Light as the source of living energy

Binet (1888) described bacteria from putrefied matter placed in a drop of water without oxygen, together with chlorophyl algae or green Euglenae¹. Nothing happened at first. Once the preparation was lit so that the chlorophyl could act, the bacteria were seen to respond, which Binet read as detection of oxygen¹. Jennings (1906) drew the consequence for green infusorians. In darkness the green bodies stop supplying oxygen, and the animal turns away from the dark². Colourless Paramecium caudatum and colourless bacteria get no more oxygen in light and do not avoid darkness². Jennings added that many coloured bacteria also reject darkness, because they need light to assimilate inorganic compounds².

Timiriazeff (1912) located carbon assimilation in the green chloroplast. He argued that the rays absorbed by chlorophyll are the ones that decompose carbonic acid, the first stage of the process³. Bayliss (1915) agreed that the pigment makes the reactions possible. Parts of variegated leaves without chloroplasts cannot photosynthesise, he noted⁴. He wrote that chlorophyll acts most strongly at wave lengths matching its absorption bands, and that formaldehyde is probably the first product, later polymerised to higher carbohydrates⁵. A substance giving aldehyde reactions splits off from chlorophyll in light and oxygen. Bayliss judged it a decomposition product of the pigment, since it forms without carbon dioxide⁵.

Loeb (1916) observed that chlorophyll, under red light, makes sugars from the carbon dioxide of the air. This suggested that chlorophyll preceded life, a conclusion he found hard to accept because chlorophyll seems a product of organisms⁶. Lotka (1925) likewise held that chlorophyll is too specialised to exist in the most primitive life forms⁷. Lotka also reported Spoehr's calculation for cultivated plants. At 1.5 gram calories per square centimetre per minute and six hours of sun, the daily income is 5400 kilogram calories per square metre⁷. Bose (1927) said plant storage had been rated below 1 per cent with defective methods. Using a magnetic radiometer for the incident energy, he reported efficiency as high as 7.4 per cent⁸.

Sousa and colleagues (2013) place photosynthesis inside a discussion of early bioenergetic evolution. They argue that ancestral sulfate reducers probably relied on sulfite reduction or sulfur disproportionation, because volcanic SO2 supplied sulfur and sulfite while sulfate was scarce before oxygen⁹. Local sulfate could form from atmospheric photolysis of SO2, or from sulfide-dependent anoxygenic photosynthesis⁹. Oceanic sulfate rose and entered the global sulfur cycle only once the atmosphere became more oxygenated⁹. In this account, light-driven metabolism appears in a form that does not release oxygen.