Where the Sun Shines Dimly

A few years ago, I measured photosynthesis of algae that inhabit the underside of translucent quartz pebbles in the Mojave Desert of California. When I first saw them in the field, I couldn’t believe that a green plant could exist under a rock! The algae under these rocks can survive in light that is only about 0.05% of full sunlight.

Light intensity is measured in photons, and the unit to describe light intensity is an Enstein, which is equivalent to a mole of photons. Full sunlight at noon during the summer has an intensity of about 2000 microEnsteins per square meter per second (2000 μmol/m2/sec). Some plants, like garden zinnias, are adapted to life in full sunlight. As long as they are well-watered, many plants will show increasing growth in response to increasing light intensity up to about half of full sunlight.

Even though they thrive in full sun, the crop plants that feed more than 7 billion humans globally typically capture only about 1 to 2% of the energy in sunlight. Clearly there are a lot of photons going to waste, but certain wavelengths of light are not readily captured for photosynthesis. Others plants, like impatiens, will die if exposed to full sunlight. The biochemistry of shade-adapted plants can’t handle the full intensity of the Sun.

Even the most shade-adapted plants can’t grow at night. Higher plants need at least 1 to 5 μmol/m2/sec to persist. The intensity of moonlight is less than 0.005 μmol/m2/sec—about a million times less than full sunlight, and less than what I measured under rocks in the Mojave Desert. A full Moon may bathe the countryside in romantic light, but moonlight does not provide enough energy to allow the growth of higher plants.

Algae grow on the underside of ice shelves surrounding Antarctic and at considerable depths in alpine snowpacks, where light intensities are 2 to 10 μmol/m2/sec. These habitats shelter these plants from drought, fluctuations in temperature, and high doses of ultraviolet light. These plants can support small communities of animals that co-occur with them. Life under the ice in Antarctica is more important than it might seem. Collectively, the small organisms support larger species, such as fishes and penguins in the Antarctic seas, especially during the winter period of 24-hour darkness. The organic materials from summer photosynthesis by the ice algae persist into the winter.

It is easy to overlook the life under sea ice in the Antarctic, but as climate warms, sea ice will diminish and so will the growth of algae that depends on it. Penguin populations in Antarctica are on the decline. The link of climate warming to losses of sea ice, ice algae, krill, and penguins may be the mechanism driving this loss of biodiversity in a special place on Earth’s surface.

References
Raven, J.A. and C.S. Cockell. 2006. Influence on photosynthesis of starlight, moonlight, planetlight and light pollution. Astrobiology 6: 668-675.

Kohlbach, D. and 8 others. 2018. Dependency of Antarctic zooplankton species on ice algae‐produced carbon suggests a sea ice‐driven pelagic ecosystem during winter. Global Change Biology doi.org/10.1111/gcb.14392

Schlesinger, W.H., J.S. Pippen, M.D. Wallenstein, K.S. Hofmockel, D.M. Klepeis and B.E. Mahall. 2003. Community composition and photosynthesis by photoautotrophs under quartz pebbles, Southern Mojave desert. Ecology 84: 3222-3231.

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