Phytoplankton are mostly single-celled photosynthetic organisms that feed fish and marine mammals. They are responsible for nearly 50 percent of the earth's annual carbon-dioxide consumption and more than 45 percent of the oxygen production. Despite the important roles of modern phytoplankton, their evolutionary origins and rise to prominence in today's oceans was an unresolved question in marine science.

In the first study that looks at phytoplankton from combined perspectives of biology, chemistry and geology, researchers from three countries, including Texas A&M University at Galveston Assistant Professor Antonietta Quigg, who specializes in algae ecology and chemistry, examined modern phytoplankton development and reviewed their evolutionary history. Findings of the project appear in the current issue of Science magazine.

Funding for the study is supported by a grant through the National Science Foundation Biocomplexity program, which aims to make new advances by bringing people together from different fields.

Despite the early origins of cyanobacteria, an essential component of modern phytoplankton, the ancestors of the majority of phytoplankton that dominate the modern seas did not appear until 250 million years ago, the researchers note.

This is fairly recent in geological terms. Cyanobateria appeared 3.8 billion years ago. A cyanobacterium is a single-celled photosynthetic organism, which with the help of sunlight could make carbon dioxide and water into oxygen and energy providing chemicals.

The researchers showed that modern phytoplankton began to form at a time when the low oxygen conditions characterized much of the world's oceans. Since a cyanobacterium was capable of producing oxygen and nutrients, another bacterium, or a one-celled organism, ate the cyanobacterium, kept a part of the cyanobacterium undigested, and let it function as an oxygen and energy generating organelle.

This added the photosynthesis function to the eater, and transformed it into the phytoplankton that would later dominate the sea.

The researchers found that changes in sea level, water chemistry and the amount of carbon-dioxide in the water, and even the evolution of grass-eating animals on land all contributed to the rise of the three dominant phytoplankton groups. For example, rising sea levels provided more ecospace for the phytoplankton, promoting increased diversity among the phytoplankton.

Quigg, coauthor of the Science magazine article said the study could help scientists understand the effects of increased carbon dioxide, or the green house gases on life in the ocean at present and in the future.

"One way to do that is to understand what was happening in the past," Quigg says. "If we have a theory or an idea, we could look in the past and check if that idea works."

She says since some algae do very well with increase carbon dioxide and some do poorly, the evolutionary history will tell, with increased carbon-dioxide, what changes there may be in the types of algae in the water and how that will affect the fish and marine mammals that eat the algae.

"If you have an ocean full of algae that use a lot of carbon-dioxide, then we may be able to resolve the problem of green house effect," Quigg says. "But if you have an ocean full of algae that do not like to use carbon-dioxide, then you are in big trouble. Carbon-dioxide will keep increasing."

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