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Mass-Spec Method IDs Defense Chemicals on Algal Surface

NEW YORK (GenomeWeb News) – A tropical red algae found in Fiji produces powerful anti-fungal chemicals, found in patches on its surface, according to a new mass spectrometry study.

The work, which appeared online last night in the Proceedings of the National Academy of Sciences, identified dozens of compounds associated with a tropical red algal species called Callophycus serratus. These compounds fell into two broad groups that correlated with algal clades.

Using desorption electrospray ionization mass spectrometry, or DESI-MS, the researchers determined that at least some of these compounds are found in distinct patches on the algae's surface, suggesting they may be used defensively to deter invasion by pathogenic microbes. And, researchers say, being able to detect such chemicals may uncover new chemicals for microbes behind human infections as well.

"Plants and animals in the wild use chemistry as a way to fight with one another," senior author Julia Kubanek, a biology, chemistry, and biochemistry researcher at the Georgia Institute of Technology, said in a statement. "Using this new technology, scientists can listen in on this fight to perhaps learn from what's going on and steal some of the strategies for human biomedical applications."

Researchers from Georgia Tech and elsewhere have been attempting to catalogue the natural compounds produced by species living in the waters around Fiji for several years. For the latest paper, they focused on C. serratus, a macroscopic red algae that seems to be relatively resistant to pathogenic microbes.

Previous research indicated C. serratus could produce chemicals called bromophycolides and callophycoic acids, which seem to inhibit some pathogens. But it was unclear where these chemicals were located and how they influenced microbial interactions.

First, Kubanek and her team looked at whether C. serratus extracts inhibited two known marine pathogens: a marine fungus called Lindra thalassiae and a bacterial species called Pseudomalteromonas bacteriolytica, which causes red spot disease. Their results suggest both bromophycolides and callophycoic acids can inhibit L. thalassiae, though neither group of compounds deterred P. bacteriolytica.

When the researchers looked at the 18S rRNA sequences for the C. serratus samples they'd collected, they identified three different haplotypes that fell into two phylogenetic clades. Their experiments suggest each clade has a distinct chemotype, which the team dubbed the bromophycolide chemotype and the callophycoic acid and callophycol chemotype.

"Together, bromophycolides and callophycoic acids represent the largest group of algal antifungal chemical defenses reported to date, adding to only a handful of previously identified antimicrobial chemical defenses from macroalgae," the authors wrote.

All of the known bromophycolide products curbed L. thalassiae growth. But only a few of the callophycoic acid and callophycol compounds showed similar L. thalassiae growth suppression. Even so, subsequent experiments revealed that, as a group, the callophycoic acid and callophycol compounds can inhibit the fungus.

In an effort to determine whether any or all of these compounds are localized on the algae's surface, the researchers used DESI-MS, a technique that involves taking compounds from the surface of materials with a solvent spray and characterizing these "desorbed" compounds by mass spec. Because the surface and, in this case, the algal tissues, remain intact, DESI-MS can distinguish between chemicals on the surface of the algae and compounds in the tissue.

"This technique allows us to examine intact organisms and see how the chemical compounds are distributed," Kubanek said. "For our research with seaweed, this is important because we'd like to understand how an organism distributes these compounds to protect itself from enemies."

Indeed, when the team used DESI-MS to assess the location of two bromophycolides, bromophycolides A and B, they found that the compounds appear in patches on C. serratus' surface. After breaking open the seaweed, the researchers detected bromophycolides inside C. serratus as well.

The researchers noted that it is possible but unlikely that algal bacterial symbionts produce the secondary metabolites, since they did not see any microbes in the algal material, and microbes isolated from C. serratus don't seem to produce bromophycolides on their own.

Down the road, the researchers plan to further characterize the C. serratus-related compounds with an eye toward developing new treatments for humans. For instance, Kubanek and one of her graduate students are currently tweaking compounds that seem to have anti-malarial activity.

"Learning about how other species avoid diseases may give us something we can use to avoid or treat our own diseases," Kubanek said.

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