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