WASHINGTON (Reuters) -- Single-celled marine organisms called phytoplankton have trim and efficient little genomes that help them work as floating solar panels, international researchers said.

A comparison of four different species of tiny plankton shows they can do their job - collecting sunlight and turning it into food - with just a few genes.

Understanding how they do this could help humanity one day better harness sunlight as a power source and even lead to ways of battling global warming.

"It behooves us to understand exactly how, with roughly 2,000 genes this tiny cell converts solar energy into living biomass - basic elements into life," said Sallie Chisholm of the lead researchers on the studies.

Three teams of scientists in the United States, France, and Israel published their work jointly in special online issues of the journals Nature and the Proceedings of the National Academy of Sciences.  They looked at three species of Prochlorococcus and one of the closely related Synechococcus.

"These cells are not just some esoteric creatures," Chisolm said.  "They dominate the oceans.  There are some 100 million Prochlorococcus cells per liter of seawater, for example."

They form the base of the food web, providing food for large range of animals.

They also "fix" two-thirds of the carbon in the ocean - meaning they take carbon from the atmosphere and use it in building their own small cells.  They also produce a huge amount of oxygen in doing so. This suggests an important role in global warming, caused in part by the release of carbon dioxide into the atmosphere.

"All life on Earth equally depends on the photosynthesis that occurs in the Earth's oceans," Donald Bryant of the biochemistry and molecular biology department at Pennsylvania State University wrote in a commentary on the work.

But they are tiny.

"A hundred of these organisms can fit end to end across the width of a human hair, but they grow in such abundance that as small as they are, they at times amount to more than 50 percent of the photosynthetic biomass in the oceans," said Gabrielle Rocap, a University of Washington assistant professor of oceanography.

Under the microscope they look very similar, but the studies show clear genetic differences.  Scientists are learning that comparing different species genetically can shed a great deal of light on how they do what they do.

"We still don't know the functions of nearly half of these organisms' genes," said Chisolm.

One thing the comparisons will do is help scientists pinpoint precisely which genes are responsible for photosynthesis, the process of turning energy from light into mass. 

"Having the completed genome in hand gives us a first - albeit crude - 'parts list' to use in exploring the mechanisms for these and other important processes," said Raymond Orbach, director of the Department of Energy's Office of Science, which helped fund some of the work.