BioEd Online

Aquatic 'dead zones', oxygen-depleted areas throughout the world's waterways, are not only suffocating marine life but may also be transforming female fish into males.

There are about 150 significant oxygen-starved patches of water throughout the world, according to a recent United Nations report¹. The most notorious is the Gulf of Mexico, in which the dead zone is approximately 20,000 square kilometres, roughly the size of New Jersey.

The oxygen deprivation can be caused by fresh river water sitting above denser salt water, capping it and preventing air from reaching the waters below. The biggest cause, however, is agricultural and industrial run-off of fertilizers and fossil-fuel waste. The nitrogen and phosphorus in these materials cause population bursts of algae and marine creatures called phytoplankton; and when these blooms die they are decomposed by oxygen-consuming bacteria. A burst in activity by these microbes robs the water of oxygen.

Such areas are well-known problem spots for the survival of aquatic life, but now it seems the lack of oxygen is having another unexpected effect: tinkering with the sex hormones of fish. "The problem is much bigger than we thought," says ecotoxicologist Rudolf Wu of the City University of Hong Kong, China, whose article in the current issue of Environmental Science and Technology² shows that hypoxia can cause gender bending.

It is well known that some species of fish, and reptiles such as alligators, can change their gender; their development depends not only on their genes but also on their environment. But this has been previously linked to factors such as acidity, temperature or chemicals in the water, rather than hypoxia.

Boys will be boys?

Wu showed that zebrafishes reared for 120 days in an oxygen-starved environment were made up of about 75% males, as compared with about 60% in normal oxygen-filled waters.

After birth, all zebrafish pass through a stage in which their gonads look like ovaries, explains Wu. Then, between the age of 10 days to a month, a zebrafish's genes and the environment will alter its hormone production, helping to determine whether it develops male or female anatomy. It seems that hypoxia can alter a fish's gene expression, says Wu, and so alter the level of sex hormones being produced.

Wu looked at some of the key enzymes involved in the synthesis of sex hormones to see what was happening. The enzymes involved in converting testosterone to oestrogen, called aromatases, were less abundant in the hypoxic baby fish, leading to higher relative levels of testosterone and a greater proportion of males.

Gasping for air

Wu says there is reason to suspect that hypoxia might similarly affect levels of sex hormones in humans. Some studies have shown that people exposed to lower oxygen levels, such as those living in high-altitude regions or those who suffer from breathing problems while asleep, have different typical sex hormone levels than those exposed to normal levels of oxygen.

"It's certainly a cause for concern and something to be followed up on," says fish endocrinologist Peter Thomas of the University of Texas in Austin. Thomas says he has seen similar findings in the croaker fish within the Gulf of Mexico, but has not yet published his research.

Wu hopes to extend his observations to real-world hypoxic areas and further determine the exact mechanism for this phenomenon.

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