't know how [food] gets into the deep sea."
     Thanks to researchers like Silver, though, we do know that marine snow is likely a major food source for life in the abyssal deep.
     Marine snow forms when plant and animal detritus floating about in the ocean sticks together. The binding substance is mucus, which ocean dwellers make a lot of. The mucus glues together scraps of crustacean shells, remnants of plants, and excrement of animals. The particles are all different shapes and denser than the surrounding ocean, but have many cavities.
     And while the particles have to be at least one-half millimeter in diameter to be considered snow, they can grow to be quite large, on the order of feet. But since the snow is fragile, the large marine snow grows only in calm waters.
     The construction of a good snow particle requires a nucleus--a solid chunk of sticky matter to serve as the basis for the growing particle. A common base for snow in the open ocean is the cast-off houses of the giant larvacean, a small, tubelike hemichordate (which is not quite an invertebrate and not quite a vertebrate) that weaves a chambered house around itself. The chambers collect food for the larvacean nestled within--so effectively, in fact, that a larvacean must drop its house up to four times a day and start again. The released houses shelter living bacteria and plankton and collect other matter on their way downward.
     Another starting point for snow--especially common in the Monterey Bay--is a colony of diatoms. These single-celled algae build walls of opal (a type of silica), and some species can form long chains. When the colonies grow old and die, they "get goopy," Silver explains. Goopy clumps are perfect for collecting dead organisms and waste.
     The turbulence of the water drives the formation of snow and determines the ultimate size of the particles. "Intermediate" turbulence favors snow growth by bumping the particles into each other. Too much turbulence, however, fragments the flimsy aggregates, Silver says.
     Small sea creatures swimming through the snow also influence the size of the particles. Alice Alldredge, a professor of marine biology at the University of California, Santa Barbara, studies how some marine organisms, such as the centimeter-long shrimp-like krill, break up the snow into smaller particles. She wants to understand the relationship between the turbulence caused by various krill activities--like swimming and eating--and the size of snow particles. If krill swimming can affect the size of snow, then perhaps other marine organisms can as well. Ultimately, the size of the particles determines how much of it reaches the bottom of the ocean.
The swirling water mingles the falling house with other abandoned houses. Many of them cling together, but the violent spinning trims fragments from the growing particle. Perhaps in the deeper, calmer waters, this piece of snow can grow to be longer than a foot. For now it is only a centimeter. Bits of feces from small animals land on the growing particle and weigh it down. Sticky colonies of dead diatoms cling to the snow. The drifting particle gains downward momentum.  
  THE SNOW particles provide a structure for marine organisms to live on. Silver describes the particles as island populations of small microbes in the ocean water, "metropolitan regions with nothing in between them." Although many organisms populate both the snow and the surrounding water, certain kinds of bacteria and plankton live exculsively on the snow.
     Some organisms arrive on the snow through their association with animal excrement. Microbes from the guts of marine animals are excreted in feces, becoming part of the snow community when the fecal matter sticks to growing snow particles.
     The particles have "a steamy little environment where a lot is going on," says Silver. The communities of microorganisms change the chemistry of the snow from that of the surrounding water. Alldredge says the microhabitats on the particles can become anaerobic, meaning without oxygen. Species of bacteria that grow under those conditions find the snow suits them much better than the water.
     Marine snow can also be a reservoir for other important elements, like nitrogen. "Nitrogen is in such low concentration that it controls the abundance of life in the sea," says Silver. The particles are also concentrated sources of carbon, and play an important role in carbon cycling, says Alldredge. Microorganisms aren't alone on the snow. The larvae of some worms and some crabs are planktonic--small enough to drift with the algae and swim with the dinoflaggelates (unicellular protozoans that move around by whipping their tails).
     The larval worms and crabs survive by eating the organisms on the particle as the snow falls to the seafloor.
     The ocean floor is host to a myriad of creatures-- mostly squishy ones like sea stars, sea cucumbers, prawns, jellyfish, sponges, urchins--but it exists in darkness, thousands of kilometers away from the energy of the sun. The phytoplankton that capture the sun's energy by photosynthesis can only do so where the sun can penetrate the ocean depth. In the clearest of waters, that is only a couple hundred meters down. "How does the deep sea sustain life?" Silver asks.
     Nutrient-rich marine snow feeds the ocean as it drifts downward. However, not every creature can eat it. The types of creatures that can eat snow are those with scraping mouth parts, like many crustaceans, explains Silver. Filter feeders--animals that collect food as the flowing ocean carries it through their highly specialized filtering mouths, like jellyfish--can't eat the snow because, like peanut butter, it would gum up their filters.
     How do the scientists figure out what eats marine snow? One way is to capture organisms and feed them marine snow in the laboratory, says Alldredge. If the organisms are fed radioactive marine snow, then the researchers can measure the radioactivity in their guts. This method not only verifies that the creature ate the snow, but it also indicates how much it ate.
     Eating marine snow provides a shortcut in the food chain for the feeding animals. Since the snow collects and concentrates nutrients, animals who eat it sustain themselves easier than without it.
Different species of bacteria and phytoplankton find themselves plastered to the surface of this falling bit of goo. The bacteria congregate in crevices. As these single-celled organisms divide and multiply, their metabolism alters the environment of their chambers. The oxygen content falls to zero in some crevices. The nitrogen and phosphorus within the snow particle increase dramatically. These elements, prized by the inhabitants of this undersea world, are likely to attract other organisms to the island habitat, to either live on it or consume it.  

 

AN IMPORTANT ecological role that snow plays is in moving atmospheric carbon from the surface of the oceans to the seafloor. By removing carbon from the atmosphere and holding onto it, "the biological pump can affect the climate over thousands of years," says Alldredge.
     The biological pump works like this: Atmospheric carbon dioxide dissolves in the ocean water, and algae take up the gas as they photosynthesize. The algae contribute to the formation of the debris that drops down in snow. "Some small percentage of the total carbon makes it as marine snow to the bottom of the ocean," she says. This percentage can be as high as 10% of the total carbon via coastal waters, and is less than 1% in the deep oceans.
     Only since the late 1970s have scientists recognized that marine snow is a vital part of the ocean ecosystem, something worth the time and energy--and money--to study. Silver pioneered research of marine snow, says Alldredge. In fact, it was on Silver's urging that undergraduates in one of Silver's classes made the first accurate studies of marine snow.
     The thought that marine snow might exist hails back to the 1950s, when Rachel Carson, author of "The Silent Spring" wrote an earlier book called "The Sea Around Us." In it Carson hypothesized that dust in the air and microbes on the surface of the ocean must--in response to gravity perhaps, or to whatever forces necessitate the weekly dusting of bookshelves--float down to the bottom of the sea. She painted a picture of all these microbial corpses wafting to the seafloor, like continuously falling snow. Carson called it "the long snowfall," and marine scientists everywhere became familiar with her idea.
     Japanese scientists were the first to notice the snowfall. After the second world war, research submersibles came into vogue. While exploring the depths of the ocean, "they looked out the window and saw all this ... stuff out there, and said, 'Oh, that must be Rachel's snow.'"
     How did snow go unstudied for two decades? To get water samples, marine scientists would fill a container with ocean water and bring it up to their vessels, Silver explains. "And to get a good representative sample,"--she holds an imaginary container full of sea creatures and plants and the fragile marine snow--"they'd shake the hell out of it." By shaking it, the scientists thought they would get a good mix of the organisms in the water, she says with a laugh. And yet these delicate houses made of dust were destroyed, and nobody was the wiser.
     Silver began studying marine snow almost accidentally, by scuba diving students. Now central to much of marine biology, diving was considered recreational in the 1970s, and "not for rigorous science." Two undergraduates, Jonathan Trent and Alan Shanks, though, wanted to use scuba diving to study invertebrates in Monterey Bay. Silver tells how they came back bemoaning the exercise as futile.
     "There was all this stuff in the water. The visibility was terrible. I told them to stop complaining and look at the stuff that's hiding the animals. I believe that you should be an opportunist."
     In other words: If life throws you ocean dustballs, study marine snow.
     Jonathan Trent is now a researcher at Argonne National Laboratory in Illinois. "Mary helped us to realize that whatever it was, nobody knew what it was. It was potentially important," he says. "She motivated us, and she gave us the opportunity to dive."
     The first question the two wanted to answer was how much marine snow was there? They devised lighted rings with propellors at the back that would float in the water. As the propellors moved the rings through the ocean, the students counted the particles of marine snow that passed through them.
     They measured the distance that a ring traveled through the water. The area of the ring, which was a circle, multiplied by the distance it traveled gave them a "virtual cylinder" with which they could determine how many particles were in a volume of water. Finally, using a simple device, scientists could measure the density of the snow in the ocean.
     They found that the density varied with the seasonal vegetative growth in the sea. The more growth, the more snowfall.
     The divers then scooped up pitcherfuls of ocean and pulled out the snow to determine what organisms lived on the dustballs. And since they knew how many particles were in the volume of water they collected, they could measure the concentration of plankton, bacteria and feces on the snow and free-floating in the water. Often, the concentrations on the snow were much higher--tens to thousands times higher.
     These initial studies of snow "helped to open up the field," says Trent. Since then, researchers have found many different organisms on the snow. Silver shows a picture of copepods--planktonic crustaceans--mating on a particle.
     Methods of studying the snow have since become more sophisticated than self-propelled rings. Alldredge measures the micro-environment of the snow by positioning small electrodes in a particle. Particles 4-5 millimeters long can be analyzed this way.
Several hundred meters lower, the island of organisms blasts through a layer of zooplankton--small creatures, including the larvae of some larger arthropods like crabs, that feed on microscopic plants and other small creatures. The particle's passage disturbs the horizontally drifting group of animals with a vertical wake, and it briefly acquires a tail like a comet. A colony of copepods--microscopic crustaceans with jointed legs, long antennae and translucent shells--find themselves adrift on the snow. Finding a store of plankton to nibble on, they settle in for some eating and mating. A larval crab emerges from the zooplankton and alights on the surface of the snow. It looks like an evil tadpole, clear with big eyes and a long pointed tail. It scrapes the phytoplankton off the surface of the snow. It feeds and grows larger as the particle zooms through the midwater, falling 30 meters every hour.  

 

CARSON'S HYPOTHESIS of "the long snowfall" predicted that particulate matter would gently float down to the bottom of the ocean. She suggested that dead phytoplankton, given their lightness and shape, should take hundreds of thousands of years to sink 4000 meters to the seafloor. And the ocean currents would carry the sinking detritus around the globe while it fell. An organism should never land directly below the place that it started.
      The nuclear bomb testing in the 1950s introduced distinct radioactive atoms of elements into the atmosphere. Uncommon forms of the elements cesium and rubidium exploded into the air, and rain carried them down to the ocean surface. There, organisms incorporated them into their cells, and upon their death, the novel elements made their way down to the seafloor.
      Looking for these elements, researchers found them in the ocean floor only two weeks after they hit the sea. Silver emphasizes the "two weeks." Two weeks is considerably faster than hundreds of thousands of years.
      In addition to their surprising speed, the organisms' debris was found underneath where they had lived. Because certain species grow only in certain areas, it is obvious, says Silver, where the dead organisms came from. Scientists wondered how the elements fell so fast.
      It was the geologists decades later who put it together, Silver says. They thought that surface critters eat all sorts of fine particles, and pass them into their waste. Their waste forms pellets--many arthropods cover their waste with membranes. The membrane-bound pellets in the ocean floor are full of geologic information derived from the ocean surface.
     To test the idea that the cesium and rubidium were transferred to the sea bottom by way of animal feces, the researchers measured feces' speed. Silver gets animated as she laughs at the scientists. "It became a fetish--take droppings of organisms and time their waste [falling through the water]. It was called the fecal express." It turned out that the fecal express moved at over a kilometer a day, straight down.
      The chemists and the government were interested in studying how droppings carried the elements down, but Silver says it should've been the biologists who were curious. The biologists, though, hadn't yet considered the problem of food getting to the ocean floor--they hadn't yet considered the problem of the ocean.
      Alldredge says that Silver helped devise sediment traps--giant cones that floated in the sea and collected sinking debris. Analyzing what got caught, the chemists and Silver learned that the fecal pellets usually came down stuck to marine snow. Marine snow used the pellets as ballast to make them sink to the floor. The combination slowed the feces but moved the snow faster.
      Silver suggested that marine snow, by aggregating the feces, was probably clearing debris from the ocean's surface, making way for growing plankton. "And I told [the government] they should fund me to study it," she says. And they did.
The growing snow particle descends into the deepest water of the ocean, slowing as the water becomes colder and denser. Grazing animals swim through the falling snow, eating some of it, breaking up other particles by their motions. Luck is with this community of organisms, though, as their island makes it untouched through the last school of deep-sea fish and lands on the ocean floor almost a week after the empty house was cast off by the giant larvacean thousands of kilometers above. The community survives the long trek, although the copepods have devoured quite of bit of the phytoplankton. The snow particle that plunks down on the benthic meadow is almost six centimeters long. The hitchhiking crab lumbers off, looking for fresh food. On the ocean floor, a hungry sea cucumber begins a laborious meter-long crawl toward the gathering snow.