Using light to describe the ancient world

Absorbance spectrum of a chlorophyll degradation product compared to a pigment from a 44 million year old leaf. This spectrum is good evidence that complex molecules like plant pigments can survive intact in the fossil record. Figure images from Dilcher et al. (1970) and Matile et al. (1999)

The fundamental difference between animals and plants is energy. Our source of energy is biotic – we consume things that were alive. The source of energy for plants is abiotic – they consume that which is not alive, namely, light. Plants harvest light and use it for photosynthesis, allowing them to convert CO2 from the atmosphere into something nutritious. Chlorophyll is the key to photosynthesis on Earth, and is the fundamental difference between animals and plants. Chlorophyll is a green pigment that strongly absorbs blue and red light. Chlorophyll, like heme, is a porphyrin, which is based on four pyrrole subunits bound together into a macrocycle. The macrocycle is a basic support frame for a whole world of chemistry – side chains can attach to the outside of the macrocycle, and the space in the middle can house a metal ion. Chlorophyll a, for example, has a specific combination of side chains and houses a magnesium ion; chlorophyll b and chlorophyll d have slightly different side chains. The macrocycle gives chlorophyll its green colouration, and the chemistry of chlorophyll is critical to life on Earth as we know it. Chlorophyll is a sophisticated molecule that is expensive to produce, and rapidly disappears when the tissue dies. Imagine, then, how exciting it would be to discover green fossil leaves in a 44±4 million year old coal deposit.

“Green-colored angiosperm leaves were reported in 1931 by Weigelt and Noack from middle Eocene brown coals of the Geisel valley near Halle, East Germany…” This is the opening line from a report by David Dilcher and colleagues, published in 1970. Dilcher and colleagues describe how “…Weigelt and Noack identified several chlorophyll derivatives from crude extracts of these green fossil leaves and their associated brown coals…”, and “…[a] reinvestigation of this material by modem techniques of chromatography and spectrophotometry has made possible a more precise separation and identification of the green pigmentation of this material…” So, green fossil leaves were found in a German coal deposit, and initial studies suggested that the green pigment was chemically similar to chlorophyll. Building on this initial work, Dilcher and colleagues set out to discover whether the remarkable green of these ancient leaves is the same pigment we find in leaves today.

Dilcher and colleagues sought information from a suite of analytical techniques, including paper chromatography, Molisch phase test, HCl number test, mass spectroscopy, and absorbance spectroscopy. I will focus on the last of these, seeing as light absorption is the biological imperative of green plant pigments. The green pigment was extracted and the absorbance spectrum was measured. Light absorption is based on a simple concept that I will over-explain. Light is part of the electromagnetic spectrum, which can be described as a continuous range of wavelengths, from high energy x-rays, to low energy radio waves (and beyond). Light is the small part of the electromagnetic spectrum that we can detect. We perceive higher energy wavelengths as blue, and lower energy wavelengths as red, but that is as far as we go. Wavelengths with slightly higher energy than blue are called ultraviolet, which we cannot see but birds can, and wavelengths with slightly lower energy than red are called infrared, which are invisible to us and birds. Now, different materials absorb different wavelengths, which give rise to different colours through a complimentary arrangement. The absorption of different wavelengths is characteristic of a material, and can be used as a spectral fingerprint. Dilcher and colleagues shined ultraviolet, visible and infrared light at their mystery green compound from the leaf fossil, and measured the wavelengths that were absorbed.

The absorbance spectrum of the green pigment was an almost perfect match with methyl phaeophorbide a, which is a degradation product of chlorophyll a. The key difference between chlorophyll and phaeophorbide is a long phytol chain, which is lost when chlorophyll begins to break down. Phaeophorbide is still green, but it is a slightly different green to chlorophyll, and the slight differences can be quantified with an absorbance spectrum. Which is exactly what Dilcher and colleagues did, allowing them to show that methyl phaeophorbide a is preserved in a 44±4 million year old coal deposit.

From Dilcher et al. (1970): “…In the Eocene the Geisel valley was a poorly drained shallow basin, receiving organic sediments from plants and animals living along its swampy margins…The preservation of such fossils as frog epidermal cells containing nuclei and bacteria in the Geisel brown coal suggests an anaerobic environment with relatively rapid deposition, in which organic decay was slow and organic accumulation extensive. The rapid burial of plant material in an anaerobic environment and the fact that brown coal has a history of low temperatures may account for the preservation of this phorbin from the middle Eocene…”

Dilcher DL, Pavlick RJ, Mitchell J. 1970. Chlorophyll Derivatives in Middle Eocene Sediments. Science 168 1447–1449.

Matile P, Hörtensteiner S, Thomas H. 1999. Chlorophyll degredation. Annual Review of Plant Physiology and Plant Molecular Biology 50:67–95

Weigelt J, Noack K. 1931. Nova Acta Leopoldina 1, 87.


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