Origin of photosynthetic systems in bacteria

1. Origin of photosynthetic systems in bacteria Photosynthetic Reaction Centers, Photosystem II and I, the Calvin Cycle

2. Recall from last lecture: Most plant matter is derived from CO2 in the air Water contributes a small amount to the dry mass of plants H+ ions from photolysis are combined with CO2 to form carbohydrates and new water Transpiration of water helps transport soil nutrients into plants where they are needed Nutrients from soil are necessary to generate complex compounds like enzymes, other proteins, nucleic acids, etc., but make up a small percentage of plant dry mass Photosynthesis consists of a light-dependent reaction that generates O2 as a by product and H+ ions (protons) and electrons that are used to generate ATP and enzymes. (exceptions include anoxygenic bacteria like purple sulfur bacteria that use S instead of O). A light-independent reaction that uses ATP and enzymes to fix CO2 and H+ into carbohydrates (CH2O). Modern understanding of photosynthesis is derived from centuries of research which continues in the present day.

3. Photosynthesis: the full catastrophe What you may have learned in BIOL 111 In plants (eukaryotic photosynthetic organisms adapted to life on land), photosynthesis looks like this:

4. Photosynthesis: the macro-evolutionary epic Evolution of light harvesting reaction center(s) Two kinds of light harvesting reaction centers assembled later in the cyanobacteria Different prokaryotes use a number of mechanisms (not just Calvin cycle) to generate carbohydrates Acquired by eukaryotes through endocytosis We’re going to retrace evolution’s steps to understand each process in the context in which it evolved Once the components are understood, we’ll assemble them to look at how the whole process works in cyanobacteria and land plants

5. Early life Believed to first evolve around hydrothermal vents (alkaline) in oceans (acidic). Stable pH gradient, stable temperature, highly reducing environment, under water and therefore protected from UV radiation at surface Used energy in chemical bonds to drive chemosynthesis – formation of carbohydrates from inorganic C sources like CO2 and CH4 Respiration evolves before photosynthesis Photosynthetic pigments thought to originally have evolved to protect against UV radiation (like your skin pigments)

6. Origin of photosynthesis:Photosynthetic bacteria 36 bacterial lineages have been identified (eubacteria) 5 of these are photosynthetic 4 anoxygenic lineages Green sulfur bacteria, purple sulfur bacteria, green non-sulfur bacteria, Heliobacteria 1 oxygenic lineage Cyanobacteria 1 Archaea lineage is photosynthetic Halobacteria (more correctly would be Haloarchaea; anoxygenic)

7. Some metabolic parameters of phototrophic prokaryotes

8. Light harvesting Photosynthetic pigments Chlorophyll a, chlorophyll b, bacteriochlorophylls, carotenoids, xanthophylls Chloro (green) + phyll (leaf) Xantho (yello), carot- (carrot) Sensitive to different wavelengths of light Incoming photon of correct wavelength excites electron to a higher energy state Immediately transferred to electron acceptor, leaving a positively charged pigment molecule if not (as in extracted chlorophyll), electron drops back down to lower energy state, releasing red light and heat

9. The photosynthetic Reaction Center There are 2 types: Reaction Center Type 1 and Reaction Center Type 2 (aka PS I and PS II) Only Cyanobacteria and derivatives have both Prototype Reaction Center New evidence: mechanism evolved from cell respiration mechanism! (use cytochrome b in a new way) Intermediate between RC1 and RC2 purple sulfur bacteria and green sulfur bacteria are most likely the earliest phototrophic life There are green sulfur bacteria that photosynthesize off the glow from undersea volcanoes doi:10.1073/pnas.0503674102 Primary functions(Switches back and forth depending on needs of cell/mass balance of environment): generate ATP to power metabolism Reduce NADH+ to NADPH for use in C fixation (electron carrier)

10. Prototype Reaction Center ancestor or purple sulfur bacteria and green sulfur bacteria Light reactions occur in the cell membrane Photons captured by antenna pigments Different types of chlorophylls or other pigments Many antenna pigments create a wider target both Spatially Spectrally Energy transferred to reaction center pigments Electron boosted to higher energy state Electron transferred to primary electron acceptor Reaction center electron is positively charged (missing an electron)

11. Prototype Reaction Center (cont.) ATP synthesis pathway (cyclic electron flow) Excited electron passes through electron transport chain (similar to as in cell respiration) Powers a proton pump in the cytochrome bf complex to pump cytoplasm full of H+ H+ flows from high concentration in cytoplasm through ATP synthase complex to low concentration in periplasm, H+ flow is like wind through a wind mill, generating ATP as top of ATP synthase complex spins

12. Prototype Reaction Center (cont.) ATP synthesis pathway (cyclic electron flow) Excited electron passes through electron transport chain (similar to as in cell respiration) Powers a proton pump in the cytochrome bf complex to pump cytoplasm full of H+ H+ flows from high concentration in cytoplasm through ATP synthase complex to low concentration in periplasm, H+ flow is like wind through a wind mill, generating ATP as top of ATP synthase complex spins NADPH synthesis pathway (non-cyclic electron flow) Alternative pathway for electrons (indicated by parantheses) Instead of going through cytochrome complex, electrons can be donated to an electron acceptor like NADH+ ? NADPH for use in carbohydrate synthesis New electrons are cleaved off H2S (also generates H+)

13. Summary so far… Photosynthetic pigments thought to originally have evolved to protect against UV radiation (like your skin pigments) 5 types of bacteria + 1 archaea use light energy for some part of metabolism Only cyanobacteria are oxygenic (chloroplasts derived from them) Prototype Reaction Center performed both functions: generate ATP to power metabolism Reduce NADH+ to NADPH for use in C fixation (electron carrier)

14. Evolutionary solutions to photosynthetic efficiency problem Problem: Natural selection would favor increased efficiency in ATP or NADPH synthesis But increased efficiency in one pathway results in less efficiency in the other pathway Solutions: RC Type 2: Purple (sulfur and non-sulfur) and filamentous green bacteria have a reaction center that uses cyclic electron flow to synthesize ATP RC Type 1: Green sulfur bacteria and Heliobacteria have a reaction center that uses non-cyclic electron flow to synthesize NADPH Linked Photosystems: Cyanobacteria have 2 different types of reaction centers linked together PS II: similar to RC 2 but instead of recycling e-, passes it to PS I PS I: similar to RC 1

15. Hypotheses on Development of different types of reaction center Selective loss hypothesis Ancestor of cyanobacteria develops two kinds of reaction centers that diverge in function, resulting in the linked phototsystems; in some descendant lineages one or the other function is lost Fusion hypothesis RC 1 and RC 2 evolve along separate lineages; lateral gene transfer between bacteria brings them together into cyanobacteria

16. Hypotheses on Development of different types of RC

17. OK, so now we have ATP and NADPH. What about photosynthesis? Bacteria use a variety of pathways to synthesize carbohydrates Reverse Krebs Cycle Calvin Cycle Non-cyclic acetyl-CoA pathway Cyanobacteria and chloroplasts use Calvin Cycle

18. In Lab Thursday: Extraction of photosynthetic pigments: Obtain 2 test tubes with 2 mL solvent 90% petroleum ether + 10% acetone Obtain a one green and one purple leaf 2 strips of filter paper 1 cm above bottom of filter paper: With a coin, squeeze plant pigments onto paper in a straight line (avoid edges of paper) Repeat to obtain a dark stripe of pigment Place paper in test tubes, set in rack, and stopper Make sure pigment isn’t submerged Let rest to extract pigments (ca. 15 minutes) See if you can identify the different pigments