Evolution of photosynthetic organisms

Life on Earth may have established itself as long as four billion years ago, a mere half a billion years after the Earth itself was formed, in the simple form of short replicating RNA strands adsorbed on pyrites (Maynard Smith and Szathmary, 1999). How complex organisms evolved from such a primitive form of organisation is only now becoming clearer from study of the gene sequences in the RNA of the Bacteria and Archea (Woese, 2000). The common chemoautotrophic ancestor of these early kingdoms of organisms probably appeared in the waters that then covered the Earth in sunless locations where highly reducing substances such as FeS, H2 or H2S were 

available to permit fixation of dissolved COz by exergonic (having negative AC) reactions such as

Chemoautotrophs feeding on such an energy-rich diet had no need of light in order to grow, maintain themselves and replicate (albeit even the simplest of organisms must possess wonderfully subtle enzymes to facilitate the successive electron transfers and other reactions involved in these vital processes). One may therefore ask why photosynthesis ever evolved. One attractive hypothesis is that it originally arose as a protection mechanism: the atmosphere of the early Earth contained almost no free oxygen, so there was then no ozone layer to screen the delicate components of living organisms from the damaging hard UV wavelengths in sunlight. A pigmented organism could survive in shallow, sunlit water where an unpigmented one could not. The protective action of a chlorin pigment such as a bacteriochlorophyll would inevitably involve formation of its electronically excited state by the absorption of light. From there it is not a great leap, in imagination at least, to the excited state of the pigment acting as the donor in the first electron-transfer step of photosynthesisand thus to the appearance of photosynthetic bacteria.
 

Photosynthetic bacteria are confined to the locations where both sunlight and their particular electron source is available. About 2.8 billion years ago, the evolutionary pressure to use less strongly reducing (and therefore more abundant) sources of electrons appears to have culminated in the symbiotic linkage of members of the two classes of photosynthetic bacteria (containing either Type I or Type I1 RCs) so that their two photosystems acted in series. Thus appeared the cyanobacteria (formerly called the blue-green algae), the earliest class of extant photosynthetic organism able to carry out oxygenic photosynthesis.
 

Cyanobacteria fall into a general class of oxygen-producing prokaryotic organisms called oxyphotobacteria. They are found throughout oceans, seas and lakes, and contribute significantly to the global oxygedcarbon cycle. They are also found on land, where they often have symbiotic relationships with eukaryotic organisms such as lichens. As oxygenic organisms, cyanobacteria contain PSI and PSI1 derived from the bacterial Type I and Type I1 reaction centres. Often called blue-green algae, the ‘cyanohhe’ colouration that gives cyanobacteria their name is due to the presence of light-harvesting phycobiliproteins, typically phycocyanin and allophycocyanin. It is not known exactly how many different species of cyanobacteria exist but it must be
many tens of thousands. They can be either single cell or multicellular filamentous organisms. Experimentally favoured representatives are different strains of Synechococcus and Synechocystis, Nostoc flagelliforme and Cyanidium calderium.

Closely related to cyanobacteria are the green oxyphotobacteria often called prochlorophytes. These prokaryotic oxygen-evolving organisms do not contain phycobilins, but chlorophyll b instead. For this reason they were once considered to represent the type of organism that could have been the precursor to the modern chlorophyll alchlorophyll b-containing chloroplast of higher plants and green algae (Lewin, 1976). However, gene sequencing (La Roche et al., 1996) has thrown some doubt on this hypothesis. Representative species of this class of oxyphotobacteria are Prochloron didemni, Prochlorothrix hollandica and Prochlorococcus marinus. The latter is a picoplankton distributed extensively throughout the world’s oceans and is thought to be the most abundant photosynthetic organism on our planet (Partensky et al., 1999). Many cyanobacteria, such as Spirulina, can fix atmospheric nitrogen and are a source of protein-rich food in some parts of the world.
 

Cyanobacteria appeared early in evolution, as described above, and evidently spread widely throughout the sunlit surface waters of Earth. At first, the oxygen they evolved was used up in oxidising the then-abundant Fe2+ and S” in the oceans and lithosphere. Once this great ‘rust event’ was complete, about 2.2 billion years ago (see Fig. 6.2), the level of oxygen in the atmosphere rapidly rose to its present day level. Non-photosynthetic eukaryotes’ evolved about 1.6 billion years ago, opening the way to the appearance of algae, created by the endosymbiosis of a cyanobacterium by a non-photosynthetic organism. Algae are oxygenic eukaryotic organisms that are mainly aquatic, although there are some groups that can live on moist soil, tree trunks and rocks. Like cyanobacteria, some algae can have symbiotic associations with lichens and thus are not solely restricted to sunlit surface waters. Together with oxyphotobacteria, algae can often be found at considerable depths in the ocean even below the photic (sunlit) zone. Algal colonies give corals their colour and attach to rock on shallow seabeds. They exhibit considerable diversity of form, size and hue, as suits their ecological niche. Green, red and brown algae take their names from their dominant antenna pigments-chlorophylls, phycobilins and fucoxanthins, respectively.
As sunlight passes through water, the longer wavelengths are absorbed first, so green algae (absorbing in the red) are found nearest the surface, while red algae and cyanobacteria (absorbing in the blue) are found at greater depths. The presence of chlorophyll b in Prochlorococcus means that this highly abundant green, marine oxyphotobacterium can also flourish at considerable depths.
 

Algae fix carbon to produce a range of substances. Green algae convert C02 mainly to starch, while other algae store oil globules, and brown and red seaweeds form sugar alcohols, polysaccharides and gummy substances such as agar and algin. Although the red tides and algal blooms that sometimes appear in nutrient-rich waters can be toxic and unsightly, many useful products come from algae, as Rosa Martinez and Zvy Dubinsky describe in Chapter 7.
 

By the beginning of the Cambrian Period 540 million years ago, the high oxygen content of the atmosphere and oceans allowed the evolution of a great variety of marine invertebrate animals deriving their energy from respiration. Air-breathing land animals appeared about 400 million years ago. 350 million years ago, the higher land plants (descended from green algae, and sharing with them the pigments chlorophyll a and b) at last proliferated, leading the way to the Carboniferous Period (345-225 million years before present), during which Earth’s deposits of fossil fuel were laid down by the large-scale growth and decay of vegetation.
 

Like oxyphotobacteria and algae, higher plants contain both Type I and Type I1 reaction centres, in the form of PSI and PSI1 respectively. The modern plant kingdom comprises over 300,000 known species. The carbon fixed by plants is largely in the form of carbohydrates (energy-storage compounds such as glucose, sucrose and starch and plant structural materials such as cellulose and lignin). As in other types of photosynthetic organisms, the pantheon of other organic molecules needed in smaller quantities to maintain cellular activity, growth and reproduction are synthesised by elegant biochemical pathways that occur in the dark, but ultimately depend on the metabolic energy derived from photosynthesis.