Light saturation and photoinhibition

Posted by Stella Kin 

Above a certain value of light intensity, a further increase in light level actually reduces the biomass growth rate (see figure) – photoinhibition – at light intensities only slightly greater than the light level at which the specific growth rate peaks.

Photoinhibition results from generally reversible damage to the photosynthetic apparatus, as a consequence of excessive light.

There are lots of papers that detail methods to overcome photoinhibition. 

Gordon (2007) advocates irradiating the algae with intense microsecond pulses of intense red light. Through turbulence, the algae are therefore cycled through light and dark zones at high frequency, allowing an increase in the algae's flux tolerance to light. Projected annual algal biomass yields could be increased from 1g dry weight m-2 h-1, to 100g.

In more detail:

• The best annual averaged bioproductivity of outdoor open algal ponds is ∼1 g dry weight m−2 h−1, while corn, sugar cane, and closed algal photobioreactors have achieved ∼2–3 g dry weight m−2 h−1
• Projected algal biomass yields are as high as 100g dry weight m-2 h-1
• This may be done through pronounced heightening of algal flux tolerance, achieved by tailoring the photonic temporal, spectral and intensity characteristics with pulsed light-emitting diodes. Such tailored photonic input is applied in concert with thinchannel ultradense culture photobioreactors with flow patterns that produce rapid light/dark algae exposure cycles
  

• In commercial algal photobioreactors, flux tolerance is typically ∼200–400 μmol photons m−2 s−1. (the naturally occurring peak terrestrial solar intensity is ∼1,000 W/m2, which corresponds to ∼2,000 μmol photons m−2 s−1 of photosynthetically active radiation (PAR—the limited spectral range of ∼400– 700 nm).
• Experimental reports of Hu et al. (1998) attest to achieving a flux tolerance of ∼2,000 μmol photons m−2 s−1.
• Hu et al. algae cultivated with averaged light/dark cycle times of the order of tens of milliseconds, produced ∼17 g m−2 h−1 at flux values up to ∼8,000 μmol photons m−2 s−1, i.e., up to 20 to 40 times the commonly perceived values of algal flux tolerance (but diminishing returns after 2000)

Flashing is sometimes understood to mean imposing an on–off character to an otherwise continuous light source, e.g., eliminating part of it at regular intervals. Pulsing involves condensing the same energy into shorter times, nominally at no energetic sacrifice. Namely, the averaged intensity comprises periodically alternating high-intensity and dark-cycle periods. The “on” fraction of the on–off cycle and its intensity are also variables in the optimization.

The strategy advocated in this paper not only irradiates the algae with a particular time sequence with pulses that persist for no longer than tens to hundreds of μs, but also increases the instantaneous photonic flux (during the “on” part of the on–off light input cycle) in the process. This in turn allows a significant increase in the averaged photonic intensity (i.e., the average over the irradiation and dark periods) while postponing the onset of light saturation to higher averaged photonic flux values. Solely chopping a continuous light signal cannot noticeably improve bioproductivity but optimal pulsing can. The optimal photobioreactor strategy must combine sufficiently rapid algae transit times (and ultradense cultures so incident light is completely absorbed) with optimally pulsed light.

Conclusion

Bioproductivity could be further enhanced with intense light pulsed at tens to hundreds of kilohertz. Flux tolerance augmentation should be possible.

In addition, photosynthesis driven with pure red light is ∼5 times more efficient than with full-spectrum sunlight. It could involve a combination of: (a) Ultra-efficient high-flux photovoltaics converting solar energy to electricity, and (b) LEDs then converting electricity to pulsed nominally monochromatic red light

Photovoltaic-LED combination can transform solar radiation into pulsed red light at conversion efficiencies of ∼20% with commercially available components

More to follow...

Reference

Biodiesel from microalgae - Christi 2007

Ultra-high bioproductivity from algae - Gordon 2007