Buried Sunlight

Chloroplasts visible in the cells of Bryum capillare, a type of moss.

On our second journey we travel again with light, particles/waves of energy, from their origin in the sun 150 million km from the earth; our companions will be photons, generated by fusion reactions deep within the surface of our near-perfectly spherical, 4.5 billion year old star. This time our journey also takes us back in time to the origins of fossil fuels – a process that began millions of years ago with the fixing of carbon using the energy of sunlight.

Part 1: Photosynthesis

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms’ activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, “light”, and σύνθεσις, synthesis, “putting together”. In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth’s atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth.

Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins containing green chlorophyll pigments. In plants, these proteins are held inside structures called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane of the cell. In these light-dependant reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are NADPH (reduced nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate), the “energy currency” of cells.

In plants, algae and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the Calvin cycle. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as RuBP (ribulose bisphosphate). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose.

The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible.

Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about eight times the current power consumption of human civilization. Photosynthetic organisms also convert around 100–115 billion tons (91-104 petagrams) of carbon into biomass per year.

Part 2: Coalification

Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements; chiefly hydrogen, sulphur, oxygen, and nitrogen. Coal is formed when dead plant matter decays into peat and is converted into coal by the heat and pressure of deep burial over millions of years. Vast deposits of coal originate in former wetlands—called coal forests—that covered much of the Earth’s tropical land areas during the late Carboniferous (Pennsylvanian) and Permian times.

As a fossil fuel burned for heat, coal supplies about a quarter of the world’s primary energy and two-fifths of its electricity. Some iron and steel making and other industrial processes burn coal. One theory states that about 360 million years ago, some plants evolved the ability to produce lignin, a complex polymer that made their cellulose stems much harder and more woody. Thus, the first trees evolved. But bacteria and fungus did not immediately evolve the ability to decompose lignin, so the wood did not fully decay but became buried under sediment, eventually turning into coal. About 300 million years ago, mushrooms and other fungi developed this ability, ending the main coal-formation period of earth’s history.

The conversion of dead vegetation into coal is called coalification. Coalification starts with dead plant matter decaying into peat. Then over millions of years the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called “brown coal”), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called “hard coal” or “black coal”) may be formed.

The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is also known from Precambrian strata, which predate land plants — this coal is presumed to have originated from residues of algae.