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.

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