Sallie Penny Chisholm - Chisholm sees big impacts from small sources
Discoverer of world’s most abundant and prolific photosynthetic organism delivers annual Killian Lecture.
David L. Chandler | MIT News Office
March 11, 2015
marine organism that Sallie “Penny” Chisholm, the Lee and Geraldine
Martin Professor in Environmental Studies at MIT, discovered back in
1988 is so vanishingly small that nobody had ever noticed it before. But
this very tiny organism, which Chisholm named Prochlorococcus, turns
out to have a huge impact, despite its diminutive size — just one
micron, or a millionth of a meter, across. In fact, it is responsible
for producing a significant fraction of the oxygen in Earth’s
Studying this remarkably abundant and diverse organism has turned out
to be the core of Chisholm’s career, and has earned her honors
including the National Medal of Science and the James R. Killian Jr.
Faculty Achievement Award, MIT’s highest faculty honor. On Tuesday,
Chisholm presented the annual Killian Faculty Achievement Award Lecture,
in which she described the long road to discovery of the abundance,
impact, and diversity of Prochlorococcus.
“Our view of life on Earth is skewed,” Chisholm said. “Photosynthesis
is the foundation of all life on Earth” — yet even well-educated people
often don’t quite understand how it works.
Chisholm illustrated that point with a survey question that few
people manage to answer correctly: In comparing a seed with a tree grown
from that seed, where does all the added mass come from? Most people
think it has to do with soil or water, Chisholm said, but the real
answer, she pointed out, is “out of thin air”: Carbon dioxide, split up
by photosynthesis and recombined into organic compounds, produces most
of the biomass.
While scientists knew, before Chisholm’s 1988 discovery, that the
oceans accounted for about half of the planet’s photosynthesis, nobody
knew which organisms were actually responsible for that output. (The
biomass generated annually by Prochlorococcus, Chisholm pointed out, is
equivalent to the weight of 5 billion Volkswagens.)
Prochlorococcus, Chisholm said, “looks like specks of dust” under a
microscope, but it produces a red fluorescent emission — a known
indicator of chlorophyll, the key chemical enabler of photosynthesis. It
was that telltale red glow that led to the discovery of this organism,
which turned out to be a form of bacterium.
But it took years of follow-up investigations, including sampling in
ocean locations around the world, to reveal the true abundance of this
creature, Chisholm said. It is now estimated that Earth’s oceans harbor 3
billion billion billion Prochlorococcus. “It’s by far the most abundant
photosynthetic cell on the planet,” she said.
Over the years, Chisholm and her students have studied in detail the
genomes of these organisms, finding them to be incredibly diverse, with
variations depending on both their geographic location and the depth at
which they live. Prochlorococcus come in two broad categories: low-light
adapted, which live in deep water, and high-light adapted, which thrive
in shallow water with plenty of light.
After analyzing the genetic makeup of these species, which contain
only about 2,300 genes, Chisholm concluded: “It’s absolutely the minimal
life form. It’s the smallest amount [of genetic coding] that can
convert inorganic compounds into organic ones.”
But for a tiny and simple creature, Prochlorococcus is remarkably
diverse, she said. Of those 2,300 genes, only about 1,200 are common to
all varieties of the organism. A vast number of other genes — perhaps as
many as 80,000 — can occur in some varieties, but not others. (By
comparison, humans have a total of about 20,000 genes).
Though phytoplankton are tiny, their impact is huge. Their
photosynthesis, which converts carbon dioxide into organic compounds —
some of which sink to the seafloor — constitutes a "biological pump"
that serves to sequester a large reservoir of carbon dioxide in the deep
ocean. If phytoplankton didn't exist, the carbon dioxide in Earth’s
atmosphere — the main agent of global warming — would be double or even
triple what it actually is.
These tiny creatures, Chisholm said, have turned out to be “the gift
that keeps on giving,” yielding ever-new insights — and new questions —
that have driven her life’s research. Ongoing research with
Prochlorococcus, she said, holds the prospect of uncovering entirely new
kinds of antibiotics, or perhaps even a new way of producing biofuels.
“To understand life, you have to study it at different scales. Each
part has emergent properties that feed back into the system,” Chisholm
said. “The complexity of this system is incredibly humbling.”