Algal Photobioreactors

A tiny green energy solution.

Microorganismal algae, perhaps most recognisable blooming on a stagnant lake, offers a great alternative to fossil fuels. Furthermore, selecting for growth traits in these plants gives the potential for ultrahigh biomass productivity, supporting a transition to a cleaner, sustainable energy source. I will discuss the most important features of algae for biofuel production here, as well how we can manipulate their environment to push growth rates even further.

Typically, biofuels are produced by conversion of corn or sugar cane to ethanol. The bioproductivity of these plants, which describes how quickly they build biomass in a given environment, are measured in grams of dry weight per metre per hour (g/m^2/h). It is essentially a measure of growth rate. These traditional biofuels are known to achieve roughly 2-3 g/m^2/h. They outperform open algal ponds, which achieve only 1 g/m^2/h. However, a number of proposed strategies predict the growth rate value of algae to increase two orders of magnitude to roughly 100 g/m^2/h.

The efforts primarily centre around increasing the flux tolerance of algae – the speed at which it can absorb energy from photons of light to produce carbon compounds through photosynthesis. One method of achieving this is by changing the environment in which the algae grows. In a 2007 publication, Gordon and Polle (see below) suggest that careful tuning of efficient LED lights may have this effect. They noted three key variables which can be controlled for maximal productivity this way: temporal (how long are the lights on), spectral (what colour are the lights), and intensity (how bright are the lights). They reported significant improvement on bioproductivity using a pulsing pattern of light to give rapid light/dark exposure cycles. Clearly the energy for these lights also need to be sourced renewably. This idea for artificial lighting somewhat reflects those used in industrial-scale hydroponic systems, where coloured LED lights are a popular and effective tool at encouraging crop growth.

A second way that scientists are looking to boost bioproductivity in algae was explained by Peter Mooij in his TEDx talk. This approach looks at the problem from reverse, and asks whether there are already algae that are naturally more bioproductive than others. He further seeks to understand what makes these especially productive the way they are. He sampled of algae from across the globe and searched those which were large, or as he calls it: “fat”. Being larger, these algae are more bioproductive. However, one must also note that this large size doesn’t necessarily indicate great speed of growth. Therefore he grows the algae in containers to study them as they grow and reproduce. He refers to this method of selecting large algae as “survival of the fattest”.

This led me to the question of genetic engineering. I knew that algae have a reasonably small genome (roughly 34 million base pairs on average), so I suspected a study to identify genetic drivers behind increased bioproductivity might already exist. I came across a publication by Radakovits and colleagues in a 2010 publication of the journal Eukaryotic Cell. In this highly detailed review, they discuss amongst other things methods for genetically altering carbohydrate and lipid metabolism, directly synthesising biofuels, and increasing photosynthetic efficiency (via increased flux tolerance). The paper made clear that whilst such engineering efforts are currently in their infancy, they hold great potential.

Finally, I wondered what made algae special in their bioproductive capabilities. Why is it that other organisms aren’t so successful in this aspect as they are? Gordan and Polle explained how a number of driving factors converged on a single organism in algae. Namely, they have minimally competitive plant functions, they reproduce quickly, don’t require many nutrients, and are convenient to work with in an artificial setting. The combination of these features is relatively unique to algae, and given their widespread abundance, they are a natural consideration for this job. Radakovits and colleagues also point to their high photosynthetic conversion efficiencies, high rates of biomass production, and abilities to both live in a range of environmental conditions, as well as to produce a large number of biofuel feedstocks. In summary, algae are a fantastic and uniquely qualified organism, perfectly primed for biofuel production.

I first came across of the idea of algae as bioreactors in the book Biodesign: nature, science, creativity by William Myers. A beautiful double page spread (which I would love to reproduce here were it not copyrighted) shows huge panels of algal photobioreactors fitted around a 1960s multi-storey construction, providing not only a clean source of energy, but also a sleek, modern, verdant appearance. To me, this sort of conceptual image serves as inspiration for the great scientific progress to come which will bring powerful biological principles into our everyday lives.