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EMSL Scientific Grand Challenge: Membrane Biology

What are cyanobacteria?

Cyanobacteria are ancient organisms often called blue-green algae, because they are photosynthetic and aquatic; however, they are related to bacteria not eukaryotic algae. In other words, cyanobacteria are oxygenic photosynthetic prokaryotes. They are the progenitors of chloroplasts, the most abundant fossils in Pre-Cambrian rocks and are found in the most diverse ecological niches in the modern world: In the ocean, freshwater lakes, soil, hot springs, even Antarctica. They are the only known bacteria with a circadian clock.

They also have played an extremely important role in the evolution of life on Earth. About 2.5 billion to 2.0 billion years ago, cyanobacteria thrived by using water, the most abundant source of H₂ in the environment. The byproduct of this metabolism was O₂ which increased in the atmosphere. Life began to change on the planet.

The Earth began to rust! Between 2.2 and 1.8 billion years ago, all Fe²⁺ became Fe³⁺ and precipitated to the ocean bottom. Huge volumes of banded iron formations were laid down. Then [O₂] built up, dissolved in water, and escaped to the atmosphere. The stage was set for the evolution of eukaryotes. These tiny organisms that exploited a wealth of CO₂, water and sunlight transformed the Earth.

Types

The cyanobacteria that we find today can live in fresh or salt water, appear as plankton blooms or in dense mats, and may be unicellular or filamentous. Cyanothece is a fresh-water, single cell organism, as is Synechocystis 6803.

Cyanothece sp ATCC 51142

Cyanothece sp ATCC 51142 was chosen as the model organism for the present work. It is a unicellular cyanobacterium capable of oxygenic photosynthesis and N₂ fixation which exhibits a robust circadian rhythm of photosynthesis during daytime and nitrogen fixation during the dark.

Keeping Time

Circadian refers to biologic rhythms or variations occuring over a period of about 24 hours. Circadian clock-controlled phenomena are found over all kingsdoms of organisms and include the sleep-wake cycle in animals and the rate of photosynthesis and seasonal reproduction in plants. Cyanobacteria are the only bacteria known to have a circadian rhythm controlling gene expression. It is interesting that although Cyanothece has a 24-hour clock, its cells grow and divide over a 10-14 hour period, and unlike other cyanobacteria, which separate photosynthesis from N₂ fixation in different cell types, Cyanothece performs both functions in the same cell.

Photosynthesis and the N₂ fixation process conflict, because N₂ fixation is sensitive to O₂. Diagrammatically, the cycle of these processes is as follows:

Carbon Sequestration and Circadian Biology

In the open ocean, all cyanobacteria experience diurnal, day and night, oscillations. Little is known about the impact of such oscillations on the efficiency of the carbon sequestration process. The Membrane Biology Grand Challenge aims to gain fundamental insights into this process, with the ultimate goal of engineering oxygenic photosynthetic microbes with enhanced carbon sequestration abilities.

Until recently, there has been little work on the specifics of the carbon sequestration cycle in cyanobacteria. We are attempting to:

• Describe the elements of the carbon sequestration system
• Deduce the networks that interrelate these elements
• Characterize the flow of information that links these elements and their networks to the circadian biology of cyanobacteria

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Last Updated: November 2009