About 2.5 billion years ago, a pigment called chlorophyll appeared in single-celled organisms, allowing them to capture energy in the form of light and convert it into sugars. This biochemical trick, called photosynthesis, transformed the biosphere. Photosynthesis later allowed plants to thrive all over the globe, and it is the basis for all food that heterotrophs—organisms that don’t produce their own food—like ourselves consume to survive.
Although the chlorophyll pigment captures short (blue) and long (red) wavelengths very well, it does not absorb the middle wavelengths of visible light as effectively. “This creates lots of untapped potential for light capture," says Jeff Dudycha, an evolutionary biologist at the University of South Carolina. (Infrared light doesn’t have enough energy to be useful for photosynthesis, while ultraviolet light can be damaging to plants.)
But an obscure and ecologically successful group of algae, known as cryptophytes, have evolved pigments that capture light where chlorophyll cannot, Dudycha and colleagues report in a series of recent papers. The extra energy absorption from more wavelengths of light has allowed these algae to thrive in a variety of diverse environments, from oceans to streams to ponds to mud puddles.
“They’re everywhere,” says Tammi Richardson, an oceanographer at the University of South Carolina and co-author of the studies. Cryptophytes have evolved not one, but a whole suite of light-absorbing pigments, called phycobilins, which allow them to diversify and live in many different ecological niches. These phycobilins are attached to proteins, forming a phycobiliprotein that absorbs light.
The origin of the phycobiliproteins pigments is inseparable from the origins of the cryptophyte algae themselves. Cryptophytes are the result of a merger between two organisms: a red alga that was engulfed by another unknown eukaryote (an organism with cells that contain enclosed nuclei). Researchers call this type of evolutionary merger between two eukaryotes a secondary endosymbiosis. The first endosymbiosis occurred when a prokaryote—a single-cell organism without a nucleus—engulfed a bacterium, producing the first eukaryotic cell about 1.7 billion years ago. Secondary and even tertiary mergers have occurred a handful of times in the history of life, creating new types of life when one organism absorbs and contains another.
“These mergers are like experiments in which you can mix and match proteins and things from different compartments in interesting ways,” says John Archibald, an evolutionary genomicist at Dalhousie University in Nova Scotia, Canada. “What you end up with is a sort of hybrid organism that has capabilities that neither one of the two original partners had.”
Together, the red alga and the unknown eukaryote that engulfed it provided the necessary parts that could be assembled into phycobiliproteins. These light-capturing proteins are made of two protein subunits, one of which is encoded in the nuclear genome of the host eukaryote, while the other is encoded in the plastid genome of the ancient red alga. (The plastid—a membrane-bound organelle—of the red alga comes from yet another, earlier merger, when the alga acquired a cyanobacteria and converted it into a plastid.)
And so by a sequence of one organism absorbing another, and then transforming into something entirely new, these unique algae formed. “The cryptophytes tinkered around with the proteins that they had available to them to take advantage of a new opportunity,” Archibald says. The result was the formation of new pigments to absorb more light for photosynthesis.
Matt Greenwold, a postdoctoral fellow in Dudycha’s lab, studies the origins of the phycobiliproteins. Part of the molecule is derived from genetic material from another pigment, called phycoerithrin, present in the ancestral red algae. But the other protein subunit that makes phycobiliprotein, encoded by a gene of the host organism, is of unknown origin. It doesn’t match the sequence of any known genes in other organisms, Greenwold says. It could have belonged to the genome of the unknown host cell, or it could have been translocated from elsewhere.
It is also possible that the mystery protein came from the red alga’s genome. It’s not uncommon for genes from one organism in an endosymbiotic merger to translocate to the host’s nucleus, Archibald says.
Although it is known that each cryptophyte has only one type of phycobiliprotein, unique to each species of algae, Richardson and her graduate student Kristin Heidenreich’s recent work suggests that the absorption spectrum of these pigments can shift based on the environment in which the algae grows. “That just blew me away,” Richardson says.
It’s still unclear how the cryptophyte algae accomplish this feat. Richardson suggests that the phycobiliprotein changes shape such that it absorbs different wavelengths. Alternatively, the cryptophytes could be changing the chromophores—the light absorbing chains on the phycobiliprotein complex—so that they are able to absorb whatever color of light that is available to them.
When eukaryotic cells first absorbed the cyanobacteria that became chloroplast—a light-absorbing organelle—photosynthesis became a powerful driver of life on Earth. But the evolution of photosynthesis may not be done yet, as secondary mergers, such as the one that produced cryptophytes, have further expanded the available pigments to capture unused portions of the light spectrum. If plants can continue to harness more and more energy from light, there is no telling how it could affect the future of the planet.