Cyanobacteria concentrate to get a carbon fix
Cyanobacteria keep carbon-fixing enzymes within compartments inside their cells in order to process carbon dioxide more efficiently.
Cyanobacteria are microorganisms that live in water and, like plants, they capture energy from the sun to convert carbon dioxide into sugars and other useful compounds. This process — called photosynthesis — releases oxygen as a by-product. Cyanobacteria were crucial in making the early atmosphere of Earth habitable for other organisms, and they created the vast carbon-rich deposits that now supply us with fossil fuels. Modern cyanobacteria continue to sustain life on Earth by providing oxygen and food for other organisms, and researchers are trying to bioengineer cyanobacteria to produce alternative, cleaner, fuels.
Understanding how cyanobacteria can be as efficient as possible at harnessing sunlight to ‘fix’ carbon dioxide into carbon-rich molecules is an important step in this endeavor. Carbon dioxide can readily pass through cell membranes, so instead cyanobacteria accumulate molecules of bicarbonate inside their cells. This molecule is then converted back into carbon dioxide by an enzyme found in special compartments within cells, called carboxysomes. The enzyme that fixes the carbon is also found in the carboxysomes. However, several important details in this process are not fully understood.
In order to explore the factors that optimize carbon fixation by cyanobacteria, Niall Mangan and Michael Brenner have extended a mathematical model of the mechanism that these microorganisms use to concentrate carbon dioxide. Carbon fixation is deemed efficient when there is more carbon dioxide in the carboxysome than the carbon-fixing enzyme can immediately use (which also avoids wasteful side-reactions that use oxygen instead of carbon dioxide). However, there should not be too much bicarbonate, otherwise the enzyme that converts it to carbon dioxide is overwhelmed and cannot take advantage of the extra bicarbonate.
Mangan and Brenner’s model based the rates that carbon dioxide and bicarbonate could move in and out of the cell, and the rates that the two enzymes work at, on previously published experiments. The model varied the location of the enzymes (either free in the cell or inside a carboxysome), and the rate at which carbon dioxide and bicarbonate could diffuse in and out of the carboxysome (the carboxysome’s permeability). Mangan and Brenner found that containing the enzymes within a carboxysome increased the concentration of carbon dioxide inside the cell by an order of magnitude. The model also revealed the optimal permeability for the carboxysome outer shell that would maximize carbon fixation.
In addition to being of interest to researchers working on biofuels, if the model can be adapted to work for plant photosynthesis, it may help efforts to boost crop production to feed the world’s growing population.
To find out more
Read the eLife research paper on which this story is based: “Systems analysis of the CO2 concentrating mechanism in cyanobacteria” (April 29, 2014).
eLife is an open-access journal that publishes outstanding research in the life sciences and biomedicine.
