7.6: On the Origins and Evolution of Respiration and Photosynthesis
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)In one scenario, abundant chemical energy on our once-cooling planet favored the formation of cells that could capture free energy from energy-rich nutrients in the absence of any O2. We initially thought that these first cells would extract nutrient free energy by non-oxidative, fermentation pathways. But they would have been voracious, quickly depleting environmental nutrient resources. Of course, this didn’t happen! A later evolution of autotrophy saved heterotrophic life from an early extinction! Why? Because autotrophs create organic molecules by extracting free energy from inorganic molecules or from light. An alternative scenario gaining traction suggests that the first cells may respired but using something other than oxygen as a final electron acceptor. In this scenario (considered in detail elsewhere), nonoxygenic “oxidative” chemistries came first, followed by the evolution of anoxic fermentative chemistries, then by photosynthesis, and finally by oxygenic respiratory pathways. In either scenario, we can assume that photosynthesis existed before oxygenic respiration.
Why is this assumption a safe one?
We also assume that oxygenic photoautotrophs that capture free energy from light would become the most abundant autotrophs, if for no other reason than sunlight is always available (during the day!), and of course, \(\rm O_2\) is now abundant in the air! Early photoautotrophs were likely the ancestors of today’s cyanobacteria, and in fact, a phylogenetic study of many genes— including genes in plastids and in nuclei that encode plastid proteins—along with a wealth of cyanobacterial genome data, suggests a common ancestry of freshwater cyanobacteria and eukaryotic chloroplasts (Ponce-Toledo, R.I. et al., 2017, An Early-Branching Freshwater Cyanobacterium at the Origin of Plastids. Current Biology 27:386-391).
Finally, think about the origins of respiratory metabolism and the endosymbiotic origins of mitochondria. How did cellular respiration co-opt the photosynthetic electron transport reactions that captured the electrons from \(\rm H_2O\) needed to reduce \(\rm CO_2\) and then direct those reactions to the task of burning sugars back to \(\rm H_2O\) and \(\rm CO_2\)? As photosynthetic organisms emerged and atmospheric \(\rm O_2\) increased, higher \(\rm O_2\) levels would become toxic to most living things. Still, some autotrophic cells must have had a preexisting genetic potential to conduct detoxifying respiratory chemistry. These would have been facultative aerobes able to switch from photosynthesis to respiration as \(\rm O_2\) levels rose. Today’s purple non-sulfur bacteria (e.g., Rhodobacter sphaeroides) are just such facultative aerobes! Perhaps we aerobes descend from the ancestors of Rhodobacter-like cells that survived and spread from localized environments, where small amounts of \(\rm O_2\) threatened their otherwise strictly anaerobic neighbors. Is it possible that the endosymbiotic critter that became the first mitochondrion in a eukaryotic cell was not just any aerobic bacterium, but a purple photosynthetic bacterium?
Contrast possible origins of mitochondria from purple non-sulfur bacteria with the endosymbiotic theory; see Alternate Mitochondrial Roots).