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American Association for the Advancement of Science

Parasites keep the water clear

Aerial view of the ecosystem sampled during the study.

Rivers and oceans all over the world are home to a variety of microorganisms. Under certain conditions, these microorganisms can grow and reproduce without limit. When the water is rich with nutrients and there are no predators or parasites to keep their populations in check, some populations of microorganisms can grow until the water seems to turn a different color – their color.

An illustrated toxic bloom event.

A "harmful algal bloom" can occur under these conditions. They happen when a population of microorganisms in the water grows without any limits. Sometimes, they are called "red tides" because the microorganisms, which are often red protozoans, grow so much that the water appears to turn red. When this happens, the protozoans can use up all the oxygen in the water, and cause other marine organisms to suffocate. The protozoans also manufacture dangerous poisons, which are even toxic to humans.

Recently, over the past couple of decades, red tides have increased all over the world. This is due to higher nitrogen levels in the water from run-off, and possibly, the warming global climate.

In the past few years, however, French researchers have noticed that a particular microscopic parasite has returned to the same waters each summer to keep populations of red tide-causing protozoans in check. Aurélie Chambouvet and colleagues sampled the Penzé River for three consecutive years, and observed that the same host-specific parasites came back each year to control the protozoans' populations.

Now, in the waters where red tides used to occur regularly, the tiny parasites keep the water clear. The protozoans are still present in the water, but red tides no longer occur because the parasites do not let the protozoans' populations grow too much.

These findings tell us that the increase of red tides around the world might be caused by disruptions between the red tide-causing protozoans and these small, natural parasites.

This research appears in the 21 November issue of Science.