Light and photosynthesis

Question 1

Oceanic and terrestrial photosynthesis

Answer 1

Land: ~54% NPP, >99% biomass, ~19 year turnover
Ocean: 46% NPP, ~0.2% biomass, 2-6 day turnover.
PPs in the ocean’s turnover >1000x faster than the land

Question 2

Photosynthesis is light dependent

Answer 2

Instantaneous rates of photosynthesis are controlled by external factors:
- temp, co2 and O concs, irradiance
Process of photosynthesis is fundamentally dependent on the availability of irradiance
Availability of light varies as a function of physical and biological processes

Question 3

Photosynthesis light curve

Answer 3

Photosynthesis-irradiance response (P vs E) divided into 3 regions:
1. light limited
2. light saturated
3. photo inhibited

Question 4

Light Limited Region

Answer 4

Linearly proportional to irradiance
* The initial slope of the P vs E curve is is often given symbol α
- where α = Pmax / Ek
- units = (µmol O2 I-1 h-1) / (µmol photons m-2 s-1 ) -1
At low irradiance O consumption > O evolution (NPP is negative)
Compensation irradiance where O is balanced (Ec) is around 5-30umol/m2/s

Question 5

Light saturated region

Answer 5

A-s irradiance increases photosynthetic rates become nonlinear and rises to a saturation level or Pmax.
-At saturation the rate of photo absorption exceeds the rate of steady state electron transport from water to carbon dioxide.
-Pmax is a function of the light independent reactions of photosynthesis and is influenced by environmental conditions influencing enzyme reactions such as temperature.

Question 6

Light saturated parameter (Ek)

Answer 6

Intersection of α and Pmax.
Represents an optimum on the P vs E curve
Ek = Pmax / α
Ek is independent of whether Pmax or α are normalised to chlorophyll, cell volume, cell C etc

Question 7

Photo inhibited region

Answer 7

Further increases beyond saturation damage rate.
Photoinhibition is dependent on intensity and duration of exposure.
Multiple mechanisms of photoinhibition remain an active area of research. Core PSII must be repaired every few hours.

Question 8

Photoacclimation

Answer 8

Phytoplankton can grow over a wide range of different environments and over a wide range of different light levels
They acclimate to different levels through mechanisms which alter capacity to harvest light:
- as irradiance increases, each cell requires less pigment to absorb the same number of photons
Characteristics of the P vs E curve can relate to the growth irradiance