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ON TRAILS OF ANCIENT SEA GRASSES

December 01, 2000

Paleobotanists from St. Petersburg have found that ancestry of sea grasses had come to sea from desert 40 million years ago. The study was supported by International Science Foundation, Smithsonian Institution, Washington D. C., and by Linnean Society of London.

It is not easy to reconstruct how life developed on the Earth - too little stuff had remained from living beings after millions of years. Luckily for scientists, leaves of sea grasses contain biological preservatives delaying oxidation. Finally grass tissues became mummified. Thus it is possible to study the structure of ancient plants and to compare them with modern species. This investigation was done by researchers of the Komarov Institute of Botany leading by Sergey V. Vikulin. They studied modern barnacle grass Zostera marina and its ancestor lived 40 million years ago, between Eocene and Oligocene.
        They found that leaves of modern and ancient plant were quite similar by appearance. But the ancestor had a very thick cuticle, a pellicle protecting leaves from sun. Moreover, the cuticle was pierced by a lot of tiny divaricate canals as in modern desert dwellers. Modern aquatic plants almost do not have any cuticle at all. What is the reason of such difference?
        Sea herbs are not algae but flowering plants. Many of them occur in littoral zone of warm seas and are blooming under salt water. For instance, Halophila is flowering near to shores of India and Indonesia at a depth of eighty metres. Barnacle grass Zostera marina occurs in Russia, in the Black, the Caspian, and the Baltic Seas. Its leaves have so much polysaccharides that they cannot burn. Just due to this compound remains of ancient Zostera can be investigated today. Sea grasses had appeared in Eocene - Oligocene presumably after global disaster obliterated almost all sea dwellers of those times.
        Botanists suggested two possible ancestors of sea grass - freshwater plants of pondweed family and coastal land plants. These results obviously argue in favor of the second theory. Ancestors of Zostera like those of modern cetacean had inhabited land in late Eocene and only later had came into sea. Thick cuticle had allowed them to survive, protecting from water evaporation and from ultraviolet radiation which was higher at the end of Eocene than now. Well-developed cuticle had served as a kind of pre-adaptation giving a possibility to adapt to salt water of seas and oceans.
        As the sun radiation had decreased during Cainozoic era the advantage of having such protection had came to naught. Water itself is a good protection against sun and thick pellicle impedes photosynthesis. Thus, as the time passed the cuticle of sea grasses became thinner and thinner and almost disappeared by now.
        The research is supported by International Science Foundation, Smithsonian Institution, Washington D. C., and by Linnean Society of London.

Informnauka (Informscience) Agency