By Charlie Whittaker
Algae are pretty cool. And when I say pretty cool, I mean ridiculously cool. They’re involved in everything from potential biofuel synthesis to novel metabolic pathway generation, but they’re also pretty special because of the unique way in which they fix carbon and generate new biomass.
Image of the algae Scenedesmus quadricauda. The pyrenoid is visible in the middle of each of the cells as the distinct circular object.
The main site of carbon fixation (the way plants and most other photosynthetic organisms, including algae, incorporate carbon dioxide to produce molecules that will eventually become new biomass) is at an enzyme called Ribulose Bisphosphate Carboxylase Oxygenase, or Rubisco for short. Rubisco catalyses the addition of CO2 to Ribulose Bisphosphate, producing a precursor that will eventually go on to generate glucose, sucrose, cellulose, and other sugars. These in turn can be respired to generate chemical energy for the cell or be polymerised to make the macromolecules constituting new algal biomass. However, as well as having the capability to interact with CO2, Rubisco can also catalyse another pathway, by which O2 is added instead. Known as the oxygenation reaction, this results in a net loss of carbon, which is highly problematic for the plant.
Algae tend to be aquatic, and this presents a number of challenges with regards to getting sufficient CO2 to supply and meet the cell’s demand. Diffusion is very slow in water, and thus it takes a long time for CO2 to enter the cell. As well as this, CO2 equilibrates with water to form bicarbonate (HCO3–) on a pH dependent basis. The pH of seawater is such that CO2 is mainly available in the form of bicarbonate, potentially representing another barrier to CO2 uptake.
In response to the challenges associated with living in an aqueous environment, algae employ what is known as a biophysical carbon concentrating mechanism to ensure CO2 supply to Rubisco, and hence carbon fixation, is not compromised. They possess a cellular microcompartment within their chloroplast called the pyrenoid, where all of the Rubisco contained within the cell is stored. In most photosynthetic organisms, plants included, Rubisco is spread throughout the entire chloroplast. By localising Rubisco to this single area, CO2 extracted from the surrounding environment can be concentrated in a single, small area.
Stylised cross section of a Chlamydomonas reinhardtii cell showing the pyrenoid and other subcellular components.
As well as this dense aggregation of Rubisco, the carbon concentrating mechanism involves a number of other proteins. CO2 is taken up from the extracellular environment into the algal cell in the form of bicarbonate (HCO3–). From there it is shuttled via a series of transporters into the chloroplast, whereupon it gets converted back to CO2 in the thylakoids by an enzyme called carbonic anhydrase, and is then subsequently delivered to the pyrenoid. The idea behind this is that in doing so, the algal cell is able to effectively exclude O2 from the pyrenoid, due to the specific nature of this CO2 delivery, and also ensure a continuous supply of CO2 to Rubisco, given the ubiquity of HCO3– in seawater. This increases the efficiency of photosynthesis, and maximises CO2 fixation.
At the moment, efforts are being made to engineer some aspects of this system into higher plants. The idea behind this is if something resembling a pyrenoid was developed in crop plants, they would be able to better exclude oxygen from the site of carbon fixation (i.e. Rubisco) and increase photosynthetic efficiency. This would translate to substantially increased yields, which is important for food security the world over, particularly in the face of increasing climactic variability and increasing global temperatures, as well as a rapidly increasing population. With more mouths to feed globally, and no concomitant increase in farmable land (if anything, a decrease due to changing weather patterns) increasing yields of key crop species such as rice, maize, cassava and millet represents an important objective for ensuring supply can meet demand globally, as well as making sure that small scale farmholders have the necessary tolerance built into their yields to allow for extreme climactic fluctuations, something that will become increasingly common in the face of climate change and global warming.