Graham Butcher, of Tawa, asks :-

If the Sun suddenly stopped shining, how long would it take for life on Earth to perish? Could some species survive?

Joe Zuccarello, a biologist at Victoria University of Wellington, responded.

If the Sun just 'switched' off and no cosmological events occurred (earth moving out of its orbit, increased comet activity) then at a certain point life would die as the earth would completely freeze. Liquid water is important for life.

Of course, the sun also is the major energy source for producing 'fixed carbon' (energy rich carbon containing molecules that other organisms can use- like sugars). These are produced by organisms that can do photosynthesis (called photoautotrophs- light-self-food, literally). A process that plants and algae do to produce, from water and carbon dioxide, sugars and other energy rich molecules. Photosynthesis captures the energy in sunlight (electromagnetic radiation) and transfers it through very many steps to organic compounds (what we call fixed carbon). We, and nearly all of the rest of life, survives on this fixed carbon.

There is another way to produce organic compounds that does not involve sunlight, this is done by chemoautotroph (chemical-self-food) and is preformed by some bacteria. They use energy (electrons) in inorganic compounds (example: sulfides, or ferrous iron) to make fixed carbon compounds. There are many different kinds of chemoautotrophs. This may be a method of making organic compounds that predates photosynthesis.

Chemoautotrophs are found in many areas of the world but are common at hydrothermal vent sites (often at great depths in the ocean where there is no light) where they sustain a wide assortment of other organisms (animals) that feed on the carbon fixed by these bacteria.

If the sun went out, photosynthesis organisms would stop fixing carbon. Animals would have to survive on what was already fixed in plants or other food and there own internal reserves. Plants would survive for a while, on stored reserves (think trees that lose their leaves in winter), but would not be able to add more to their growth.

Deep-ocean chemoautotrophic-driven ecosystems would continue thriving until the deep oceans froze completely (it make take a while).

If the sun was switching off and could come on again, then the question is how long could organisms survive. Many organisms have long lived dormant stages (seeds, spores) that can last many years and would grow again if the Sun came on again. Some plants can also survive longish periods without light. High-latitude mosses in Antarctica that can survive several months in winter in the freezing dark. Chemoautotrophs in the deep ocean would not be bothered at all and may evolve to take over the world.