Food chains

      Question
      Broadly speaking, plants obtain energy through photosynthesis of
sunlight and animals by eating those plants or each other. Surely evolution
will have experimented with other means. Is there any fossil evidence to
suggest what this might have been?

      Brian Wall , Ferndown, Dorset, UK

      Answers
      Evolution may have experimented with primary sources of energy other
than sunlight, but at present no authenticated trace of such organisms
exists on Earth. Anaerobic photosynthesis seems to have underpinned life for
effectively all of the early aeons of its terrestrial existence, before
oxygenic photosynthesis emerged some 2 billion years ago.

      Today, lifelong cave-dwelling organisms, subterranean microbes,
deep-sea hydrothermal vents and suchlike may never actually encounter
sunlight, but they depend on it nonetheless because they use oxygen
generated elsewhere by photosynthesis. Such oxygen is also used by bacteria
that exploit chemical transformations ­ such as those involving sulphur or
iron compounds ­ to "chemosynthesise" their organic matter. A few anaerobic
bacteria can chemosynthesise organic matter using energy from the oxidation
of hydrogen by sulphate, carbonate or nitrate instead of oxygen, but their
hydrogen is generated by other bacteria from previously photosynthesised
organic matter.

      Nevertheless, there is no reason in principle why "mineral" hydrogen,
which could be generated chemically from water, might not act as a primary
energy source independent of sunlight, supporting an ecology based wholly on
chemosynthesis. Such a process, based on the geochemical decomposition of
water, has been suggested as a means of supporting deep subterranean
microbial life, but the basic chemistry remains speculative. A comparable
process could conceivably support life on a warm, wet celestial body,
perhaps even on our distant neighbour Europa, one of the Jovian moons.

      John Postgate , Lewes, East Sussex, UK

      There are food chains that do not depend upon photosynthesis for the
initial input of energy into the system. Chemoautotrophs are microorganisms
that obtain energy by oxidising sulphides in inorganic minerals.
Hydrothermal vents that pump hot, chemical-laden water onto the ocean floor
support flourishing ecosystems far from the influence of sunlight.

      These ecosystems derive their energy entirely from the chemicals in
the vented water. Nevertheless, the oxygen they utilise to oxidise the
sulphides is dissolved in the sea water and is present as a by-product of
photosynthesis far above at the depths to which sunlight can penetrate. So
even these communities can be said to be indirectly dependent on sunlight
and photosynthesis.

      Jonathan Wallace , Newcastle upon Tyne, UK

>From New Scientist

Posted by
Robert Karl Stonjek
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