Following the worst mass extinction event on Earth, the land was not entirely barren of life. In the wake of this cataclysm, when forests mostly disappeared and many familiar plant species were lost, a unique group of plants emerged and proliferated across the planet.
These plants were lycophytes. They were spore-producing plants. Recent findings indicate that they survived by utilizing a different approach to photosynthetic carbon assimilation. Specifically, this approach involved utilizing the cooler hours of the night for part of their carbon intake.
This hypothesis regarding the effect of extensive volcanism in the Siberian Traps that contributed to the Permian-Triassic Mass Extinction, approximately 252 million years ago, the event commonly referred to as The Great Dying, is particularly significant. It was not merely an environmental disaster.
The Great Dying marked the extinction of 81% of all marine species and the extinction of nearly 89% of all tetrapod genera on the terrestrial surface. Consequently, this event was accompanied by intense volcanism, significant increases in atmospheric carbon dioxide, and extensive periods of persistently elevated global temperature.
Over many millions of years, the planet continually remained dangerously hot. Equatorial sea temperature had exceeded a maximum of 35 °C. In addition, the temperature on land at equatorial latitudes exceeded 45 °C. In some areas where lycophytes occur in the model, the modeled average surface temperature exceeded 50 °C.
On land, following the event, a substantial turnover of ecosystems occurred. In lowland settings, prior to the catastrophic extinction event, the dominant type of ecosystem was the broad forested habitat. This habitat consisted of multiple species of large tree forms and many other plant forms. In contrast to these diverse forest types, the post-extinction landscape was eliminated. Low-diversity habitats consisting of only very small herbaceous forms of lycophytes replaced them.
Researchers from the University of Leeds conducted a study to investigate the process associated with this turnover of ecosystems. What allowed these relatively simple organisms to survive when so many others did not? And how did these organisms manage to proliferate during one of the most extreme greenhouse periods in Earth’s past history?
To arrive at a conclusion regarding this matter, the team used several pieces of evidence. This included studying the evolutionary history of lycophytes, a group of plant species. They studied a total of 485 specimens of fossil and living sporophylls, the leaf-like structures that produce spores.
Furthermore, they analyzed carbon isotopes from fossilized plants found in southern China. They also compared fossil distribution patterns with climate model simulations. Combining paleontological, physiological, and climatic modelling allowed for the deduction of a probable explanation for these observations.
The authors contend that lycophytes had most likely evolved to use CAM photosynthesis, crassulacean acid metabolism, as their primary mechanism for photosynthesis and water conservation. Currently, this form of photosynthesis is mainly associated with plants found in hotter and drier climates.
A nocturnal mechanism for surviving extreme daytime heating is employed with the CAM photosynthetic pathway. Unlike most modern plants that open their stomata during the day, CAM plants open their stomata at night when the atmosphere is cooler, and therefore there are lower rates of water loss. They absorb carbon dioxide during the night and convert it to an acid. This acid is then used during the daytime photosynthetic process while keeping the stomata closed.
This nocturnal adaptation to high-temperature environments is extremely advantageous under extreme conditions. Today, modern CAM plants make up a relatively small proportion of the world’s vegetation. However, they are abundant in arid landscapes where high heat and drought conditions limit the ability of typical photosynthetic processes.
In this study, fossilized lycophyte carbon isotopes were observed to be distinct from fossilized carbon isotopes from plants living during the same time frame. The gap in environmental conditions during the extinction phase was especially significant. However, as the environment improved, the gap decreased and eventually disappeared.
The authors of this research caution that while the fossil record is not a definitive record of metabolism from ancient times, it does provide insight into the metabolic processes of living organisms through the use of stable isotopes. For example, the current use of malic acid during the night is one way that scientists confirm CAM photosynthesis in extant species.
The authors acknowledge that there are very few fossils from the Early Triassic period and that various influences can create variability within carbon isotope signatures. However, using a combination of stable isotope data, plant morphology, and modeling of climate, the authors maintain that, from the evidence presented, the best explanation of the data is that these lycophytes exhibited some form of CAM as an adaptive mechanism to extreme heat and lack of water.
Another important clue supporting the findings of this research comes from living relatives of these ancient lycophytes. The authors observed that there is a strong connection between the modern-day Isoetes, quillworts, and the fossilized remains of the Permian-Triassic lycophyte Tomiostrobus.
Research indicates that quillworts still grow in many parts of the world, including Scotland. Some species of quillworts can adapt their metabolic pathways. Under periods of environmental stress, they can alternate their photosynthetic strategies between C3 and CAM pathways.
The authors believe that the findings presented in this article indicate that ancient lycophytes were able to develop similar metabolic pathways to adapt to extreme environmental conditions. These conditions would exceed the tolerances of modern-day C3 plants.
The authors conclude by stating that the flexibility of metabolic strategies has a long evolutionary history. This may help explain the success of the Isoetaceae family of plants. Modern C3 plants usually thrive in temperatures ranging from 10 °C to 35 °C. However, as temperatures increase, they have difficulty due to water loss, photorespiration, and enzyme damage in photosynthesis processes.
A modelling study conducted on climate conditions during the Triassic period indicates that it was much harsher than the temperature ranges stated above. Fossils from South China, North China, Xinjiang, Europe, Australia, India, and Argentina indicate that the average daily maximum temperatures of these areas must have exceeded 40 °C. The absolute daily maximum temperatures exceeded 45 °C to 60 °C.
According to the study’s lead author, Dr. Zhen Xu from the University of Leeds School of Earth and Environment, “Our data suggest that, in the event of an increase in temperature globally, plants that possess CAM photosynthesis characteristics will become increasingly valuable.”
Professor Barry Lomax from the University of Nottingham notes, “Our analysis takes many disparate scientific disciplines and brings them together into one cohesive study examining how this unique class of plants survived the Great Dying and then subsequently thrived in an incredibly difficult environmental regime.”
By bringing these data together, researchers can better understand how plants developed their adaptations to ancient climatic emergencies, and thus better understand the resilience of the Earth’s system to climatic changes.
According to Professor Benjamin Mills, also at the University of Leeds, “Understanding how the diversity of strategies within the physiological characteristics of plants generated ecosystems of the past will help to understand how vegetation may reorganize itself in future climate scenarios, and because plants are the foundation of food chains on land, changes to the dominant strategies of photosynthetic plants can affect the functionality of the entire Earth system.”
Research findings are available online in the journal Nature Ecology & Evolution.
The original story “Scientists discover how primitive plants survived Earth’s worst mass extinction” is published in The Brighter Side of News.
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