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Cyanobacteria Study Seeks to Reveal Evolution of Oxygen on Earth

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In Hope Johnson’s Dan Black Hall laboratory, she and student researchers are growing cultures of cyanobacteria — bacteria that produces oxygen during photosynthesis. The bacteria cultures are growing inside glass flasks as brilliant emerald liquid and as tiny green specks in small petri plates.

Cyanobacteria could hold the key to understanding how oxygen evolved on Earth billions of years ago and what it means to the planet in the future as pollutants contaminate the environment.

“One of the things that makes us human is our desire to reflect on how we came to be, why we have such abundant life on Earth, and where else in the universe could life survive and thrive,” said Johnson, associate professor of biological science. “The presence of oxygen on our planet is so integral to life, shouldn’t we try to understand how that happened?”

Their study focuses on understanding how photosystem II, the enzyme that produces oxygen in cyanobacteria and in plants, evolved to produce oxygen from water. A National Science Foundation award is supporting Johnson’s work for “EAGER Collaborative Research: Manganese Phototrophy in Cyanobacteria.” The two-year grant is expected to total $172,258, with first-year funding of $82,350.

Johnson has a team of student researchers working on the project, including biology graduate student Yvonne Dimagiba and biological science undergraduates William Kim, Joshua Phelan and Sara Sadek. The researchers are collaborating with Woodward Fischer, professor of geobiology at Caltech, where Johnson was on sabbatical in his lab in spring 2015, and R. David Britt, Winston Ko chair and distinguished professor of chemistry at UC Davis.

“Exactly how cyanobacteria gained the ability to convert water to oxygen gas is a mystery,” noted Dimagiba, who has been working on the research since January 2017 for her thesis. “It’s surprising that the evolution of atmospheric oxygen, perhaps the most crucial process that allows us all to persist, is still uncertain.”

Earth began as an oxygen-free environment and only organisms that don’t require oxygen for growth were present, Johnson explained. About 2.3 billion years ago was the Great Oxidation Event, when oxygen started to accumulate in the environment due to oxygen-producing photosynthesis by cyanobacteria.

“This greatly changed our planet. It changed the chemistry of Earth, leading to a great diversity of cellular metabolisms, including respiration with oxygen, which ultimately led to multicellular plants and animals, and the life we have today,” Johnson said.

The researchers are testing a hypothesis that photosynthesis can occur using the element manganese — a transition metal that can exist in different forms or oxidation states — instead of water. So far, their research has found that oxidized manganese is produced by cyanobacteria and that oxidation is dependent upon photosystem II.

“This is the first step to understanding if manganese may have been oxidized by photosystem II before it evolved to make oxygen,” Johnson said.

The research project has given Dimagiba the opportunity to learn lab techniques and skills used by scientists and valued by future employers. It also has taken her out of her comfort zone to develop critical-thinking and time management skills, and to find solutions for research problems.

“I’ve developed a huge amount of patience. I can’t tell you how many times I’ve had to repeat an experiment,” she said. “Collaborating and communicating with your adviser and lab mates are key, and you learn skills that you normally wouldn’t have the opportunity to learn in any of your classes.”