Several natural compounds made by and that protect Callophycus serratus, a red seaweed, against marine fungi also inhibit the malarial parasite Plasmodium but not Candida albicans, a yeast that is an opportunistic pathogen of humans, according to biologist Julia Kubanek of the Georgia Institute of Technology (GIT), Atlanta, and her collaborators. Although the link between antifungal and antimalarial activity is not understood, these dualacting natural compounds might work via a common mechanism, she says. "Learning how other species avoid diseases may provide us with tools we can use to avoid or treat our own diseases."
Kubanek and her collaborators are analyzing natural compounds from more than 800 marine organisms that were collected near the Fiji Islands. Recently, they came to focus on the red seaweed C. serratus, which produces at least 28 distinct chemical compounds to block growth of the common marine fungal pathogen Lindra thalassiae. More than half- 18-of the antifungal compounds are bromophycolides, while the other 10 are callophycoic acids. The bromophycolides and callophycoic acids form the largest group of algal antifungal compounds reported to date.
These antifungal compounds are found in discrete patches along the surface of the seaweed, and at concentrations sufficient to inhibit fungi, according to Kubanek's colleague Facundo Fernandez at GIT. He and members of his lab group are using desorption electrospray ionization mass spectrometry (DESI-MS) to analyze features of the surface chemistry of intact organisms. "We hypothesize that the patches are wound sites where the compounds are made or leak out to create a barrier against fungal invasion," Kubanek says. Details appeared online in the Proceedings of the National Academy of Sciences on May 5, 2009.
Although none of the antifungal compounds kill C. albicans cells, three of the bromophycolides have antimalarial activity, possibly through a common mechanism of action such as interfering with cell cycle functions, according to Kubanek. Other bromophycolides show modest anti-HIV and anticancer activity, she points out.
Kubanek finds it surprising that this seaweed produces so many-namely, 28-structurally related antifungal chemicals. "These compounds may have other functions that we have not yet discovered," such as staving off predators or preventing fouling by spores and larvae of other microorganisms, she says. Alternatively, enzymes with low specificity may generate diverse bromophycolides and callophycoic acids. "Evolution is not perfect, and some enzymes do not synthesize the exact molecule every time."
From seaweed samples belonging to the same species, a population nicknamed "bushy" makes only bromophycolides, while another population makes only callophycoic acids, according to Kubanek. Findings of this sort could provide insights into seaweed and microbial ecology, she says.
The surface-associated bromophycolides and callophycoic acids are secondary metabolites that govern biological interactions among organisms sharing an ecosystem, according to Paul Jensen, a microbiologist at the Scripps Institute of Oceanography in La Jolla, California. These secondary metabolites have "been the black box of microbial chemical ecology," he says. Now Kubanek and her collaborators "cracked open this box with the application of DESI-MS to directly determine the location and concentration of specific secondary metabolites on algal surfaces." Not only do the findings confirm that marine algae use antimicrobial defenses, he adds, but DESI-MS also provides an exciting new tool to explore questions in microbial chemical ecology.
Carol Potera is a freelance writer in Great Falls, Mont.