Penny Chisholm
There are discoveries so fundamental, so quietly omnipresent, that they become part of the very fabric of our understanding—so deeply woven into our knowledge that it’s hard to imagine a time when we didn’t know them. Penny Chisholm’s discovery of Prochlorococcus—the smallest, most abundant photosynthetic organism on Earth—is one of those.
I photographed Penny at her office and lab at MIT in the fall of 2023, and I’ll never forget the moment we stepped into a refrigerated room deep inside her lab. The chill was immediate, the hum of machinery a steady undercurrent. She reached into a rack and lifted two small test tubes, each glowing a faint, otherworldly green. “This,” she said, holding them up to the light, “is the foundation of the ocean’s food web. The most abundant phytoplankton on the planet.”
That microscopic speck of green—Prochlorococcus—had once been invisible to us, despite existing in vast numbers across the world’s oceans, quietly producing oxygen, fueling marine ecosystems, and shaping the chemistry of the planet. It was Penny’s work, her persistence and curiosity, that revealed it to science.
The story of that discovery stretches back to the late 1980s, a time when oceanographers still largely relied on traditional methods—net tows, microscopy, and chemical analyses—to study phytoplankton. But the problem was that these tools missed something. The ocean’s primary producers weren’t just the larger phytoplankton that could be seen under a microscope; something much smaller was at work. Penny, working alongside colleagues, turned to flow cytometry, a technique borrowed from medical research, to analyze seawater one cell at a time.
When they did, they saw something unexpected—tiny, green fluorescent cells in astonishing numbers. At first, no one quite believed it. How could an organism so small—barely a fraction of a micrometer across—be responsible for such a significant fraction of the ocean’s primary production? But as they sequenced its DNA and studied its ecology, a new picture of the ocean emerged.
It turned out that Prochlorococcus wasn’t just abundant—it was ubiquitous. Trillions upon trillions of these microscopic cells drifted through the oceans, thriving at different depths, subtly adapting to varying light conditions, forming an invisible yet essential foundation of the marine ecosystem. It was as if an entire unseen rainforest had been hiding in plain sight, only now revealed.
But Penny’s impact reaches far beyond Prochlorococcus. She has spent her career advocating for a deeper understanding of the ocean as a living system, not just a resource to be exploited. She has pushed back against the idea of carbon as a simple commodity, warning of the unintended consequences of geoengineering schemes that treat the ocean as a dumping ground for excess CO₂. She understands something fundamental: that nature is not a machine to be tuned but a complex, interwoven system.
And then there’s her role at MIT—not just as a scientist but as a mentor, a leader, an advocate. Penny was a key ally in one of the most significant battles for gender equity in academia: the movement led by molecular biologist Nancy Hopkins to expose gender discrimination at MIT in the 1990s. It started quietly, with a small group of senior female faculty—including Penny and Nancy—comparing notes on the subtle but persistent disparities they faced: fewer resources, smaller labs, lower salaries. It wasn’t just perception; it was measurable.
Nancy, ever precise, collected the data—evidence that left no room for doubt. When they presented their findings to MIT’s administration, something remarkable happened: MIT admitted it. The institution publicly acknowledged gender bias, an extraordinary moment of institutional self-reflection that rippled across academia. Penny was part of that fight, standing beside Nancy and the other women who reshaped the culture of MIT.
Sitting in her office, surrounded by books on oceanography, ecology, and climate change, Penny talked not just about her own discoveries but about what it means to be a scientist—to see the world clearly, to fight for truth, whether it’s hidden in a drop of seawater or buried in institutional norms.
As I packed up my camera, I kept thinking about those test tubes glowing green in the cold room. A lifetime of work distilled into something so small, so deceptively simple. But Penny Chisholm had shown the world that the smallest things—the unseen, the unnoticed—can hold the greatest power.
Penny Chisholm
There are discoveries so fundamental, so quietly omnipresent, that they become part of the very fabric of our understanding—so deeply woven into our knowledge that it’s hard to imagine a time when we didn’t know them. Penny Chisholm’s discovery of Prochlorococcus—the smallest, most abundant photosynthetic organism on Earth—is one of those.
I photographed Penny at her office and lab at MIT in the fall of 2023, and I’ll never forget the moment we stepped into a refrigerated room deep inside her lab. The chill was immediate, the hum of machinery a steady undercurrent. She reached into a rack and lifted two small test tubes, each glowing a faint, otherworldly green. “This,” she said, holding them up to the light, “is the foundation of the ocean’s food web. The most abundant phytoplankton on the planet.”
That microscopic speck of green—Prochlorococcus—had once been invisible to us, despite existing in vast numbers across the world’s oceans, quietly producing oxygen, fueling marine ecosystems, and shaping the chemistry of the planet. It was Penny’s work, her persistence and curiosity, that revealed it to science.
The story of that discovery stretches back to the late 1980s, a time when oceanographers still largely relied on traditional methods—net tows, microscopy, and chemical analyses—to study phytoplankton. But the problem was that these tools missed something. The ocean’s primary producers weren’t just the larger phytoplankton that could be seen under a microscope; something much smaller was at work. Penny, working alongside colleagues, turned to flow cytometry, a technique borrowed from medical research, to analyze seawater one cell at a time.
When they did, they saw something unexpected—tiny, green fluorescent cells in astonishing numbers. At first, no one quite believed it. How could an organism so small—barely a fraction of a micrometer across—be responsible for such a significant fraction of the ocean’s primary production? But as they sequenced its DNA and studied its ecology, a new picture of the ocean emerged.
It turned out that Prochlorococcus wasn’t just abundant—it was ubiquitous. Trillions upon trillions of these microscopic cells drifted through the oceans, thriving at different depths, subtly adapting to varying light conditions, forming an invisible yet essential foundation of the marine ecosystem. It was as if an entire unseen rainforest had been hiding in plain sight, only now revealed.
But Penny’s impact reaches far beyond Prochlorococcus. She has spent her career advocating for a deeper understanding of the ocean as a living system, not just a resource to be exploited. She has pushed back against the idea of carbon as a simple commodity, warning of the unintended consequences of geoengineering schemes that treat the ocean as a dumping ground for excess CO₂. She understands something fundamental: that nature is not a machine to be tuned but a complex, interwoven system.
And then there’s her role at MIT—not just as a scientist but as a mentor, a leader, an advocate. Penny was a key ally in one of the most significant battles for gender equity in academia: the movement led by molecular biologist Nancy Hopkins to expose gender discrimination at MIT in the 1990s. It started quietly, with a small group of senior female faculty—including Penny and Nancy—comparing notes on the subtle but persistent disparities they faced: fewer resources, smaller labs, lower salaries. It wasn’t just perception; it was measurable.
Nancy, ever precise, collected the data—evidence that left no room for doubt. When they presented their findings to MIT’s administration, something remarkable happened: MIT admitted it. The institution publicly acknowledged gender bias, an extraordinary moment of institutional self-reflection that rippled across academia. Penny was part of that fight, standing beside Nancy and the other women who reshaped the culture of MIT.
Sitting in her office, surrounded by books on oceanography, ecology, and climate change, Penny talked not just about her own discoveries but about what it means to be a scientist—to see the world clearly, to fight for truth, whether it’s hidden in a drop of seawater or buried in institutional norms.
As I packed up my camera, I kept thinking about those test tubes glowing green in the cold room. A lifetime of work distilled into something so small, so deceptively simple. But Penny Chisholm had shown the world that the smallest things—the unseen, the unnoticed—can hold the greatest power.