other sources.
The most common form of photosynthesis involves chlorophyll‐type pigments and operates using light‐driven electron transfer processes. The organisms that we will discuss in detail in this book, including plants, algae, and cyanobacteria (collectively called oxygenic organisms because they produce oxygen during the course of doing photosynthesis) and several types of anoxygenic (non‐oxygen‐evolving) bacteria, all work in this same basic manner. All these organisms will be considered to carry out what we will term “chlorophyll‐based photosynthesis.” The retinal‐based form of photosynthesis, while qualifying under our general definition, is mechanistically very different from chlorophyll‐based photosynthesis, and will not be discussed in detail. It operates using cis–trans isomerization that is directly coupled to ion transport across a membrane (Ernst et al., 2014). The ions that are pumped as the result of the action of light can be either H+, Na+, or Cl− ions, depending on the class of the retinal‐containing protein. The H+‐pumping complexes are called bacteriorhodopsins, and the Cl−‐pumping complexes are known as halorhodopsins. No light‐driven electron transfer processes are known thus far in these systems.
For many years, the retinal‐based type of photosynthesis was known only in extremely halophilic Archaea (formerly called archaebacteria), which are found in a restricted number of high‐salt environments. Therefore, this form of photosynthesis seemed to be of minor importance in terms of global photosynthesis. However, in recent years, several new classes of microbial rhodopsins, known as proteorhodopsin, heliorhodopsin, and others, have been discovered (Béjà et al., 2000; Pushkarev et al., 2018; Inoue et al., 2020). Proteorhodopsin pumps H+ and has an amino acid sequence and protein secondary structure that are generally similar to bacteriorhodopsin. The proteobacteria that contain proteorhodopsin are widely distributed in the world's oceans, so the rhodopsin‐based form of photosynthesis may be of considerable importance. Recent evidence suggests that the proteorhodopsins are responsible for a significant amount of primary productivity in the ocean (Gómez‐Consarnau et al., 2019).
As mankind pushes into space and searches for life on other worlds, we need to be able to recognize life that may be very different from what we know on Earth. Life always needs a source of energy, so it is reasonable to expect that some form of photosynthesis (using our general definition) will be found on most or possibly all worlds that harbor life. Photosynthesis on such a world need not necessarily contain chlorophylls and perform electron transfer. It might be based on isomerization such as bacteriorhodopsin, or possibly on some other light‐driven process that we cannot yet imagine (Kiang et al., 2007a, b; Schweiterman et al., 2018).
1.2 Photosynthesis is a solar energy storage process
Photosynthesis uses light from the Sun to drive a series of chemical reactions. The Sun, like all stars, produces a broad spectrum of radiation output that ranges from gamma rays to radio waves. The solar output is shown in Fig. 1.1, along with absorption spectra of some photosynthetic organisms. Only some of the emitted solar radiation is visible to our eyes, consisting of light with wavelengths from about 400 to 700 nm. The entire visible range of light, and some wavelengths in the near infrared (700–1000 nm), are highly active in driving photosynthesis in certain organisms, although the most familiar chlorophyll a‐containing organisms cannot use light with a wavelength longer than 700 nm. The spectral region from 400 to 700 nm is often called photosynthetically active radiation (PAR), although this is only strictly true for chlorophyll a‐containing organisms. Recently, some oxygenic photosynthetic organisms that utilize radiation outside the PAR region have been discovered. These are discussed in detail in Chapters 2 and 7.
Figure 1.1 Solar irradiance spectra and absorption spectra of photosynthetic organisms. Solid curve: intensity profile of the extraterrestrial spectrum of the Sun; dotted line: intensity profile of the spectrum of sunlight at the surface of the Earth; dash‐dot line: absorbance spectrum of Rhodobacter sphaeroides, an anoxygenic purple photosynthetic bacterium; dashed line: absorbance spectrum of Synechocystis PCC 6803, an oxygenic cyanobacterium. The spectra of the organisms are in absorbance units (scale not shown).
The sunlight that reaches the surface of the Earth is reduced by scattering and by the absorption of molecules in the atmosphere. Water vapor and other molecules such as carbon dioxide absorb strongly in the infrared region, and ozone absorbs in the ultraviolet region. The ultraviolet light is a relatively small fraction of the total solar output, but much of it is very damaging because of the high energy content of these photons (see Appendix for a discussion of photons and the relationship of wavelength and energy content of light). The most damaging ultraviolet light is screened out by the ozone layer in the upper atmosphere and does not reach the Earth's surface. Wavelengths less than 400 nm account for only about 8% of the total solar irradiance, while wavelengths less than 700 nm account for 47% of the solar irradiance (Thekaekara, 1973).
The infrared wavelength region includes a large amount of energy and would seem to be a good source of photons to drive photosynthesis. However, no organism is known that can utilize light of wavelength longer than about 1000 nm for photosynthesis (1000 nm and longer wavelength light comprises 30% of the solar irradiance). This is almost certainly because infrared light has a very low energy content in each photon, so that large numbers of these low‐energy photons would have to be used to drive the chemical reactions of photosynthesis. No known organism has evolved such a mechanism, which would in essence be a living heat engine. Infrared light is also absorbed by water, so aquatic organisms do not receive much light in this spectral region.
The distribution of light in certain environments can be very different from that shown in Fig. 1.1. The differing spectral content, or color, of light in different environments represents differences in light quality. In later chapters, we will encounter some elegant control mechanisms that organisms use to adapt to changes in light quality. In a forest, the upper part of the canopy receives the full solar spectrum, but the forest floor receives only light that was not absorbed above. The spectral distribution of the filtered light that reaches the forest floor is therefore enriched in the green and far‐red regions and is almost completely lacking in the red and blue wavelengths.
In aquatic systems, the intensity of light rapidly decreases as one goes deeper down the water column, owing to several factors. This decrease is not uniform for all wavelengths. Water weakly absorbs light in the red portion of the spectrum, so that the red photons that are most efficient in driving photosynthesis rapidly become depleted. Water also scatters light, mainly because of effects of suspended particles. This scattering effect is most prevalent in the blue region of the spectrum, because scattering by small particles is proportional to the frequency raised to the fourth power. The sky is blue because of this frequency‐dependent scattering effect. At water depths of more than a few tens of meters, most of the available light is in the middle, greenish part of the spectrum, because the red light has been absorbed and the blue light scattered (Falkowski and Raven, 2007; Kirk, 2011). None of the types of chlorophylls absorb green light very well. However, other photosynthetic pigments, in particular some carotenoids (e.g. fucoxanthin, peridinin), have intense absorption in this region of the spectrum and are present in large quantities in many aquatic photosynthetic organisms. At water depths greater than about 100 m, the light intensity from the Sun is too weak to drive photosynthesis.
