. 2017 Nov 21;42(2):205–231. doi: 10.1093/femsre/fux056

Table 2.

Key transitions in physiological evolution and primary production.

Relative age	Tetrapyrrole^a	Physiology^a	Modern group	Electron sources
Modern	Chlorophyll	Chlorophototrophy with RCI and RCII	Cyanobacteria	H₂S, H₂O
Advanced	Chlorophyll	Chlorophototrophy with RCI	Cyanobacteria, chlorobia	H₂S^b
Intermediate-2	Porphyrins	Mixotrophy	None	H₂S
Intermediate-1	Heme	Anaerobic respiration	Sulfur reducers^c, autotrophic ε-proteo^c	Organics^d, H₂
Primordial	Cobalamin^e	Chemolithoautotrophy	Acetogens, methanogens^f	H₂

Heme before chlorophyll: Granick (1965); cobalamin before heme before chlorophyll: Decker et al. (1970). Chemolithoautotrophs before anaerobic respiration before anoyxgenic photosynthesis: Decker et al. (1970).

Before the advent of photosynthetic H₂S oxidation, H₂ was the sole reductant driving primary production. Chlorophotosynthetic use of the same reductant (H₂) would not substantially increase primary production (see text) and would be restricted to environments where H₂ from serpentinization was discharged in photic environments. Of course, H₂ is also generated by anaerobic fermentations of reduced carbon compounds, but that does not permit a net increase in primary production.

Here, the term sulfur reducers designates organisms that gain energy by S, sulfite or sulfate reduction, a very broad definition (Rabus et al.2015). Many sulfate reducers grow chemolithoautotrophically using the acetyl-CoA pathway for carbon metabolism and sulfate reduction with H₂ for energy metabolism, or chemoorganoheterotrophically on acetate or lactate (Rabus et al.2015). H₂-dependent epsilonproteobacteria that use the rTCA cycle for autotrophy and anaerobic respiration via sulfur reduction, for example, would be another kind of early intermediate. See the text.

Of course, use of organics as electron donors does not increase primary production, but it increases metabolic flexibility and permits specialization of carbon and energy metabolism.

Decker et al. (1970); Sousa et al. (2013).

In acetogens and methanogens, the acetyl-CoA pathway is the central pathway of carbon assimilation and energy metabolism, whereas the ion gradient that drives ATP-synthase is generated during the process of CO₂ reduction with electrons from H₂ (Thauer et al.2008; Schuchmann and Müller 2014; Sousa and Martin. 2014).