A hungry red tide is a dangerous red tide

By John Upton

When fertilizer or sewage runs into a waterway, or when phosphorous and nitrogen rise up from the ocean depths, algae can converge and feast and mushroom on the buffet of growth-inducing nutrients.

But scientists have discovered that starving a poisonous red tide of its nutrient supply can trigger a very dangerous and counterintuitive response.

Red tides are freaky types of algae blooms. They often occur in the ocean or in salty bays, and they frequently produce poisons. Scientists prefer the term “harmful algal bloom,” since a red tide isn’t always red and it is most certainly not a tide.

Illustrated by Perry Shirley
Illustrated by Perry Shirley

The most common type of algae in Gulf of Mexico red tides is a dinoflagellate called Karenia brevis. The neurotoxin produced by these single-celled creatures help protect them from predation: Would-be hunters can die if they take a mouthful. But as the red tides break down, the poison escapes from the plankton cells and it can drift through the marine environment, poisoning it. The toxin can even spray into the air, aerosolized by crashing waves, where it can get into lungs and trigger serious ailments in people and other animals. The Floridian West Coast is often the worst affected.

Concentrations of the poison in each of the algae cells varies widely — from a mild 1 picogram per cell to a treacherous 68 picograms per cell. Needless to say, figuring out what causes a bloom to be especially poisonous would be valuable for public health officials.

Since Karenia brevis uses nutrients to grow, one may assume that starving them of phosphorous and nitrogen, such as by preventing fertilizer or sewage runoff into the Gulf, would protect the environment from their poisons.

But that’s only true up to a point. If you can keep nutrients out of the water, a bloom will not materialize, so there will be no danger of the waterway being poisoned by it. But if the nutrient supplies suddenly dry up, an existing bloom will switch into a defensive mode, stop growing and become very toxic.

The ecological theory to describe this response comes to us from botany. It is called the carbon:nutrient balance hypothesis.

North Carolina scientists grew samples of the dinoflagellate in water taken from the Gulf in a laboratory. Some samples were fed plenty of phosphorous, but others received very little. The scientists found that K. brevis strains living with limited phosphorous supplies produced 2.3 to 7.3 times more poison than did those that had plenty of phosphorous available.

“Because PbTxs [K. brevis brevetoxins] are potent anti-grazing compounds, this increased investment in PbTxs should enhance cellular survival during periods of nutrient-limited growth,” the scientists wrote in their paper, published last month in PLoS ONE.

The algae samples living without much phosphorous put their carbon to a defensive use, since it couldn’t be used as effectively for growth. The proportion of carbon that each cell used to produce poison as much as doubled when phosphorous was limited.

This is consistent with the carbon:nutrient balance hypothesis. When vegetation has lots of carbon and lots of nutrients available, it invests those building blocks of life into fast growth. But when nutrients, be they phosphorous or nitrogen, are in short supply, the carbon is put to a different use: Defense against predators.

It also helps explain some of the late season bursts in toxicity noticed in the red tides: They become poisonous after they have greedily slurped down the last of the available nutrients.

This research was limited to phosphorous. But previous research uncovered a similar red tide response when nitrogen was limited.

The discovery could help public health managers predict the potency of red tides in the Gulf of Mexico. By measuring the amount of phosphorous in the ecosystem, it could become possible to determine how dangerous the red tides will become.