Light Requirements

From Phyco.org

Algal Light Requirements attempts to cover in detail the constraints of building an algae production system from a lighting perspective, and ideal light conditions for growing algae in general.

It may be helpful to read the Physics of Light Brief to further understand what is covered in this article.

Contents 1 Factors that Affect Light Requirements of Algae 1.1 Algal Species 1.2 Light Color 1.3 Light Intensity 1.4 Photoperiod 1.5 See Also 2 Effects of Altering Culture Lighting 2.1 Cell Size 3 Utilized Spectrum 3.1 Various Algae use Different Wavelengths 4 Light Intensity 4.1 Lighting and Stirring Algae 4.2 Limits on Bright Light 4.3 Limits on Dim Light 5 Light Sources 5.1 Fluorescent Lighting 5.2 Light-Emitting Diodes 5.3 Natural Sunlight 5.4 Directed Sunlight 6 See Also 7 References

[edit] Factors that Affect Light Requirements of Algae

Plant cells use only a small fraction of the sunlight that hits them, in two ways. Algae use only some colors of light. Moreover, sunlight is more intense, at all colors, than necessary for optimal algal growth. There are many factors that determine algal light requirements, including the topics covered below.

[edit] Algal Species

Blue-green algae (also called Cyanophyta and cyanobacteria) contain phycocyanin (a pigment), which photosynthesizes deep red light, about 680nm, more efficiently than other pigments that use other wavelengths.

[edit] Light Color

Algae can only use certain colors (wavelengths) of light, and among the ones algae can use, some colors are more efficient than others. In other words, some colors of photon will cause photosynthesis with little wasted energy and other colored photons release more waste energy as heat. This is known as Photosynthetically-Available Radiation, and makes up only 43% of the solar radiation that hits the earth. This is typically the red and blue portions of the visible spectra, depending upon the type of chlorophyll. This is also why the chlorophyll pigment appears green in color.

[edit] Light Intensity

Excessive light can damage or kill algae, because it overloads their photosystems and can even bleach out their pigments. Adequate mixing/agitation is very important for this reason, among others (promotes better gas exchange, for example).

Moreover, once a cell has absorbed enough energy, it needs a dark period to use the energy via photosynthesis. Additional light hitting the cell will be converted into heat. This leads to the discussion of photoperiod.

[edit] Photoperiod

Photoperiod, or the number or hours an organism gets exposed to light during a 24-hour period, is an important factor to be considered when building a photobioreactor. Photoperiod cannot be controlled with raceways and similar environments, which may lead to the lower yields typical of those systems.

The effect of alternating light and dark is not well understood, because there is too little information. Algae has been grown using no dark period, and up to eighteen hours dark. Bright light may damage algal antennas, which may extend required dark periods to effect repair. Long dark periods result in biomass loss and slower growth rates as algae consume carbohydrates and oxygen during photorespiration. There is no consensus about light and dark durations.

The reason a photoperiod is necessary is due to a the nature of photosynthesis. The reason involves the Calvin-Benson cycle, which is the cycle that produces organic compounds from CO2 and H2O using high-energy ATP and NADPH created by the photosystem of the organism. This cycle is separate from the so-called light reactions, which takes place in the photosystem, producing energy.

Plants are naturally accustomed to the 12-hour photoperiod that takes place on our planet. Changing the photoperiod could result in higher amounts of some substances than others, and could either optimize the process of photosynthesis, have no effect, or have a detrimental effect. Regardless of the effect of changing the photoperiod, one must also take into consideration the effect on net energy input and the actual fiscal feasibility of altering the amount of light the culture receives.

[edit] See Also

• ^[1]
  • This paper suggests that using Light-Emitting Diode lighting to extend daylight in winter uses less electricity to get growth similar to sunlight. However, before one invests in LEDs, one should evaluate the initial lighting investment, the cost of electricity for several lighting options.

[edit] Effects of Altering Culture Lighting

Some possible effects of altering the lighting of an algae culture are described here.

[edit] Cell Size

It is understood that algal growth under nearly ideal lighting grows smaller algae, but faster than other colors. Consequently, the total algal mass is about the same as using a broad spectrum light, such as sunlight.

[edit] Utilized Spectrum

[edit] Various Algae use Different Wavelengths

Summary

This article explains differences in light absorption of several algae species among chlorophyta (green algae), phaeophyta (brown algae) and rhodophyta (red algae).

Both green and brown algae use photosynthesize light most efficiently around 435nm and 675nm. However, red algae photosynthesize most efficiently using other wavelengths. Moreover, red algae use little or no light of 435nm and 675nm.

Some of the light used by red algae is blue-green, which penetrates up to 10m deep in seawater.

This study suggests that a multi culture of algae that use different wavelengths of light will produce more algae than a monoculture. Moreover, a multi culture that uses a greater portion of the light spectrum for photosynthesis will assure the algal medium warms slower than a monoculture in bright sunlight.

^[2]

[edit] Light Intensity

[edit] Lighting and Stirring Algae

Relevant Information

The authors summarized the practical information as follows:

"Practical Aspects: The entirely practical question, toward which the current work was directed, is the possible contribution of turbulence of culture to increase in growth rate in dense cultures. The light intensity of 23x10⁴ erg/cm²-sec of light used is about 0.6 of the maximum intensity of full sunlight in the visible region. It can be seen from the curves of Figures 5 and 6 that a dense culture growing under sunlight will experience a significant increase in growth if cells are moved in and out of the high intensity of the front surface at such a rate as to give flash times between 0.001 and 0.1 second. It is also clear that the culture should be thick enough or dense enough so that almost all the light will be absorbed in the first 10% and the dark time will be about ten times as long as the light flash. These considerations lead to the conclusion that almost any attempt to grow algae in sunlight will experience some gain by turbulence. The feasibility of increasing turbulence will depend on the extent of the gain in growth as compared to the increased power requirement of stirring or pumping the suspension."

Refer to the paper for Figures 5 and 6.

This paper explains why some say sunlight is ten times brighter than algae need, as it says they need about 1/10 sunlight light and 9/10 dark per second.

^[3]

[edit] Limits on Bright Light

Summary Though this paper is about the effects of temperature on algal growth, it also contrasts the amount of light needed to grow algae with sunlight. These experiments showed that their cultures did as equally well with light intensity of 2,500 lux and 10,000 lux. Moreover, 15,000 lux inhibited growth. Bright sunlight has an intensity of 80,000 lux. Note that the fraction light used for these experiments compared to sunlight is following:

1/32 = 2,500 lux / 80,000 lux

^[4]

[edit] Limits on Dim Light

Summary Three common fresh water algae species, a diatom, green, and blue-green algae, demonstrated they reacted differently to various levels of dim light and amounts of phosphorous. Given sufficient phosphorous, researchers found that the blue-green algae grew slower in dim light than the diatom and green algae. Though, dim light slowed growth in all three of the species. The paper suggested that this difference between the blue-green algae and the other two may be the reason that blue-green algae are naturally prolific in summer; whereas, the diatom and green-algae are naturally prolific in spring and fall.

^[5]

[edit] Light Sources

Various sources of light can be used to provide the light energy required to sustain photosynthesis.

[edit] Fluorescent Lighting

[edit] Light-Emitting Diodes

[edit] Natural Sunlight

[edit] Directed Sunlight

Diffusers, light pipes, and optical fibers can be used to channel light within a photobioreactor. A system such as that implemented by Sunlight Direct is one example of this technique.

[edit] See Also

• Parabolic Trough Lighting System - A sunlight distribution system for photobioreactors using parabolic troughs and acrylic glow plates.
• Solar Glow Plate Photobioreactor - A photobioreactor with a heavy emphasis on light requirements.

[edit] References

1. ↑ Study of Light Requirements of a Photobioreactor - Anil Kommareddy and Gary Anderson, North Central ASAE/CSAE Conference, Winnipeg. (2004) [1]
2. ↑ Photosynthetic Action Spectra of Marine Algae, 1949, F. T. Haxo and L. R. Blinks, Hopkins Marine Station of Stanford University, Pacific Grove. [2]
3. ↑ Growth Rate of Chlorella in Flashing Light, 1953, J. Neal Phillips, Jr. and Jack Myers, Research: University of Texas, Austin, Publication: Plant Physiology (date and vol. unknown) pp 152-161. [3]
4. ↑ Effect of Temperature on Blue-Green Algae (Cyanobacteria) in Lake Mendota, 1978, Allen Konopka and Thomas D. Brock, Applied and Environmental Microbiology, Oct 1978, pp 572-576. [4]
5. ↑ Photosynthetic and Growth Responses of Three Freshwater Algae to Phosphorous Limitation and Daylength, 2003, Elena Litchman, Daniel Steiner and Peter Bossard, Freshwater Biology, (2003) 48, pp. 2141-2148. [5]