September 18, 1997 

Important points from last lecture 

Fate of light energy absorbed by pigment 
Redox reactions: pigments and electron donors 
Z scheme of oxygenic photosynthesis (PS) 
Arrangement of 3 components of PS in membranes 

 

H⁺ balanced by Mg⁺² and Cl^-. End result is no gradient in charge. Unlike oxidative phosphorylative, PMF solely due to proton gradient. Also, direction H⁺ pump opposite to what mitochondria do. But otherwise, ATP synthase very similar. 

Balance between PSI and PSII  

      Depends on NO₃^- or NH₄⁺ as N source 
      NO₃^- + xNAD(P)H ----> NO₂^- + yNAD(P)H ----> NH₄⁺ 
                                    nitrate                                nitrite 
                                reductase                           reductase 

In words: NO₃^- needs to be reduced to oxidation state of NH₄⁺ by NAD(P)H. 

A more complete answer to: What determines balance between PSI and PSII? [From R. Geider] 

     The balance between in vivo PSI and PSII reaction rates should be related to relative demands for NADPH (which requires linear electron flow and involves both PSII and PSI for transfer of electrons from H2O to NADPH) versus ATP (which can be generated by either non-cyclic photophosphorylation involving proton transfer associated with donation of electrons from PSII to the cytochrome b₆/f complex [together with release of H+ from water splitting] or cyclic photophosphorylation involving donation of electrons to the b₆/f complex from PSI). 

     What, you may ask, determines the relative requirements for NADPH and ATP? Well, biochemistry determines the relative demands. This includes biochemical composition of cells, ion transport across the cell membrane, nitrate vs ammonium as N source, operation (or not) of a CO₂ concentrating mechanism. 

     Of course, it is not that simple, because the ratio of H⁺ transfer through the ATP synthetase to ATP synthesis is expected to vary, and respiration may play a role in providing reductant (NADH) and/or ATP even under illuminated conditions. The Mehler reaction (PSI mediated electron transfer to oxygen) can also lead to pseudocyclic photophosphorylation. The Mehler reaction requires that oxygen first be liberated at PSII before it can subsequently be consumed at PSI. The Mehler reaction is not insignificant, and may equal 50% of gross oxygen evolution in cyanobacteria at light saturation. 

     A good text is D. Lawlor's new revised (1996?) edition with a title something like "Photosynthesis: mechanisms metabolism and molecular biology." Of course, it is directed at terrestrial vascular plants rather than aquatic photosynthesis. 

Use of fluorescence in marine biology and oceanography  

A. Chl concentrations 
      1. Extract cells within acetone 
      2. Measure fluorescence in fluorometer 

            Excitation 
            ----------> sample 
                light            |       Emission 
                                 v 
                    Photomultiplier tube 

           Spectrofluorometer: wavelengths can be changed 

 

          Chl a approximate measure of algal carbon = 50* [chla] 
 
B. Photosynthesis rate 
      - several methods to get at PS rate 
by monitoring fluorescence properties of cells 
      - mentioned by paper 
      - basis is to measure quantum yield  
    =    O₂ produced (or CO₂ fixed)  
                   mole of photons absorbed 

 Photosynthetic rate proportional to   for low light 

 

^B = a*  Mn 
                     | 
                    absorption cross section Mn²(chl a)^-1 

In words: P = (light intensity)*(area absorbing light)*(PS activity per area per mole of photons) 

Light harvesting by aquatic plants 

Special problem with living in water 
      -attenuation of both low and high wavelengths 
             -see handouts 

      I_z = I_o e^-zk where 
     I = irradiance ("light intensity") at depth z 
   K = extinction or attenuation coeffective, which dependent on wavelength (lambda) 

Three major solutions: "accessory" pigments 
      Use light between 450 and 600 nm 
 
  
 

General comments on structure of pigments  
    Many alternating double bonds 
    Tetrapyrroles 
        ring structure with Mg⁺² in chla 
             note similarity with heme, e.g. in cytochromes, which has Fe²⁺ ----> Fe³⁺ instead of 
            Mg²⁺; see p 629 in V&V. 

How is light energy transferred to reaction centers (RC)?  
      Remember: 
            L.H. pigments ----> PSII RC 

One example: Marine cyanobacteria 

      Cyanobacteria = blue-green algae 
      that is, they are bacteria, but have oxygenic photosynthesis, just like higher plants 

Important marine cyanobacteria 
     Trichodesmium (Oscillatoria): chains, macroscopic tuffs of cells N₂ - fixing 
     Prochlococcus: single cell 
            - recently discovered; has form of chl b 
     Synechococcus: single cell, ca. 0.8 m 
            - very abundant 
            - in tropical waters, especially, accounts for large (>50%) of primary production 

Light harvesting pigments in Synechococcus 

          allophycocyanin (APC) 
          phycocyanin (PC): blue 
          phycoerythyrin (PE): red 
 
phycobiliproteins = phycobilin + protein 
      Covalently linked together 

phycobilin: light absorbing part = chromosphore 

What is sequence of energy transfer, eventually to chl a? 
      Hint: all types of cyanobacteria have PC & APC, especially freshwater strains, but some do not have PE 

    Marine Synechococcus has PC, APC, but lots of PE 

    Pure cultures are blood red 

Distribution of pigments in different organisms 
      1. Unlikely that different organisms have different energy transfer 
      2. Data suggest: PE ---> PC & APC ---> Chla 

Absorption and fluorescence characteristics 
      PE ---> PC ---> APC ---> chla in reaction centers in PSII 

      this is direction by which energy is transferred but it's not by fluorescence (too slow), but by excitons. 

Phycobiliproteins in complex: phycobilisome "Structure follows function".