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

单细胞绿藻可以在地球上大多数自然形成的水体中生长和游泳。它们很小--一万只可以放在针头上--它们倾向于朝着特定的方向游泳，比如朝向光线或远离重力，以提高它们的生存机会。事实上，红色的形状是你有时在融化的雪上看到的粉红色光泽的原因。当它们聚集在流体的上部区域时，富细胞流体的密度大多高于密度较低的流体，这可能导致在短短几十秒内推翻并令人惊讶地错综复杂的自我延续模式。物理学家和数学家，包括我自己，多年来一直在研究稀释悬浮液中的这些所谓的生物对流模式，并提出了预测发生模式的某些方面的方法。这项建议的一个次要方面是使用几何图像处理技术来研究图案的其他统计属性，我希望使用曲率来开发这些技术。该系统是一个很好的例子，说明了针对个体的简单规则如何扩大到产生个体大小的许多倍的结构，同样的方法也可以用于其他生物体，如细菌。事实证明，绿藻还有其他诀窍。当它们缺乏硫时，每个电池内部的一个新电路就会启动，将光合作用中的多余电子与质子一起转化为氢。这将是一个极好的消息，因为如果不是因为这个电路对光合作用的另一种产物氧气极其敏感，它最终可能会提供一种无污染、具有竞争力的氢燃料来源。为了产生氢气，你还需要使培养物中的氧气匮乏。这在一段时间内效果很好，因为光合作用释放的所有氧气都被呼吸回路耗尽了。除了产生氢气外，细胞还会改变形状和结构，从而对环境做出行为反应，这意味着藻类会产生不同类型的大规模模式，这反过来又会影响每个细胞产生的光合作用和氢气量。然而，几个小时后，细胞开始饥饿，它们关闭了。硫磺和氧气是使藻类恢复其原始状态所必需的。事实上，饥饿的细胞、产生的模式和氢气的产生之间存在着微妙的平衡。有理由预测，对该系统的更好理解可以产生更好的氢气产量。为了理解整个过程，我们必须为每个方面建立数学模型，并将它们粘合在一起，使它们有意义。我最近的研究论文集中在细胞稀释悬浮液产生的模式上，但我现在有一些强有力的想法，关于如何处理从单个细胞动力学到稀释悬浮液中的大规模模式，通过简单的细胞之间的相互作用到非常集中的培养的各种行为。在意在使用技术从概率和流体和多孔结构的研究。我还建立了一个实验室，在那里我可以探索机制并测试数学理论，以确保它们与现实完全一致。人们希望，有一天我们可能会用绿藻以环保的方式生产氢气汽车，但这项研究产生的方法和结果无疑将应用于从制药到渔业的许多其他系统。

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.