Cyanobacteria: Past, Present and Future

Cyanobacteria, like plants, are photosynthetic organisms that use sunlight to power the conversion of inorganic forms of carbon (e.g. atmospheric carbon dioxide, CO2) into organic forms (e.g. sugars). This process also releases oxygen into the environment. In fact, cyanobacteria are credited with playing a major role in generating the oxygen in the Earth’s atmosphere, enabling life on Earth as we know it. Moreover, because cyanobacteria are the ancestors of chloroplasts, they provide a valuable model system to study photosynthesis.

Cyanobacteria growing in a 20 liter bioreactor.

     (Image Credit: Ryan Leverenz and Cheryl Kerfeld)

We routinely use two cyanobacterial strains in our lab: Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803. They have fast doubling times, their genomes are sequenced and, because they are widely used model organisms (“lab rats”), they have well-established molecular tools. The following images show some important cellular features: the carboxysomes (where carbon fixation occurs) and the thylakoid membranes (where light energy capture happens). Part of the Kerfeld group studies photoprotection, how cyanobacteria avoid damage by excess sunlight.

Electron micrograph of the cyanobacterium Synechococcus elongatus PCC 7942.
This cell is ~2.5 µm long and 750 nm wide.

     (Image Credit:  Raul Gonzalez and Cheryl Kerfeld)

Electron micrograph of the cyanobacterium Synechocystis sp. PCC 6803.
Cells of this species are smaller and rounder (~1.5 µm diameter).

     (Image Credit: Raul Gonzalez and Cheryl Kerfeld)

Cyanobacteria are promising microbial factories for biofuels and other valuable compounds. They use a “carbon concentrating mechanism” (CCM) to concentrate inorganic carbon in the cell for subsequent “fixing” into sugars. Efforts are underway in many labs to install carboxysomes and other components of the CCM to increase plant photosynthesis.

Uptake of inorganic carbon from the environment by cyanobacteria is the first step of the CCM. One can think of this as the cell acquiring the raw materials for photosynthetic sugar production, a process that begins in a specialized subcellular factory, the carboxysome. Two forms of inorganic carbon are utilized: CO2 and bicarbonate (HCO3-). CO2 diffuses through the cell membrane and enters the cytoplasm where it is quickly converted into HCO3- by the CO2 uptake systems.  This prevents the CO2 from diffusing back outside the cell; cell membranes are freely permeable to CO2 but not to HCO3-. This is also why HCO3- needs to be pumped across the plasma membrane by specialized bicarbonate transport proteins.

Simplified scheme explaining carbon concentrating mechanism (CCM) in cyanobacteria.
The workers’ cartoon images used for depicting different enzymes taken from 123RF.

     (Image Credit: Sandeep B. Gaudana, Raul Gonzalez and Cheryl Kerfeld)

Both the CO2 uptake and HCO3- transport involves expending cellular energy. The transporters and uptake systems (shown as trucks/wheelbarrow in the simplified scheme) effectively function as the supply lines for the sugar production that occurs in a specialized subcellular factory, the carboxysome.

Carboxysomes consist of enzymes for the first steps of production of sugar enclosed in a protein shell. The shell selectively lets in HCO3- and other raw materials for sugar production. Inside the carboxysome (enzymes = workers) ribulose 1, 5-bisphosphate carboxylase/oxygenase (RuBisCO) and carbonic anhydrase (CA) are encapsulated. The CA transforms HCO3- into CO2 within the carboxysome, elevating the concentration of CO2 around RuBisCO; this enhances the efficiency with which RuBisCO can begin the conversion of CO2 into sugar.

In the Kerfeld lab we use approaches referred to as synthetic biology to re-engineer various aspects of cyanobacterial CCM. Among other projects, we are aiming to generate bicarbonate transporters that are driven by light instead of cellular energy. We are also reengineering the carboxysome to make it more portable, and to make it easier to install into other types of cells. Enhancing the CCM or installing it into other types of microbial cell factories will help develop “green” approaches—avoiding the use of petroleum—to producing industrially important commodities. Cyanobacteria played a major role in shaping the Earth’s environment and today they can help us address pressing global issues related to food and fuel security and climate change.

- Sandeep B. Gaudana and Raul Gonzalez

This work by Sandeep B. Gaudana, Raul Gonzalez, Ryan Leverenz, and Cheryl Kerfeld is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.