Photosynthetic water oxidation driven by near infra-red light

光合作用是将太阳能转化为生命所需的化学能的过程。这种光被用来分解水，去除其中的一些电子，并利用它们从大气中吸收二氧化碳，以制造生命的基石和燃料。当水以这种方式分裂时，会释放出质子（氢离子）和氧气。氧气在大气中积累，与紫外线反应形成保护臭氧层。氧气也提供了一个允许呼吸发生的反应环境。氧的这两个作用对于多细胞生物的发展都是至关重要的，即我们所知道的生命。最重要的光合酶是光系统II (PSII)，即水分解酶。正是这种酶改变了地球。水很不活泼，很难分解。一种能够分解水的酶似乎只进化过一次，而所有产生臭氧的光合作用生物，从最古老的蓝藻到橡树，都使用同一种酶。这种复杂的化学反应需要大量的能量，而这些能量来自阳光。光的能量取决于它的颜色，PSII利用一种叫做叶绿素a的色素吸收红光。直到最近，人们还认为所有的PSII都以叶绿素a为核心。关于红光（680nm）是进行水氧化所需的最低能量的原因，人们已经讨论了几十年：这被称为红色极限。当发现一种被绿海鞘遮蔽的海洋细菌的叶绿素d在710nm左右进行光化学反应时，红色极限受到了质疑。最近发现了一种波长更长的色素叶绿素f。这一次，它不仅仅是在一个奇怪的生态位中出现的一个奇怪的一次性现象，叶绿素f被发现存在于广泛的常见蓝藻中。然而，叶绿素f只有在它们生长在接近黑暗的环境时才会产生，这种环境不受可见光的影响，但暴露在远红光/近红外光下，例如，在温泉的细菌垫深处，或在一些岩石中。叶绿素f的作用通常被认为只用于聚光，而不用于光合作用的光化学部分。我们现在发现叶绿素f似乎确实在PSII中进行光化学反应。这个令人惊讶的结果代表了红极限的重大扩展。这些奇怪的远红色PSII进行正常的PSII化学反应，但它们在能量方面与正常的PSII有很大不同。在目前的项目中，我们打算研究这个长波光合作用的新世界，跟踪我们的惊人发现，了解它是如何工作的，评估发生了什么变化，使PSII以更少的能量运作，并看看向低能量的转变是否会带来更好的能源效率。既然似乎不太可能有免费的午餐，我们还将测试能源效率的提高是否伴随着对光线强度变化的适应能力方面的惩罚。该项目将涉及使用一系列生化和生物物理方法研究活细胞、膜和分离酶中的PSII。这一证明了氧光合作用远远超出了既定的红色界限，将我们带入了一个很大程度上未被研究的领域；然而，更长波长的光合作用已经是一个备受瞩目的工程目标，旨在提高作物和生物能源的效率。正常的光合作用是低效的，人们花了很多精力想办法改进它。设计出更长波长的光合作用似乎是一个遥远的梦想，但现在事实证明，大自然已经完成了这项工程。我们的目的是确定向远红光光合作用转移是否会提供一个有用的技术目标，改善能量预算，并测试它是否会失去弹性，这可能会限制工程长波光合作用在特定生长条件下的使用。

Photosynthesis is the process that converts solar energy into the chemical energy that powers life. The light is used to split water, removing some of its electrons and using them to pull down carbon dioxide from the atmosphere to make the building blocks and fuel for life. When water is split in this way, protons (hydrogen ions) and oxygen are released. The oxygen accumulates in the atmosphere, reacting with UV to form the protective ozone layer. The oxygen also provides a reactive environment that allows respiration to occur. Both of these roles of oxygen were crucial for the development of multicellular organisms: life as we know it.The most important photosynthetic enzyme is photosystem II (PSII), the water splitting enzyme. It is the enzyme that changed the planet. Water is very unreactive and splitting it is hard to do. An enzyme capable of splitting water seems to have evolved only once and all O2-producing photosynthesizers, from the most ancient cyanobacterium to the oak tree, use the same enzyme.Such difficult chemistry requires a lot of energy and this comes from sunlight. The amount of energy in light depends on its colour and PSII uses red light absorbed by a pigment called chlorophyll a. Until recently it was thought that all PSIIs have chlorophyll a at the heart of the process. There have been decades of discussion about why red light (680nm) is the lowest energy needed to perform water oxidation: this is known as the red-limit.The red limit was questioned when it was found that a marine bacterium, which was shaded by a green sea-squirt (!), had chlorophyll d performing the photochemistry at around 710nm. An even longer wavelength pigment, chlorophyll f, was discovered recently. This time it was not just a quirky one-off in a weird ecological niche, chlorophyll f was found to be present in a wide range of common cyanobacteria. However the chlorophyll f is only made when they grow in near-darkness, shaded from visible light but exposed to far-red/near-IR light, e.g. deep in bacterial mats in hot springs, or in some rocks. The role of chlorophyll f is generally considered to be only for gathering light but not for the photochemical part of photosynthesis. We have now found that the chlorophyll f does seem to perform photochemistry in PSII. This surprising result represents a major extension of the red limit.These strange far-red PSIIs perform normal PSII chemistry and yet they are quite different from normal PSII in energy terms. In the present project we intend to study this new world of long-wavelength photosynthesis, to follow up our surprising discovery, to understand how it works, to assess what changes have occurred that allow PSII to function with less energy, and to see if the move to lower energy gives better energy efficiency. Since it seems unlikely that there is such a thing as a free lunch, we shall also test if the improved energy efficiency comes with penalties in terms of it resilience to variations in light intensity, for example. This project will involve studying PSII in living cells, membranes and in the isolated enzyme using a range of biochemical and biophysical methods.This demonstration of oxygenic photosynthesis working well beyond the established red limit, takes us into a realm of the subject that is largely unstudied; and yet longer wavelength photosynthesis is already a high profile engineering target aimed at making crops and bioenergy more efficient. Normal photosynthesis is inefficient and much effort goes into thinking up ways of improving it. Engineering longer wavelength photosynthesis seemed a far-off pipedream but now it turns out that nature has already done the engineering. Our aim here is to determine if moving to far-red photosynthesis will provide a useful technological target with an improved energy budget and to test if it comes with a loss of resilience that could restrict the use of engineered long-wavelength photosynthesis to specific growth conditions.