Identical virus, host populations coexist for centuries

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A scientist analyzing ancient plankton DNA signatures in sediments of the Black Sea has found that the same genetic populations of a virus and its algal host can persist--and coexist--for centuries. The findings have implications for the ecological significance of viruses in shaping ecosystems in the ocean, and perhaps in fresh water.

"The finding that the DNA of viruses and their algal hosts can be preserved in the geologic record is of great interest to microbial ecologists," said Marco Coolen of the Woods Hole Oceanographic Institution (WHOI).

"It offers unprecedented insights into long-term algal, viral and host population dynamics between globally important algae and their viral pathogens in the ocean."

A paper by Coolen reporting the results appears in this week's issue of the journal Science.

The research was funded by grants from the National Science Foundation (NSF).

"We know that viruses play an important role in phytoplankton population dynamics in today's oceans," said David Garrison of NSF's Division of Ocean Sciences. "This research shows that this was also true in the distant past."

In examining a 7,000-year continuous genetic record preserved in sediments under the Black Sea, Coolen discovered that the DNA of both a Coccolithovirus and its host, Emiliania huxleyi, a phytoplankton that plays a major role in the global carbon cycle, have been preserved for thousands of years.

"Biologists now have a picture of long-term viral/host dynamics in the ocean," Coolen said. Previous laboratory work had confirmed such co-existence for only a few successive years.

Coolen added that long virus/host records, such as the ones he studied, could answer such important questions as:

What factors are involved in controlling viral infection of these globally important marine algae, and how long can the same host and virus populations coexist? Were past algal populations only controlled by the prevailing environmental conditions, or did viruses also play a role?

The latter is of particular interest, Coolen said, because "no one has long-term records of viruses. Ecological shifts in past algal communities are usually explained by changes in climate and environmental conditions."

Now it seems possible, he said, that viruses also played a part.

"This is important for E. huxleyi, which performs photosynthesis--just like plants," said WHOI scientist Benjamin Van Mooy. "It consumes carbon dioxide."

In doing so, it reduces the amount of carbon dioxide released into the atmosphere, and forms a calcium carbonate shell, which also helps regulate the carbon cycle.

But DNA viruses of the genus Coccolithovirus kill off large populations of E. huxleyi, particularly in the north Atlantic Ocean.

Van Mooy has traced this phenomenon to lipids, or fatty compounds, in certain viruses. If viruses are killing off phytoplankton, this can increase greenhouse emissions, Van Mooy said.

"If viruses infect a whole bunch of [phytoplankton] cells," he said, "then they can't perform photosynthesis, they can't take up carbon dioxide."

Coolen said that his data buttress Van Mooy's work by suggesting a significant role for viruses in affecting the algal population, and carbon cycling, in the past.

He observed, for example, major shifts in the types of Coccolithovirus and E. huxleyi in Black Sea sediments over centuries.

Environmental conditions almost certainly had a role in selecting successful E. huxleyi genotypes, but Coolen believes viruses also may have played a part.

"Until now, shifts in past plankton species [identified through the microscopic analysis of preserved diagnostic cellular fossils] have mainly been linked to changes in environmental conditions and climate," Coolen said.

However, Coolen believes, understanding the virus's role in controlling past algae is needed to improve the interpretation of past climate records. "This can now be studied using DNA," he said.

The continuous absence of oxygen in the bottom waters of the Black Sea over the last 7,500 years enabled Coolen to study sediments far back in time.

"This lack of oxygen facilitated the preservation of organic material in general, and ancient viral and algal plankton DNA in particular," he said.

Unpublished data from Coolen's lab "show that Black Sea sediments older than 7,500 years contain well-preserved DNA of different algae adapted to lower salinities and freshwater environments." These sediments also likely harbor the DNA of the algae's viral pathogens.

"Comparable studies could be employed in a wide variety of marine and lake ecosystems," Coolen said.

"In a different and broader context," he added, "it may be possible to reconstruct the historical spread of human viral diseases, since a variety of human viral infections are also caused by DNA viruses."

Source: National Science Foundation