Cyclopoid copepod. (Photo: School of Forest Resources & Conservation)

Viruses affect copepod populations: study

UNITED KINGDOM
Friday, January 18, 2013, 05:20 (GMT + 9)

A new study provides unprecedented evidence of viral infections in copepods, or tiny marine crustaceans. 

Viruses could account for part of the up to 35 per cent of the zooplankton's mortalities, whose causes are currently unknown but suspected to be harmful algae, environmental stressors, parasites and diseases. Researchers used genomic techniques to support the hypothesis that viral infections are a major cause of copepod deaths.

The study was published in the Proceedings of the National Academies of Science.

"This is the first evidence of viruses in marine zooplankton," said Ian Hewson, Cornell University assistant professor of microbiology and senior author of the paper. 

Copepods are critical in oceanic food webs and ocean carbon cycling, which helps regulate the Earth's climate. They also consume most of the oceans’ phytoplankton, which seize about half of the carbon dioxide pulled from the atmosphere and fixed in plant cells. 

As copepods defecate phytoplankton in the form of pellets that sink to the ocean floor, they help block atmospheric carbon there for thousands of years, mitigating climate change. Such information is useful to researchers who work with computer models to climate change in relation to population dynamics and food webs.

"We now have identified another aspect of the microbial food web that affects the amount of atmospheric carbon that potentially gets fixed," Hewson said. 

The team used microscopic and molecular techniques to track viral infections in the natural populations of two common types of copepods, Acartia tonsa, a herbivore commonly raised for fish meal, and Labidocera aestiva, a large omnivore crucial to estuary ecosystems.

Using a genomic technique, the team was able to remove viruses found in copepod tissues, sheared their DNA and identified known viral DNA sequences. In this way, they indentified two previously undocumented viruses, each unique to one of the two copepod species, that belong to the Circovirus group, previously only seen in pigs, birds and flying insects. 

According to the study, infection rates varied by population and season, with average viral loads of hundreds of thousands of viruses per copepod and 100 per cent of some L. aestiva groups infected. These preliminary findings suggest that the virus that infects L. tonsa may emerge in the spring and autumn and dissipate in the summer and winter along with the growth and reduction of copepod populations.

Using electron microscopy, the researchers checked that these viruses were not associated with gut parasites that may carry them.

Future studies will evaluate the viruses' pathology and copepods mortality rates, as well as routes of infection and ecological effects.

The study was funded by the National Science Foundation.

By Natalia Real
editorial@fis.com
www.fis.com

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