Origin of Photosynthesis
The advent of photosynthesis was a key evolutionary event in the history of life that enabled living organisms to harness light energy into free chemical energy needed for metabolic purposes. Except for plants and some unicellular protists, which are secondarily photosynthetic due to the presence of chloroplasts that have originated from cyanobacteria, all other photosynthetic organisms are prokaryotes. Thus, this key fundamental innovation which either directly or indirectly sustains all eukaryotic organisms, originated within the prokaryotes.
Within prokaryotes, photosynthetic capability is present within five major groups of bacteria:
- Firmicutes or the low G+C Gram-positive bacteria (Heliobacterium),
- Chloroflexi or Green nonsulfur bacteria,
- Chlorobi or Greensulfur bacteria,
- Proteobacteria and,
- Cyanobacteria.
Of these only Cyanobacteria, which contains two different reaction centers (RC) RC-1 and RC-2 (or PS I and PS II) linked to each other, are capable of carrying out oxygenic photosynthesis. All other photosynthetic bacteria carry out only anoxygenic photosynthesis and contain a single reaction center. Of these Heliobacteria and Chlorobi contain Fe-S type of reaction centers (RC-1) whereas Chloroflexi and Proteobacteria have a pheophytin-quinone type of reaction center (RC-2). The similarities of these RCs in component parts and the mechanisms of charge transfer indicate that they have evolved from a common ancestor. To understand the origin of photosynthesis and which of these reaction centers first evolved, it is essential to understand the branching order of different photosynthetic phyla from a common ancestor, which is not resolved by traditional phylogenetic means. However, based upon the signature sequence approach the branching orders of different bacteria phyla can now be reliably deducted (see diagram on above)
Inferences Based on the Observed Branching Order
1. Earliest Branching Photosynthetic Bacteria
Firmicutes (Heliobacterium) are indicated to be earliest branching photosynthetic bacteria. The ancestral nature of this group is also supported by a number of other observations:
- Unlike other photosynthetic bacteria, both antenna and reaction center activities are present within a single protein in Heliobacteria;
- The reaction center complex in Heliobacteria (and also green sulfur bacteria) has a simpler homodimeric structure as opposed to being heterodimeric in other photosynthetic bacteria;
- The RC in Heliobacteria contains a unique photosynthetic pigment Bchl g, which is indicated to be primitive in comparison to the pigments found in other photosynthetic organisms.
- Of the different photosynthetic bacteria, only Heliobacteria are bounded by a single unit lipid membrane (monoderm cell structure), which is indicated to be an ancestral characteristic in comparison to the cells containing both an inner and outer cell membranes (Diderm cell structure).
2. The Second Photosynthetic Bacteria
Following Heliobacteria, Chloroflexi are indicated to be the next group of photosynthetic organisms that branched off from the common ancestor. The branching of both Heliobacteria and Chloroflexi prior to Cyanobacteria provides evidence that both RC-1 and RC-2 had already evolved prior to the emergence of Cyanobacteria, which contain both of these reactions centers linked to each other.
3. Anoxygenic Photosynthesis vs Oxygenic Photosynthesis
The bacterial groups utilizing anoxygenic photosynthesis mode evolved much earlier than those capable of oxygenic photosynthesis. This is in accordance with the observation that change in atmosphere from anoxygenic to oxygenic occurred much later (between 1.5-2 billion year) after the evolution of earlier organisms. This observation indicates that the earlier prokaryotic fossils probably do not correspond to Cyanobacteria but some other groups of photosynthetic bacteria.
4. Later Branching Photosynthetic Bacteria
The later branching photosynthetic phyla which contain either one or both of these RCs could have acquired such genes from the earlier branching lineages by either direct descent or by means of lateral gene transfer.
5. Speculations About the Earliest Organism
The presence of photosynthetic ability in the earliest branching bacterial phylum indicates that photosynthesis evolved very early in evolution and it is possible that the earliest organism that evolved were photosynthetic.
Selected References
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