Plants 250 million years ago were able to "transform" the earth

Recently, the plant paleoecology team led by Professor Feng Zhuo of the Institute of Paleontology of Yunnan University used a variety of techniques and combined comparative studies of living plants and fossil plants to confirm that plants 250 million years ago already had strong silicon biomineralization capabilities, providing direct fossil evidence for the transformation of plants in the process of earth system evolution for the first time. The relevant results were published in the famous academic journal National Science Review.

Since plants "landed" 470 million years ago, they have been silently transforming the various spheres of the earth in their unique way, making the earth gradually evolve into our livable home today. On the one hand, plants absorb carbon dioxide and release oxygen through photosynthesis, which is well known to us. This physiological metabolic process changes the atmospheric composition of the earth's atmosphere, that is, reducing the carbon dioxide content while increasing the oxygen content, which has completely transformed the high carbon dioxide and oxygen-poor environment that existed on the early earth for a long time. On the other hand, plants also have a special physiological metabolic ability-absorbing silicon elements in the surrounding environment and precipitating silicon dioxide in the body or on the surface. This process is called the "silicon biomineralization" of plants. The solubility of silicon dioxide after plant mineralization is greatly improved. After the death of plants, these silicon dioxides enter the ocean circulation through surface runoff, providing nutrition for planktonic microorganisms (especially diatoms) in the ocean, thereby promoting the carbon cycle process in the ocean and further consuming carbon dioxide in the atmosphere.

Figure 1. Late Permian (about 250 million years ago) Selaginella leaf fossil

The biomineralization of silicon in plants also has many benefits for the plants themselves. We must still remember the story of Lu Ban inventing the saw when we were young: Lu Ban was inspired by a grass cut on his hand and invented the saw. This is because Gramineae plants have a strong ability to biomineralize silicon, so a large amount of microscopic "glass slag" is deposited in the leaves. These sharp glass slags are the metabolites of plant mineralization - phytoliths. With phytoliths, the toughness and strength of the plant body are greatly enhanced, and the plant's ability to resist lodging and pests and diseases is also significantly improved.

Figure 2. Phytoliths preserved in situ in a Selaginella leaf from the Late Permian (about 250 million years ago)

Through morphological, anatomical and genomic studies of living plants, scientists have found that in today's plant kingdom, Selaginella, Equisetum, Poaceae, Cyperaceae and some ferns have strong silicon mineralization capabilities and can form a large number of phytoliths in and on their bodies, while gymnosperms such as pines and cypresses have very weak silicon mineralization capabilities. Molecular biology and fossil records confirm that Selaginella has been very conservative and has hardly changed in its 370 million years of evolutionary history. Therefore, scientists have proposed that plant silicon biomineralization existed in the early stages of plant evolution and had a profound impact on the evolution of the early Earth system. However, due to the lack of direct fossil evidence, we have not understood the evolution of plant silicon biomineralization and its role in transforming the Earth system.

Researcher Feng Zhuo from the Institute of Paleontology at Yunnan University led a team to discover more than 100 beautifully preserved Selaginella leaf fossils in the Permian strata in Fuyuan, Qujing, Yunnan. Based on morphological, anatomic, and in-situ elemental spectroscopy (SEM-EDX) analysis, combined with comparative studies of living Selaginella, it was confirmed that these 250 million-year-old fossil Selaginella already had a strong ability to biomineralize silicon, providing direct fossil evidence for the first time for plants to transform the early evolution of the Earth system.

Figure 3. SEM-EDX spectrum analysis of Selaginella leaf fossils from the Late Permian (about 250 million years ago)

It is worth mentioning that the fossil Selaginella discovered by Feng Zhuo et al. is a specimen preserved in the form of a cuticle. In traditional research methods, in order to speed up the experimental progress, this type of preserved fossil material is treated with high-concentration strong acids (hydrofluoric acid, hydrochloric acid) and strong alkalis (potassium hydroxide, sodium hydroxide) and other reagents. Since silicon dioxide is extremely soluble in hydrofluoric acid, traditional experimental methods cannot obtain phytolith specimens preserved in situ. During the research process, Feng Zhuo et al. avoided hydrofluoric acid and directly used hydrochloric acid to treat the samples, and appropriately heated the samples. The method used by Feng Zhuo et al. is much more "time-consuming" than the traditional method. The experimental cycle usually takes several months to complete, but it can obtain complete phytolith specimens preserved in situ. The experimental method used by Feng Zhuo et al. provides an important reference for future scientists to reveal the origin and evolution of silicon biomineralization in plants and its role in the evolution of the earth system.