Toxins released by algae and launched into the air by bursting bubbles and breaking waves can accumulate to much higher levels than are found in seawater, and can then be inhaled and potentially impact human health.
Scientists used a tank that simulates crashing waves to investigate how algal toxins travel from water to air in the laboratory and on a beach in Belgium, and found that the concentration of toxins in the air could reach levels several hundred times higher than those of the water. The team reported the findings April 12 in Environmental Science & Technology.
"When we see harmful algal blooms, we know we can't swim in the water, we can't drink the water, but what our results are showing is ... that it might also be harmful to our health just to walk on a beach when there's a harmful algal bloom," said Jana Asselman, an assistant professor in blue growth and blue economy at Ghent University in Belgium and a co-author of the study.
In both coastal and freshwater environments, microorganisms can be trapped by air bubbles created by the churning water, then hurled into the air when bubbles pop. During harmful algal blooms such as the red tides sometimes seen in Florida, these sea spray aerosols can cause respiratory symptoms such as coughing and wheezing when breathed in, particularly in people with asthma.
However, with the exception of two groups called brevetoxins and ovatoxins, little is known about how toxins produced by coastal algae become aerosolized.
"All these things led us to focus on ... what is in sea spray aerosols that could cause positive and negative health effects?" Asselman said.
She and her colleagues examined seven common phycotoxins. Among these were okadaic acid and dinophysistoxin-1, which can contaminate shellfish and cause food poisoning when eaten. Another, called homoyessotoxin, is under study as a potential anticancer drug.
For their experiment, the researchers "spiked" tanks of saltwater and natural seawater with the algae that produce these toxins, then measured the concentration of toxins in the water and the air above.
The concentrations of toxins that were launched from the saltwater by the tank's artificial waves ranged from 488 to 1,769 times as high as those from beneath the surface. The concentrations of airborne toxins were 78 to 520 times greater than the concentrations measured in the natural seawater. These lower concentrations were likely related to bits of organic matter or other seawater properties that affected the aerosolization process, the researchers said.
The team then brought its wave-making tank to a beach in the coastal city of Ostend and repeated the experiment outdoors. Because the researchers used seawater without adding extra algae, they saw lower phycotoxin concentrations than those measured in their previous natural seawater and saltwater experiments.
The abundance of airborne toxins was tied to their build-up in a sticky, slimy layer on the surface of the water.
"These molecules stick in there and they enrich in that area," Asselman said. "When waves break these, bubbles will burst and this will lead to high levels of phycotoxin [in the air], much higher than what you find in the large body of water."
This could explain why even relatively low concentrations of algae in the water can cause respiratory symptoms during red tides, she says.
She and her team also sampled the air at the beach.
"The concentrations at that time were below the detection limit of the machine because there wasn't enough wind or waves," Asselman said. "If there is no wind, if the sea is pretty calm, there is little to no aerosolization."
Plus, since the researchers had collected air near the shoreline rather than in the enclosed space of the tank, the aerosols were more diluted.
"To make extrapolations about what we can find in the real world," Asselman said, they measured the sodium concentration in the air, which gave them a hint about the magnitude of the salty aerosols. They estimated that the outdoor aerosol production was something in between what was measured in their previous natural seawater and saltwater experiments.
"It shows us that these toxins indeed can become aerosolized even though we cannot measure them in the air itself due to weather conditions and sampling conditions," Asselman said.
Additionally, Asselman says, the researchers did not account for the fact that when the wind blows from inland, it will sweep the aerosols back out to sea and away from people on the coast.
She and her colleagues are now investigating potential human health effects of different concentrations of the algal toxins by exposing them to human cell cultures. They're also expanding the work to include seaborne bacteria and viruses.
"We're always talking about harmful algal blooms and toxins, but it is always a question of dose," Asselman said. Whether naturally occurring algae build up to harmful levels depends on our own actions, she says.
"Harmful algal blooms are driven by climate change and pollution, which is something that we have under our control," Asselman said. "How toxic the ocean becomes to us really depends on how well we care for the ocean."
The study, "Phycotoxin-enriched sea spray aerosols: methods, mechanisms, and human exposure," published April 12 in Environmental Science & Technology, was authored by Emmanuel Van Acker, Steve Huysman, Jana Asselman, Karel A. C. De Schamphelaere, Lynn Vanhaecke and Colin R. Janssen, Ghent University; and Maarten De Rijcke, Flanders Marine Institute (VLIZ).