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Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
Does a full moon reflect enough sunlight to drive photosynthesis? Is there a cut-off point below which the process simply stops?
Mike Follows
Sutton Coldfield, West Midlands, UK
Some plants can photosynthesise using nightglow, traditionally comprising moonlight, starlight and the light reflected from planets. Increasingly, artificial light at night, known as light pollution, contributes to this background illumination.
However, as plants already receive ample sunlight during the day, there has been little evolutionary pressure to depend on nightglow as a primary energy source.
The currently accepted theoretical light intensity required for photosynthesis is approximately 6 quadrillion photons (6 x 1015) per square metre per second. This may sound substantial, but it is about 50,000 times lower than the average daytime photosynthetically active radiation (PAR), but about 10 times the photon flux from full moonlight. PAR, which has wavelengths between 400 and 700 nanometres, constitutes approximately 37 per cent of total solar energy.
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A year-long research study recorded photosynthesis by microalgae at depths of up to 50 metres beneath Arctic sea ice
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A year-long research study recorded photosynthesis by microalgae at depths of up to 50 metres beneath Arctic sea ice, shortly after the polar night, despite illumination just four times the minimum level required.
In another extreme case, green sulphur bacteria (GSB1) were discovered at a depth of 2500 metres off Mexico’s Pacific coast, surviving on the faint infrared glow from hydrothermal vents.
Understanding photosynthesis under low-light conditions may help extend growing seasons in high-latitude agriculture sites and inform strategies for crop cultivation in space or extraterrestrial habitats.
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