Wayne Coats (right) and Mario Sengco inspect a culture in the lab’s growth chamber.

 

This story first appeared in the SERC quarterly Newsletter Winter 2006/2007

Small World Big Impact

Last summer, astronomers redefined one of the most basic structures in their field of study, the planet. With Pluto now demoted to a new category, that of dwarf planet, it has became very publicly evident how fluid the science of classifying our world can be.

So it is in the complex world of single-celled organisms, where nomenclature continually shifts in an elusive attempt to describe the order of the universe in a drop of water. What distinguishes and defines these tiny beings—protists as they are collectively called—is not merely a question for academics, for they are among the most abundant forms of life on earth. Some of the most infamous of the protists include the parasitic sporozoan that causes malaria, the insidious amoeba responsible for dysentery, and, perhaps more germane to the developed world, the many species of dinoflagellate responsible for harmful algal blooms such as toxic red tide.

What goes on between these elusive single cells in their enigmatic world ripples up through the fabric of their universe into the very tangible reality of ours. Understanding how is critical to the hope that we may some day exert control in the areas of human health and economics where the protists reign.

For Wayne Coats, SERC's Principal Investigator in the Protist Ecology Lab, that hope is translated from the study of what drives and controls the populations of various species of microalgae in the Chesapeake Bay.

It's well known that nutrients flowing into the water from the land after a heavy rain can cause a bloom of algae. But what determines the species that blooms at any given time remains a mystery. What causes blooms to recede is equally unclear. However, Coats has demonstrated that parasites may play an important role.

Indeed, he has shown that they hold great potential for helping us manage harmful algal blooms. To be useful, a parasite would need to infect only the species of algae that we want to manage without having other impacts on the environment. It would need to be harnessed, cultured in the lab in great quantities, and kept live at the ready should a bloom begin to form. These are exactly the pieces of the puzzle that Coats has been laying on the table.

A few years ago, he began investigating the role that microalgae of the genus Amoebophrya, played in the waters of the Chesapeake Bay. A nasty parasitic algae that enters its single-celled host and destroys it from within, Amoebophrya has been known to infect at least 30 species of microalgae around the world. It had been linked in the field to declines in red tides, and clearly played an influential role in the environment. But, with so many species susceptible to infection by Amoebophrya, it was dismissed as a potential tool for controlling harmful blooms.

Coats, however, was determined to understand the details of the relationship between Amoebophrya and its hosts. After careful lab and field studies, he discovered that Amoebophrya was not comprised of a single species, but rather a species complex with a number of variants, all very specific as to their target hosts. His work re-invigorated scientific interest in the use of parasites to control blooms.

That interest drew Mario Sengco to Coats' lab a few years ago when he was a graduate student at the Massachusetts Institute of Technology. Sengco has since earned his Ph.D. and returned to work in Coats' lab. Recently, he has isolated the Amoebophrya strain that infects the red-tide algae responsible for paralytic shellfish poisoning in the Gulf of Maine.

With his live cultures, Sengco intends to conduct controlled laboratory experiments to find out exactly how host specific his strain is, what happens to the host toxins after an Amoebophrya infection and what environmental conditions seem to influence a bloom and a subsequent infection by Amoebophrya.

Isolating specific strains and maintaining live cultures has been a technological challenge, but Sengco's culture is just one of seven different live strains of Amoebophrya housed in Coats' lab. "It's the only place in the world with such a collection,” Coats said. His cultures enable him and his colleagues to ask questions that are not feasible in the field. They also provide the framework for creating harmful bloom control tools. "If parasites are ever going to be used to control blooms,” Coats said, "the source would have to be a culture.”

Though his research is laying the groundwork for a very practical future, Coats is fond of saying that he is driven by a basic curiosity about how things work. Following up on his earlier studies, he set out to understand how parasites manage to infect toxic algal species without themselves falling victim to the toxins.

The bloom-forming micro-algae Karlodinium veneficum, (formerly called Karlodinium micrum) has a compound in its tissues that essentially pokes holes through cell walls. As water floods in through the new pores, infected cells swell to the point of rupturing. When Karlodinium veneficum gets into the gills of certain fish, it's deadly, and has been linked to fish kills in the Chesapeake Bay since the early 1950s. Coats and his colleagues presumed that a successful parasite of K. veneficum would either have some means of inactivating the toxin, or would mimic its host's protective mechanisms. They discovered that it was the latter. Amoebophrya utilize the same defense mechanism as the toxic algae, namely their cell membranes contain a certain fatty acid that renders them impenetrable to the hole-punching toxin.

The next question for Coats was how well the parasite succeeded in infecting K. veneficum of varying toxicity levels. His work surprised even him, and suggest some solution to the paradox of why the toxic algae comes in a variety of strengths in the first place.

The species varies from non-toxic to very toxic, and Coats presumed that the Amoebophrya parasite would naturally prefer the non-toxic variety. However, his lab experiments showed a distinct preference for the toxic K. veneficum. To Coats, this discovery revealed an elegant system of interaction and adaptation.

High toxin levels serve the species very well when grazers are dominating the neighborhood. With few predators willing to risk a meal, toxic K. veneficum could be primed for a bloom whenever conditions are ripe. But once their numbers reach some critical mass, conditions become right for an outbreak of any pathogen able to take advantage of high population density. Enter the parasite Amoebophrya. As we know from human history, outbreaks can be devastating to a population. Here is where the low-toxicity K. veneficum shine. Because they are less desirable hosts for Amoebophrya, they avoid the outbreak, and ensure the survival of the community. Varying toxicity, it seems could be K. veneficum's answer to alternating pressures from grazers and parasites.

"There are lots of neat things about this,” said Coats. "It helps answer that paradox of why there are toxic and non-toxic strains. There is evolutionary selection for both to persist.”

It also points to a potential tool for controlling these types of toxic blooms with no ill effects, by introducing a parasite which only impacts K. veneficum and yet does not wipe out the species.

Others, like Sengco, will surely build on this work, searching for practical ways to influence harmful blooms. But in the meantime, Coats continues to lay down the foundation. Understanding the role of parasitic species in mitigating blooms may offer the promise of a potential management tool. But the most important tool Coats and his colleagues are creating is knowledge, the basic knowledge of how organisms in a universe too tiny to see with the naked eye behave and interact to influence the world around us.

For more information, or to reach Dr. Coats, please contact SERC science writer Kristen Minogue.