“Red Tides” May Be Linked to Bacterias
November 6, 2006
Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334
If California Sea Grant researchers are right, bacteria that live symbiotically with toxic algae may be just as important as nitrogen and phosphorous, the active ingredients in fertilizers, in sparking some toxic algal blooms.
Most “red tides” are harmless but some produce toxins that taint seafood with potentially deadly toxins. For more information, visit http://seafood.ucdavis.edu/Pubs/natural.htm.
Credit: Washington Department of Health
Professor Carl Carrano of San Diego State University and colleagues Frithjof Kuepper and David Green at The Scottish Association for Marine Science in Oban, Scotland, are examining compounds made by bacteria that they believe transform otherwise biologically useless iron compounds into forms that can be used by marine algae. Because iron is so essential for growth, it is these iron compounds, they believe, that trigger sudden bursts in the number of algae in an area, also known as algal blooms.
Most algae are harmless and, as the base of the marine food chain, are vital for maintaining the ocean’s productivity. Some algae, however, produce toxins that can taint fish or shellfish. These can cause harmful algal blooms. For reasons that largely elude scientists, harmful algal blooms have become more common and more intense in recent decades. Mainstream science blames fertilizers spread on crops and lawns – and washed to sea via rivers and runoff – as the “MiracleGro” feeding marine algae and hence triggering harmful algal blooms.
Polluted waters may also be high in another nutrient that marine algae need – iron. The Sea Grant researchers’ main point is that symbiotic bacteria may be making this iron – or other iron sources – available. Taken a step further, their ideas imply that bacteria may be actively changing the chemistry of seawater in ways that fundamentally influence primary productivity in the sea.
As a case study for testing their ideas, Carrano and colleagues are investigating compounds made by bacteria associated with the toxic algae Gymnodinium catenatum. This toxic algae has some notoriety and significance for public health and the seafood industry because it produces a toxin that causes paralytic shellfish poisoning, a potentially life-threatening neurological syndrome contracted by eating contaminated seafood. The California Department of Health Services annually issues a quarantine on all sport-harvested mussels for the entire California coast from May 1 to Oct. 31 to protect the public from paralytic shellfish poisoning and domoic acid, a nerve toxin produced by another kind of marine algae.
“The marine bacteria we are studying have evolved a complex mechanism for obtaining iron,” Carrano
San Diego State University professor Carl Carrano studies the chemistry of an organism whose toxins cause paralytic shellfish poisoning. His research investigates a possible link between harmful algal blooms and iron compounds made by certain bacteria. Credit: San Dego State University
said. They produce compounds known as siderophores that convert insoluble, inorganic forms of iron into stable, soluble, biologically available iron compounds. Receptors on a bacterium’s outer membrane recognize these compounds, bind to them and shuttle them inside the cell, where the iron is stripped and used for a variety of essential cellular processes.
The researchers’ working hypothesis is that this toxic algae, and likely others, can utilize the bacteria’s iron-acquisition machinery.
Now eight months into their two-year Sea Grant project, they have preliminary findings that lend exciting support to the theory. For one, they have found that all the bacteria associated with the toxic algae produce the same siderophore – a compound known as vibrioferrin. “No matter where we collect the algae, we find their symbiotic bacteria always produce vibrioferrin,” Carrano said. “We would not expect this unless the siderophore was providing some function.”
It is also impossible to grow the toxic algae without these same bacteria. “Clearly, the bacteria provide a critical function,” he said.
It is no stretch to guess that this function is related to iron. “The specific forms of iron are variable and complicated in the ocean,” said marine chemistry professor Kathy Barbeau of Scripps Institution of Oceanography. “We don't understand how much of this iron is biologically available. This is where siderophores might be important. They could be changing the form of iron.”
Even more exciting has been the discovery that the siderophore vibrioferrin binds to the element boron. This is significant because natural products containing boron are very rare, Kuepper said. “The most notable bacterial product using boron is a quorum-sensing molecule.”
Quorum sensing is basically a way for bacteria to communicate among themselves – a kind of chemical signaling mechanism. It allows bacteria to coordinate gene expression when a sufficient population of bacteria is reached. Researchers have long speculated that quorum sensing is involved in bioluminescent algal blooms (algal blooms that glow at night).
GLOWING WAVE: Bioluminescent marine algae light up a breaking wave at midnight in Carlsbad, Calif. The blue light is a result of a luciferase enzyme. The phenomenon is thought to have something to do with quorum sensing. Credit: This is Public Photo downloaded from www.Flickr.com.
Their work could potentially identify the specific compound involved in signaling blooms. “We believe that the toxic algae effectively wire taps the bacteria's communication system,” Kuepper said. “The boron compound tells the algae it is OK to grow because there is enough iron.” In this scenario, the production of the siderophore precedes algal blooms. This is the type of scientific information that could be used to develop novel early algal-bloom detection systems.
Though the scientists’ ideas remain in the realm of speculation, their ideas dovetail well with other cutting-edge investigations of marine algae. Professor Alison Butler of UC Santa Barbara, a former California Sea Grant researcher who led some of the first studies of marine siderophores, notes that others have already demonstrated the ability of some eukaryotic cells (cells with a nucleus) to reduce iron from iron-siderophore complexes. Professor Mark Wells of the University of Maine in Orono, meanwhile, recently published findings suggesting a link between iron and copper and the toxin domoic acid, which causes amnesic shellfish poisoning.
This Sea Grant research would be the first to link bacteria and marine siderophores to algal blooms, and this would represent a major discovery. “To our knowledge, no other group in the world is currently studying or planning to study the role of siderophores in forming harmful algal blooms,” Carrano said.
to restore commercial salt ponds at the South San Diego Bay National Wildlife Refuge. “This is an internationally recognized area for bird nesting and migration,” said Johnson, who earned a master’s in marine ecology from San Diego State University.
Prior to joining SCWRP, Johnson was a biologist with the environmental consulting firm Merkel & Associates in San Diego for four years. Before that, she was a California Sea Grant State Fellow in 2001 with the Coastal Commission in Ventura, where she worked on some of the very earliest stages of implementing the Marine Life Protection Act, including mapping out where to site marine protected areas.
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