Natural dust may be harmful to ocean organisms

by Carolyn Gramling
Thursday, January 5, 2012

Although human-made aerosols sometimes are toxic, natural aerosols from deserts and volcanoes are important contributors to ocean fertilization. Now, new research suggests that natural aerosols have their downside too — and the impact of natural aerosols on ocean organisms could influence predictions of future climate change.

Scientists studying the link between the ocean’s biogeochemical cycles and natural aerosols — tiny particles suspended in the atmosphere that originate from non-human sources, such as volcanic eruptions, soil dust from deserts or soot from forest fires — tend to focus on iron and its possible fertilizing effect on ocean organisms, says Katherine Mackey, a doctoral student at Stanford University in Palo Alto, Calif. But Mackey, Stanford undergraduate student Joshua Chan and biogeochemist Adina Paytan of the University of California at Santa Cruz are looking at the flip side of the issue: whether the metals in natural aerosols can also have a toxic effect on ocean organisms.

Mackey and Paytan previously researched the effect of natural aerosols on a range of organisms in a sample of seawater. In that study, Mackey says, they found that “some aerosols had a fertilizing effect and some a toxic effect” on some marine organisms.

The team suspected that the metal particles in the aerosols might be responsible for the toxicity, because certain metals were at much higher concentrations in the toxic dust than in the fertilizing dust, particularly silver, copper and nickel. But which of these metals was the most harmful to the algae — and how did its impact vary for algae from different parts of the ocean?

To answer these questions, the team studied Synechococcus, a “ubiquitous genus that is incredibly diverse,” Mackey says. Synechococcus are cyanobacteria (often called blue-green algae) that thrive around the world in both open oceans and freshwater, and even in extreme environments such as the hot springs of Yellowstone National Park in Wyoming. In the earlier seawater study, Synechococcus were particularly susceptible to aerosols, so the team decided to study the effects of excess metals on four cultured strains from different ocean environments: two from coastal waters and two from the open ocean.

They added several metals to each of the cultured strains, including silver, nickel, cadmium, lead, copper and zinc. Lead and copper were highly toxic to all strains of the algae, but for most of the metals, the coastal-dwelling algae were more resistant than the algae from the open ocean. That, Mackey says, could mean that strains of algae living close to the shore are more evolutionarily adapted to cope with these metals. The team presented their results at the annual meeting of the American Geophysical Union in San Francisco, Calif.

Why some strains of the algae are hardier when it comes to toxic metals is an interesting chicken-or-egg question, Mackey says. “Are they more resilient because they evolved there, closer to land, closer to the source? Or is it that of all the available populations that lived there, those are the ones that survived?”

The question is of particular interest from a climate change perspective, Mackey says. As the climate changes, it is not yet clear whether dust, and therefore natural aerosols, will be more or less prevalent, as some parts of the world may become drier and others may become wetter.

“And the second thing is, is that a good or a bad thing for the ocean?” she adds. Climate change models have predicted anywhere from a 300 percent increase to a 60 percent decrease in desert dust in the future. What impact the dust would have on photosynthesis is also not clear: More dust could fertilize the ocean and spur photosynthesis, but an increase of toxic metals could instead inhibit photosynthesis. It is clearly important to better understand this effect, as photosynthesis is itself a driver of climate change: It draws more carbon out of the atmosphere and into the ocean, thereby helping to cool the planet, she says. “Climate warming will have many implications, and those on dust and ocean productivity are still unclear.”

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