The uncanny connections between methanogenesis and photosynthesis

I have become quite interested in the evolutionary relationship between methanogenesis in archaea and photosynthesis in bacteria.

The first evolutionary link comes from the homology between the two enzymes of chlorophyll and bacteriochlorophyll synthesis, Protochlorophyllide reductase (BchLNB or ChLNB) and Chlorophyllide reductase (BchXYZ), and the enzyme required for the synthesis of the Ni-tetrapyrrole cofactor, coenzyme F430, of methyl-coenzyme M reductase, which is essential for methanogenesis. 

Another enzyme of chlorophyll synthesis, BciB (8-vinyl reductase) may also be related to another enzyme of methanogenesis (FrhB). See this paper.

Now a new paper by Kono et al (2017) seems to find that this evolutionary link goes much deeper, as it seems that methanogenic archaea not only have a proper rubisco, but also have phosphoribulokinase (PRK), previously considered to be unique to photosynthetic organisms that use the Calvin-Benson-Bassham cycle for carbon fixation.

I know that some enzymes of methanogenesis seem to trace back to the last universal common ancestor (LUCA), but the same isn't true for photosynthesis. From this perspective it would seem that photosynthesis emerged perhaps by borrowing a few components from methanogenesis.

Recently, Martin et al (2017) said in a recent perspective: "Let us presume, just for a moment, that the first bacteria and archaea were acetogens and methanogens respectively. On an uninhabited planet, they have no competitors, and life multiplies quickly given ample growth substrates. The founders of their respective domains would have bubbled off into the ocean bottom waters to be spread around by currents and eventually to be introduced back into hydrothermal systems in the crust [...]"

I would add to this, that life could have only spread around the globe and take a hold during the early Archaean after the evolution of photosynthesis. In fact, I would go as far as to say that it was the innovation of photosynthesis what allowed the early microbes to escape the hydrothermal vents and the crust... but other may disagree! :)

Stromatolitic-chert member, Strelley Pool Formation, middle to lower Paleoarchean, 3.35 to 3.46 Ga. Photo by James St. Johns.