Photosynthetic water oxidation driven by near infra-red light

光合作用是将太阳能转化为为生命提供动力的化学能的过程。光被用来分解水，移除水中的一些电子，并用它们从大气中吸收二氧化碳，以制造构成生命的积木和燃料。当水以这种方式分裂时，质子(氢离子)和氧气被释放出来。氧气在大气中积累，与紫外线反应形成保护性的臭氧层。氧气还提供了一个允许呼吸发生的反应环境。氧的这两个作用对我们所知的多细胞生物体的发展至关重要。最重要的光合酶是光系统II(PSII)，它是一种分解水的酶。正是这种酶改变了地球。水是非常不活泼的，很难分解。一种能够分解水分的酶似乎只进化了一次，所有产生氧气的光合作用分子，从最古老的蓝藻到橡树，都使用相同的酶。如此困难的化学需要大量的能量，而这些能量来自阳光。光中能量的多少取决于它的颜色，而PSII使用的是被一种名为叶绿素a的色素吸收的红光。直到最近，人们还认为所有PSII都有叶绿素a处于这一过程的核心。关于为什么红光(680 Nm)是进行水氧化所需的最低能量，人们已经讨论了几十年：这被称为红色限制。当发现一种被绿色海鞘(！)遮挡的海洋细菌在710 nm左右进行光化学时，红色限制受到了质疑。最近发现了一种波长更长的色素--叶绿素f。这一次，这不仅仅是在一个奇怪的生态环境中的一次奇特的一次性事件，在广泛的常见蓝藻中发现了叶绿素f。然而，只有当它们生长在接近黑暗的环境中，遮蔽在可见光下，但暴露在远红光/近红外光下时，例如在温泉或一些岩石的细菌垫中，才会产生叶绿素f。叶绿素f的作用通常被认为只是收集光线，而不是光合作用的光化学部分。我们现在已经发现，在PSII中，叶绿素f似乎确实执行了光化学。这一令人惊讶的结果代表了红色界限的主要扩展。这些奇怪的远红PSII执行正常的PSII化学，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长光合作用的新世界，跟进我们令人惊讶的发现，了解它是如何工作的，评估发生了哪些变化，使PSII能够以更少的能量发挥作用，并看看转向更低的能量是否会带来更好的能源效率。由于似乎不太可能有免费午餐这样的事情，我们还将测试能源效率的提高是否会带来惩罚，例如，它对光照强度变化的适应能力。这个项目将包括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。这一证明氧气光合作用的工作远远超过既定的红色极限，将我们带入一个在很大程度上尚未被研究的领域；而更长波长的光合作用已经是一个旨在提高作物和生物能源效率的备受瞩目的工程目标。正常的光合作用是低效的，要想出改善它的方法需要付出很大的努力。设计更长波长的光合作用似乎是一个遥不可及的梦想，但现在事实证明，大自然已经完成了这项工程。我们在这里的目的是确定转向远红外线光合作用是否会提供一个有用的技术目标，改善能源预算，并测试它是否会带来弹性的损失，这可能会限制工程长波长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.