Most of us met photosynthesis as a tidy equation: sunlight and carbon dioxide go in, sugar and oxygen come out. The equation is correct, as far as it goes. What it leaves out is everything a cell has to do to keep the transaction from going wrong.
Adrien Burlacot, a physicist and engineer by training who now studies photosynthetic organisms at Carnegie Science, put the gap bluntly in a Q&A published by the institution: "The textbook view is already fairly complex, but it is simple compared to reality." What makes it complicated, he says, is not the core chemistry but the crowd of proteins and regulatory activities that keep energy production matched to what the cell actually needs.
Why algae are the test case
Burlacot's lab works mostly with Chlamydomonas reinhardtii, a single-celled green alga. It has two practical virtues. It is fast — a population in a tank can double in 16 hours or less — and it has been a model organism for decades, so there is a large body of existing knowledge and finely tuned genetic tools to work with.
The bigger reason is scale. Microalgae carry out roughly half of the photosynthesis on Earth, and they are not confined to the ocean. Dig up soil in a backyard, put it under a microscope, and you will find algae; they live in soil, sea ice, lakes, ponds and oceans, and some manage photosynthesis in harsh conditions. That range is what makes them useful for asking what keeps photosynthesis robust in the first place.
A cloud is a control problem
Too much photosynthetic energy can damage a cell. Too little and, in Burlacot's words, "you are basically going to die." So a cell is constantly adjusting output to demand, and the environment refuses to hold still.
His example is ordinary weather. A cloud passes and light drops sharply. The cloud moves on, full sun returns, and suddenly the cell has more energy than it can use. Some of the proteins involved in this balancing act are known. Burlacot is candid that the list is far from complete: "I believe we are still pretty far from understanding all the knobs that tune photosynthesis so the cell gets exactly what it needs."
Seconds, hours, minutes
The response depends on how much time the cell has. Within seconds to minutes of a jump from low to high light, it activates mechanisms that dump excess energy — using less light to make chemical energy, or discarding much of what it does make. Over longer periods it changes how much protein it builds, for instance adjusting the number of light-absorbing structures to suit the conditions it is actually in.
And over minutes, Chlamydomonas can simply leave. It has flagella, small arms that let it swim, so it can move toward the light level that suits it. As Burlacot points out, finding the right spot is easier than endlessly shedding excess energy or rebuilding the photosynthetic machinery.
The carbon pump and its power bill
Light is only half the problem. Carbon dioxide dissolves poorly in water — the reason seltzer bubbles — and photosynthesis needs it. Many microorganisms therefore evolved, independently of one another, a carbon concentrating mechanism that pumps CO2 from the surrounding water into the cell. In some photosynthetic bacteria this raises the internal concentration about 1,000-fold compared with outside; without the mechanism, most such cells would die or fail to grow.
Pumping costs energy, and Burlacot is exploring the idea that the algal version is expensive indeed. The hypothesis is that dedicated energy-producing structures sit around the place where CO2 is concentrated, generating power locally to keep the gas from leaking back out. It points to a broader biological question: when a process is energetically costly, does the cell need its power plants next door? That remains a hypothesis under investigation, not a settled mechanism.
An unplanned discovery
One result came from a project nobody planned. A Stanford PhD student asked to study diatoms, another kind of microalga. Burlacot said yes without working out first where the money would come from. The lab borrowed a tool from mammalian cell biology, applied it to diatoms, and used it to watch how they handle fluctuating light. The work led to a previously unknown protein, now called STROBE1, that appears critical for surviving fluctuating light in a large fraction of the phytoplankton on Earth. Its precise function is not yet known, which is exactly what makes it interesting rather than finished.
What it would take to matter
Burlacot would like to see algal photosynthesis used for large-scale carbon capture, energy production and food production. He is clear that this is not economically viable today: fossil fuels are cheap, and algae-based systems are hard to make competitive. Better understanding could raise productivity and help tune photosynthesis to a given environment, and might make such approaches viable later. Capturing carbon in a durable form — perhaps at the bottom of the ocean — at a scale that offsets human emissions is described as something that could follow from understanding, not as a demonstrated result.
That honesty is the useful part. Photosynthesis is less an equation than a control problem, solved continuously by organisms that have had a long time to practice. The research is the slow work of finding the dials, and admitting how many are still unlabeled.