Oceanography researchers discover toxic algae in open water

The findings were published in the Nov. 8 edition of the Proceedings of the National Academy of Sciences (PNAS).

Harmful algal blooms, or HABs, are reported as increasing both geographically and in frequency along populated coastlines. Bargu's research shows that the ubiquitous diatom Pseudo-nitzschia -- an alga that produces the neurotoxin, domoic acid, or DA, in coastal regions -- actually also produces DA at many locations in the open Pacific. The presence of these potent toxins in deep water environments is worrisome, given that in coastal waters, where the phenomenon has been studied, DA can enter the food chain, forcing the closure of some fisheries and poisoning marine mammals and birds that feed on the contaminated fish. The main concern, though, is that the adding of iron to ocean waters -- one of the most commonly proposed strategies to reduce global warming -- appears now to likely result in promoting toxic blooms in the ocean.

Because both natural and artificial iron additions in open ocean waters can result in phytoplankton blooms in large areas of the sea that are deficient in the metal, and also because phytoplankton take up carbon dioxide and iron in addition to seawater, it has been suggested as a remedy to combat global warming. Basically, the new growth of photosynthetic cells draws carbon dioxide into the sea, reducing the amount of this gas in the atmosphere and thus reducing its warming effect. However, Pseudo-nitzschia-like cells have been known for a while to be common responders in a number of iron fertilization experiments conducted at sea over more than a decade.

To address the possible presence of the neurotoxin DA in regions where Pseudo-nitzschia occur in the open ocean, Bargu and one of the paper's co-authors, Mary Silver from University of California, Santa Cruz, or UCSC, sought and received funding in 2007 from National Science Foundation to look for the toxin producers in the open sea. They joined Ken Bruland, professor of ocean Sciences at UCSC, on a research cruise to study iron chemistry in oceanic waters of the Gulf of Alaska. During this expedition, they collected water samples and found the algae and its toxin in nearly all of the natural oceanic environments throughout the region. This prompted them to examine older, stored samples from other sites around the Pacific, and again they found the toxin in most samples.

Then, with the help of Kenneth Coale, director of Moss Landing Marine laboratories and principal investigator on several cruises that conducted classic iron enrichment experiments in the Pacific, they retrieved samples and found both Pseudo-nitzschia and substantial amounts of toxin. Their findings show that iron enrichment indeed promotes high levels of toxins in the open sea, sometimes as high as those in coastal regions, where deaths of seabirds and mammals occur. The authors of this PNAS paper also noted that iron enrichments can occur naturally, suggesting that the high levels of toxins may also have occurred when iron was added by wind-blown dust and other climate and geological processes.

Bargu, a principal investigator on the Gulf of Alaska phytoplankton study, was responsible for identification of all the Pseudo-nitzschia species and the toxin testing in the project. In addition to Bargu and her former graduate student Garcia, the coauthors of the PNAS paper include Silver, Coale, Ken Bruland, Susan Coale and Kathryn Roberts at UCSC; and Claudia Benitez-Nelson and Emily Sekula-Wood at the University of South Carolina. This research was funded by the National Science Foundation, U.S. Department of Energy and the U.S. Office of Naval Research.

Impacts from HABs have been also the subject of regional interest in the northern Gulf of Mexico for many decades. Among the many HAB impacts in this region, those due to coastal blooms of the diatoms genus Pseudo-nitzschia and the associated DA are of particular concern. Studies of Louisiana coastal and estuarine waters have documented the occurrence of toxic Pseudo-nitzschia, often at bloom concentrations with annual occurrence of spring blooms exceeding a million toxic cells per liter. Current thinking suggests the annual spring peak in Pseudo-nitzschia probably occurs in response to the spring flood of the Mississippi River.

An ongoing study by Bargu's lab is studying the extent of Pseudo-nitzschia and its toxin, finding that that DA-producing Pseudo-nitzschia are frequently present in nearshore and offshore Louisiana waters. Her research group has already detected high levels of DA in water and at moderate levels in gulf menhaden from Terrebone Bay, La., the second largest fishery in the U.S.

Bargu is specifically studying the impact of these blooms on the food web, trying to better understand the impact such algae growth can have on the next level up the food chain. She argues that it is now critical to understand the extent of toxic algal blooms in the area and their causes.

"Given our dependence on the seafood industry here, it is critical to recognize and then reduce the causes of toxic algal blooms," she said. "Only by monitoring and addressing the causes of the blooms can we safeguard our fisheries and the people who make a living working in them."