roots to the seabed, planktonic creatures mostly drift, though
as those World War II pings revealed, some are masters at
swimming up and down. Many forms of plankton are so tiny
you need a microscope to see them, but they are the unsung
heroes of the planet’s air.
You can thank species of sun-loving plankton for the
breath you just took. Until about two billion years ago, the
planet’s atmosphere was breathlessly devoid of oxygen. But
then a distant cousin of today’s blue-green algae began using
the sun’s rays to split water into hydrogen and oxygen. Earth
hasn’t been the same since.
Today, half of the atmosphere’s oxygen comes from ocean
plankton—every other breath. Plankton comes in all shapes
and sizes, but scientists divide them into two categories.
Phytoplankton are the microscopic algae and other cells
that drift in the sun-infused upper layer of the ocean. Think
of them as the plants of the sea, the oxygen producers.
Zooplankton are the larger animals that typically feed on
the phytoplankton. Think of them of the sea’s insects, snails,
and worms.
Small and large, plant and animal, they do amazing things.
One zooplankton called phronima chomps on smaller
plankton and uses their body parts to make protective
cellulose barrels. The mother lives in the barrel, zealously
guarding her young, while males bolt at the slightest danger.
zooplankton species has little blue sails; washing up on the
beach, their colonies look like a regatta of tiny blue boats.
Another jellyfish can literally reverse its aging process; its
nickname is “the immortal jellyfish.”
When phytoplankton die, they emit cloud-forming
chemicals that give beaches their intoxicating and briny smell.
Some phytoplankton also are killers: Shaped like glistening
needles, they secrete neurotoxins that find their way into
shellfish and animals that feed on them. After one toxic algal
bloom in 1961 in Monterey Bay, Calif., thousands of poisoned
seabirds dive-bombed houses and cars and piled up dead on
streets. The phenomenon inspired Alfred Hitchcock’s movie,
“The Birds.”
Counting plankton is like counting stars, though instead
of stars, some species look like Christmas ornaments and the
Leaning Tower of Pisa. A single teaspoon of seawater might
contain a million phytoplankton bacteria and 100 million
planktonic viruses that feed on them. Viruses alone have
an overall biomass in the oceans of 75 million blue whales.
Mostly unseen, these hordes of planktonic algae and viruses
are in a constant state of biochemical warfare: The viruses
attack the algae while the algae develop special plates of armor.
Much of what we know about plankton has been
discovered since World War II, thanks in part to those
curious Division 6 scientists, but also because of people
like Dennis Allen, who one morning in
January, stands in a johnboat in a South
Carolina marsh, wondering: Why does
the water look so weird?
Curiosity and the Long View
Phronima’s fierce appearance is the inspiration for the
creature in the movie “Alien.”
Some algae and bacteria have red pigments and grow
so dense they color vast areas of the ocean; floating pink
blankets of bacteria gave the Red Sea its name. One
26
Dennis Allen is resident director of the
University of South Carolina’s Belle
W. Baruch Marine Field Laboratory,
a scie