In the dramatically changing conditions of the ancient Earth, organisms were forced to develop new strategies to keep pace. From the middle of the oligocene, about 30 million years ago, to the Middle Miocene until about 5 million years ago, the carbon dioxide concentrations in the atmosphere dropped by about a third. At that time he saw the emergence of a new form of photosynthesis in a subset of plants, path C4. At present in a subset of plants, the C4 path completed the earlier photosynthetic pathway C3, meaning that these species harvested energy from the sun using two different strategies.
Researchers have long believed that a reduction in the level of carbon dioxide caused the origin of plants with this innovation, but a new study at Proceedings of the National Academy of Sciences, Based on biochemical models by a group led by the University of Pennsylvania's biologists and paleoclastic models by a Purdue University group, suggests that water availability may be the critical factor behind the emergence of C4 plants.
"The initial source of C4, which happened when carbon dioxide in the atmosphere was still very high, seems to be driven by limiting water," says Haoran Zhu, a graduate student at the School of Arts and Biology and the Department of Biology and the first author on paper. "Then, about 5 to 8 million years ago, there is a large expansion of C4 areas, this is because carbon dioxide was getting lower, lower carbon dioxide and light intensity were actually limiting factors in favor of C4 at the same time."
"What we're showing," says Errol Akshi, assistant professor of biology at Penn University, "is that the increased water efficiency of the C4 pathway is enough to give it an initial ecological advantage in relatively arid environments if you were looking only at temperature and carbon dioxide , You may miss this role of water and light. "
The researchers' work also indicates that C4 plants had a competitive advantage over C3 plants even when carbon dioxide levels in the atmosphere were still relatively high in the late oligocene.
"The entanglement is that C4 could have developed quite a bit earlier than we had previously thought," says Brent Lalker, a biology professor who, along with Akshi, is a consultant for Gaucho. "It supports some molecular clock estimates when C4 evolved as well."
In plants with photosynthesis path C3, the first stable compound produced in photosynthesis contains three carbon atoms; In C4 plants, the first compound has four carbon atoms. The C3 path evolved first, functioning efficiently when the atmosphere was rich with carbon dioxide. However, C4 plants independently evolved C3 plants dozens of times, capable of photynthethesize efficiently despite low carbon levels thanks to an additional step in the process used to pump carbon from the air into the inner layer of cells where the remainder of the cycle. By applying this "closed" system, where the photosynthetic machines do not exist directly with the external air, photosynthesis C4 allows plants to make more food with less water loss than C3.
Today, about a quarter of the planet's vegetation cover consists of C4 plants. Some important crop species, including corn and sugar cane, have a C4 track. The fossil and isotopic studies have helped scientists assess when this pathway developed, although these estimates were later than those suggested by molecular clock data from the phylogenetic data of different plant species, leading to some confusion about when and when the route was expressed in certain ecosystems .
To look closely at the factors that might have favored the expansion of the pathogenesis of photosynthesis C4, the Acai and the Lycra have created a multi-layered model. They thought about the variables that affect photosynthesis along with those affecting the hydraulic system, where the plants "decide" to devote more energy to the roots and strands to take water, or to build more leaf material that can help take light from carbon dioxide but also exposes them to greater water loss. In addition, plants can determine the optimal balance of carbon gain and water loss. Coupling these two systems, the scientists model included four factors that could favor lines C3 or C4: concentration of carbon dioxide, light, temperature and water availability.
According to their model, C4 evolution seems to play in two stages. When carbon dioxide was still high, C4 appeared in areas of the Earth that became warmer and more dry. But it did not reap its competitive advantage over C3 plants until a few million years later, when carbon dioxide was very low and the expansion of grazing areas provided open habitats with abundant light. In these areas, C4 expanded spaces and replaced C3 grazing.
To see how this model interacts with the paleoclimate in the early days of C4 plants, the Penn team collaborated with Matthew of Purdue University and Matthew Hoover, a paleoclimate model funded by the National Science Foundation to the Miocene climate model, and graduate student Ashley Dix. Using climate model data and paleoclimate data, including carbon dioxide, temperature, and rain, the researchers predicted the geographic distribution of C3 versus C4 during the period between the late Oligocene and the early Meocene, about 30 to 5 million years ago. They found two previously unrecognized areas where C4 plants were expected to dominate after the first development due to their water efficiency: North West Africa and Australia.
"These two previously unrecognized pockets of the world where C4 plants could have had an ecological advantage really took over," says Akçay.
"This was a really exciting opportunity," Hoover says, "when the Penn group came to us because it is a very innovative application of the production of a paleoclastic model, it helps to create the link between the climate models that tell us about the climate of the past and the future.
Although research does not investigate what may happen in the future as atmospheric carbon levels rise again, it can help improve understanding why plants are distributed as they are today and how they may react to future conditions.
"Climate conditions that were present when C4 evolved may still be important today," says Lillieker. "If the lineage of C4 plants evolved mainly because of water limitations when carbon dioxide was high, then these plants may be found in dry environments today, whereas if it was more carbon dioxide that led to their evolution and dominance then these plants can be found in soft spots today."
In addition, some scientists believe that other species of agricultural importance, such as rice, which have photosynthesis of C4, may help increase food production so that the model can help predict where such plants can grow optimally.
Like climate change, plants can not suck carbon from the air fast enough
Haoran Zhou et al, C4 photosynthesis and climate through the lens of optimism, Proceedings of the National Academy of Sciences (2018). DOI: 10.1073 / pnas.1718988115