Bioconvection: hydrogen production and high concentrations in suspensions of swimming micro-organisms

Single celled green algae can be found growing and swimming in most naturally occurring bodies of water on Earth. They are small - 10,000 could fit on a pinhead - and they tend to swim in particular directions, such as towards light or away from gravity, to improve their chances of survival. Indeed, a red form is responsible for the pink sheen that you can sometimes see on melting snow. When they accumulate in upper regions of the fluid, the mostly high density of the cell-rich fluid above less dense fluid can lead to overturning and amazingly intricate self-perpetuating patterns in just tens of seconds. Physicists and mathematicians, including myself, have been studying these so-called bioconvection patterns in dilute suspensions for some years and have come up with ways to predict some aspects of the patterns that occur. One minor aspect of this proposal is to study other statistical properties of the patterns with geometric image processing techniques that I hope to develop using curvature. The system is a great example of how simple rules for individuals can scale up to produce structure many times the individuals' size, and the same methods can be used with other organisms such as bacteria. It turns out that green algae have other tricks up their sleeves. When they are starved of sulphur, a new circuit internal to each cell kicks in to convert spare electrons from photosynthesis together with protons to hydrogen. This would be fantastic news, for it might ultimately provide a pollution-free and competitive source of hydrogen fuel, were it not for the fact that this circuit is extremely sensitive to oxygen, which is another product of photosynthesis. In order to produce hydrogen you also need to starve the culture of oxygen. This works well for a while, as all the oxygen released from photosynthesis gets used up by the respiration circuitry. As well as producing hydrogen, the cells change shape and structure, and thus their behavioural response to the environment, which means that the algae produce different types of large-scale pattern and this in turn effects the amount of photosynthesis and hydrogen that each cell produces. However, after some hours the cells begin to starve and they shut down. Sulphur and oxygen are required to bring the algae back to their original condition. Actually there are fine balances between starving the cells, the patterns produced and hydrogen production. It's reasonable to predict that a better understanding of the system can produce better yields of hydrogen. To understand the whole process we must make mathematical models of each aspect and to glue them together so that they make sense. My recent research papers have concentrated on the patterns produced by dilute suspensions of cells, but I now have a number of strong ideas on how to deal with the range of behaviour from individual cell dynamics to large scale patterns in dilute suspensions, through simple cell-to-cell interactions to very concentrated cultures. In intend to use techniques from probability and the study of fluids and porous structures. I also have set up a laboratory where I can explore mechanisms and test the mathematical theories to make sure that they are fully consistent with reality. The hope is that one day we may have cars fuelled by hydrogen produced in an environmentally friendly way using green algae, but the methods and results produced from this research will undoubtedly have application in many other systems from pharmaceuticals to fisheries.