From Larry Rivera,
Your Guide to Australia / New Zealand Travel.
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A. They look like rocks, you'd think they were rocks.

The stromatolites found at their most diverse and in great abundance at Hamelin Pool, Shark Bay, Western Australia, may look like rocks but they are in fact living things, believed to be the oldest living organisms on earth.

In photosynthesis in the daytime, they actually wave around like rocks in some kind of slow motion.

Favorable salinity

Predators and other marine life which feed on the bacteria and algae of which stromatolites are composed cannot tolerate the salinity of Hamelin Pool (twice as saline as usual sea water). This high salinity is caused by a bar at the mouth of the bay which traps the salt in the pool when rapid evaporation occurs in hot conditions.

As a result Hamelin Pool is one of the best places on earth to find living marine stromatolites.

Building blocks

Primitive one-celled organisms including cyanobacteria (blue-green algae) grow through sediment and sand, binding sedimentary particles together, and increasing in size layer by layer in extremely slow fashion. Through long periods of time, stromatolites do turn into rocks.

For much of the earth's history stromatolites were the building blocks of reefs, creating large masses of calcium carbonate.

Through photosynthesis, stromatolites produce carbohydrates from carbon dioxide and water, in the process liberating oxygen into the air.

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