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  • The formation of organic matter from inorganic carbon (CO2) with light as the primary energy source
  • 6 carbon dioxide + 6 water = 1 glucose + 6 oxygen 

  • 6 CO2 + 6 H20 = C6H12O6 + 6 O2
  • Two reaction steps: 
    • (1) Light reaction: photophosphorylation: production of O2 and energy from H2

    • (Where does the O2 come from, H2O / CO2?) 
    • (2) Dark reaction: carbon fixation: CO2 to glucose
  • Phytoplankton and algal photosynthesis = primary production
  • Organisms that perform photosynthesis = primary producers = autotrophic = phototrophic organisms
  • All phototrophic organisms possess chlorophyll a and several accessory pigments (chl. b, c, carotenoids), which serve as antenna pigments to capture light energy and transfer electrons to the photosynthetic reaction center
  • Each pigment has a distinct absorption spectrum 
  • Photosynthesis most efficient in blue and red light, according to absorption maximum of chlorophyll (action spectrum)
    Photosynthesis and Light
  • P vs. I curves (photosynthesis versus light intensity): shows photosynthetic adaptation
  • Gross production = total production; net production = gross prod. respiration
  • Compensation point: photosynthesis = respiration, 

  • net production = 0
  • Pmax = maximum production; depends on dark reaction (unlimited growth) or limiting resources
  • Initial slope a: photosynthetic efficiency (how good is low light used), also quantum yield f = DP/DI; depends on light reaction
  • Ik: summarizes key characteristics Pmax and a in one term; shade-adapted cells have lower Ik than high-light cells


Note: Species (1) and (2) have the same Ik despite different Pmax and a. The lower Ik of (1) and (2) as compared to (3) reveals them as shade-adapted species.

Sverdrup's Model of Critical Depth

  • Photosynthesis decreases exponentially with depth due to decrease in light availability
  • Respiration is unaffected by light and remains constant with depth
  • Phytoplankton is mixed by turbulence and experiences different light intensities over time, sometimes above and sometimes below compensation point
  • Critical depth = depth at which photo-synthesis of the total water column phytoplankton population equals their total respiration

A phytoplankton population can only proliferate if mixing is shallower than the critical depth. Only then is the population net production >0