An ancient lineage of cyanobacteria is helping biologists uncover an early evolutionary stage of the mind-boggling process that turns light into life.
Son of Alan for Quanta Magazine
Introduction
E very second, trillions of watts of solar energy — more than 10,000 times the energy used by modern humans — blast the Earth’s surface. Around 2.4 billion years ago, life took an evolutionary leap when bacteria learned to harness these photons to break apart water molecules and stitch carbon atoms into sugars. Along the way, they flooded Earth’s atmosphere with oxygen and rewrote the rules of life.
“The oxygen-evolving capability was a big innovation. I sometimes call that a singular event,” said Robert Blankenship , a retired biochemist from Washington University in St. Louis. “By all accounts, it only happened once during the process of evolution, and that really set up the world for becoming oxygenated and the wholly aerobic world that we live in now.”
However, the set of chemical reactions we call photosynthesis has bewitched and befuddled scientists for generations. It requires the coordination of dozens of proteins and hundreds of pigments that harvest photons, all embedded in a cellular structure less than one-thousandth the width of a human hair. Electrons pinball across membranes and between compounds to drive molecular turbines that rebuild air and water into sugars to provide the energy and raw materials that cells need to grow.
We now know this process in fundamental detail; advances in microscopy and cell biology mean that researchers can essentially track a single electron through photosynthetic proteins to illuminate the full molecular mechanism. This level of detail dims, however, as scientists attempt to travel back in time to understand how photosynthesis could possibly have first evolved in single-celled organisms called cyanobacteria over 2 billion years ago.
“It’s now pretty clear that all the photosynthetic [protein] complexes descend from a single common origin,” said Blankenship, who spent his career studying the molecular mechanisms of photosynthesis. “But the nature of that very first organism is not very well understood.”
To solve such riddles, biologists often turn to organisms that share many, but not all, of the traits they want to understand. But for years, they believed that nearly all modern cyanobacteria evolved in a single, closely related cluster, offering little variation that might reveal mechanisms of early photosynthesis. The discovery of Gloeobacteria, a group of photosynthetic bacteria that branched off from other cyanobacteria over 2 billion years ago, changed this. Although Gloeobacteria haven’t remained at an evolutionary standstill — no organism has — they seem to have changed little over billions of years, making them a sort of genetic time capsule.
“[Gloeobacteria] tell us a little bit about what the earliest cyanobacteria might have looked like,” said Christen Grettenberger , a geochemist and microbiologist at the University of California, Davis. “It’s not some weird one-off species. It has a real pattern of retaining these tools.”
The most recently identified Gloeobacteria species, Anthocerotibacter panamensis , harvests light using a different set of proteins than modern cyanobacteria — but converts sunlight into chemical energy within protein complexes that vary only slightly from those in other Gloeobacteria. These traits add new color to the long, strange evolutionary story of photosynthesis.
A Photon Capture Machine
Before striking a plant leaf, a solar photon travels 93 million miles through empty space. The most dynamic part of this journey happens in the last few billionths of a meter, as a Rube Goldberg machine of proteins and pigments converts the photon’s light energy into chemical energy.
Mark Belan/Quanta Magazine
The leaves of modern land plants are packed with chloroplasts, oblong organelles that are themselves stuffed with stacks of coin-shaped compartments known as thylakoids. Thousands of proteins and pigments stud the thylakoid membrane, creating a sprawling biochemical circuit with a single purpose. A large protein complex there, named photosystem II, hosts light-harvesting “antenna” complexes on its outer ring that maximize the number of photons the plant can snag. Chlorophyll and other pigments embedded within the antennae absorb the energy from captured photons. Then, as in a game of hot potato, chlorophyll and other pigment molecules funnel this excess energy to the reaction center of the photosystem.
Energy is lost every time the photon hops between pigments, but it retains enough to jolt electrons loose from nearby water molecules, releasing oxygen as waste. These liberated electrons then flow through a series of membrane-bound proteins, known as an electron transport chain, where their energy pumps protons and spins molecular turbines. This molecular assembly line generates life’s energy currency, a molecule known as adenos…
Read the full article at Quanta Magazine →
United States