Increasing the saltiness of freshwater changes the number and types of bacteria that are launched into the air by breaking waves and bursting bubbles, which could have implications for the risk that people will be exposed to toxins from harmful algal blooms in the future.

Scientists sprinkled varying amounts of salt in pond water samples and found that airborne bacteria peaked at salinity levels that reflect conditions sometimes seen in real-world habitats. The researchers reported the findings April 5 in Environmental Science & Technology.

Lakes, streams and other freshwater bodies are becoming increasingly salty in many areas because of human activities such as roadway de-icing, fertilizer runoff from agriculture and sewage and industrial waste. Additionally, increased flooding related to climate change and sea level rise is expected to send more salty runoff and seawater into lakes and coastal waters.

"Our infrastructure isn't necessarily designed to experience these extreme events," said Hosein Foroutan, an assistant professor of environmental and water resources at Virginia Polytechnic Institute and State University and last author of the new paper. 

Freshwater salinization is detrimental to water quality and aquatic biodiversity. 

"Increasing salinity can induce shifts in the structure of native freshwater bacterial communities, which could disturb their role in mediating basal ecosystem services," Foroutan and his colleagues wrote in the paper. 

He and his team have previously found that increasing the salt concentration of water also affects a process called aerosolization, which involves small bubbles that form underwater, rise to the surface, and pop.  

"Even if you don't go into the water and you spend maybe half an hour to an hour by the beach ... you feel that your face is kind of salty, you feel the taste of salt in your mouth," Foroutan said. "That's salt that's being ejected from the surface of the water through the exact same mechanism — basically, small droplets of water getting airborne because of this wave-breaking." 

Salt isn't the only thing that can be wafted into the air; as they rise, bubbles can also "scavenge" waterborne microbes. When a harmful bloom of cyanobacteria or other algae is underway, people can inhale aerosolized toxins that cause breathing problems and asthma attacks. 

"These droplets could eject anything that's in the water — bacteria, viruses, any ocean pollution," Foroutan said. Once a bacteria-laden droplet is launched from the water, it can be carried hundreds of meters inland. This makes it crucial to understand how changing salinity affects how these microbes are aerosolized. 

As water becomes saltier, the air bubbles that drive aerosolization become more numerous, surface foams become bigger and smaller bubbles emerge, Foroutan has found. To find out what this means for airborne bacteria, he and his colleagues gathered freshwater from Pandapas Pond in Blacksburg, Virginia. 

They then added to the samples different amounts of salt, which ranged from 0 to 35 grams per kilogram of water, and placed them in a tank that sloshed the water about to mimic breaking waves. Finally, the researchers assessed the abundance and identities of the bacteria that became airborne.

The team found that the airborne bacterial community was significantly different from the waterborne community. Additionally, the aerosolized bacteria increased and finally peaked at concentrations of around 1 gram of salt per kilogram of water, which match those observed in some freshwater habitats.  

It's not clear why bacteria were most plentiful at this salinity level. However, the peak may be related to the fact that some kinds of bacteria can be launched into the air more easily than others, Foroutan says. Additionally, changes in bubble size due to increasing saltiness might also affect which bacteria become aerosolized. 

"Depending on this size ratio — the size of bacteria to the size of bubbles — they may scavenge more bacteria from a specific group, or less, to the surface," Foroutan said.  

He and his colleagues looked at broad families of bacteria and only sampled from one pond, which means that the researchers couldn't determine whether the microbes pose a health hazard, or whether others that become aerosolized in saltier waters are more likely to be harmful to humans.

However, stormwater runoff is known to feed harmful algal blooms and can become contaminated with manure that hosts pathogens such as E. coli, which seems to thrive in salty freshwater. 

"What we are trying to say is that the pollution doesn't stay just in the water; it may get airborne," Foroutan said. Salinization, he added, "is affecting what can get airborne, and it's important to look into the combined water and air quality altogether." 

In the future, he plans to study more precisely how bubble size affects which bacteria become airborne and whether different types of salt and water pollution have different impacts on this process, as well as to pinpoint which species of bacteria are most affected. 

"These are things that we can control, hopefully, by controlling the amount of salt that we let go into the freshwater system," Foroutan said. 

The study, "Increasing freshwater salinity impacts aerosolized bacteria," published April 5 in Environmental Science & Technology, was authored by Charbel Harb, Jin Pan, Stephen DeVilbiss, Brian Badgley, Linsey C. Marr, David G. Schmale, III and Hosein Foroutan, Virginia Polytechnic Institute and State University.