How sea-dwelling microbes help form clouds
The bubble-bursting role of bacteria in sea spray has a profound influence on the global climate. Andrew Masterson reports.
The link between oceanic bacteria and cloud formation is far more direct than previously assumed, new research shows.
Molecules produced by bacteria and phytoplankton affect the size and durability of tiny bubbles caused by the actions of waves and ripples.
Chemical reactions catalysed by the molecules in turn affect the ways in which the bubbles burst and release a vapour known as sea-spray aerosol. Movements and densities of these aerosols have a marked influence on the creation of clouds and therefore, ultimately, climate.
Scientists led by Vicki Grassian of the Centre for Aerosol Impacts on Climate and the Environment at the University of California, San Diego (UCSD), discovered the influence of oceanic bacteria and phytoplankton means that each of the trillions of bubbles formed by wave action is unique – and so too is the vapour released when a bubble fractures.
"We were surprised by how distinct the changes in the chemicals in the individual aerosol particles were," Grassian says.
Molecules derived from seaborne microbes were found to interact with chemicals and salts trapped during bubble formation. In some cases the interaction reduced the bubble’s ability to hold water, and thus lowered the amount of aerosol released into the air.
To reach their findings, she and her colleagues made use of an artificial ocean constructed at the Scripps Institution of Oceanography on the UCSD campus.
They induced a phytoplankton bloom and then measured the aerosol content of burst bubbles at various stages through the microbial bust-and-boom cycle. They found the bubbles were most adversely affected during periods when the phytoplankton was under predation from bacteria.
"This is a great result for us as it helps us see more clearly how sea spray aerosol is involved in climate," Grassian says. "It's important to understand the impact natural processes have on the climate so we can build up a more accurate picture of climate change."
The choice of using of the Scripps artificial ocean instead of performing fieldwork in a real sea was deliberate, but not for reasons of convenience.
The water in the campus facility is unpolluted, and the team wanted to establish the influence of naturally occurring aerosol modifiers as a baseline measurement.
The next challenge for the researchers is to investigate how the aerosolising process is affected by the presence of common ocean pollutants, including ozone, fly ash, soot and nitrogen oxides.