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Green Plants, Pink House

August 5, 2013

The Earth was described as a "pale blue dot" by astrophysicist, Carl Sagan. The dot was a sub-pixel image of the Earth taken at a distance of 3.7 billion miles (6 billion kilometers) by the Voyager 1 spacecraft in 1990. The blue color came mostly from Rayleigh scattering from clouds in the atmosphere.

A blue dot might be how extraterrestrials would first image the Earth. However, for most people here on Earth's surface we live on a green planet because of all the vegetation.

There's a simple reason why vegetation is green. It's a consequence of the specific chemicals, chlorophyll a and chlorophyll b, which plants use to harvest solar energy. The green wavelengths of light are not useful for photosynthesis, so they are not absorbed by plants, and they are instead reflected (see figure). This may be why human vision evolved to be especially sensitive to green light. If you see it, it might be dinner.

Spectra of chlorophyll a and b.

Spectra of chlorophyll a and chlorophyll b.

Green light, which is nominally 550 nm, is not absorbed.

(Modified image from Wikimedia Commons.)


Colonists on Mars, for example, would be wise to heed the evidence of science when it comes to the efficient growth of food. Mars is significantly less insolated than the Earth, as its lower temperatures show. This is a simple consequence of the inverse-square law for the distance from the Sun. For this reason, Martian greenhouses need to be both thermally insulated and supplied with artificial light. As the spectra in the above figure indicate, red-rich and blue-rich lighting would be most efficient.

Scientists at Purdue University, the Kennedy Space Center, and Orbital Technologies Corporation have just published results of their experiments on tomato (Solanum lycopersicum) culture using red and blue light-emitting diodes (LEDs).[1-3] Light-emitting diodes are far more efficient than other light sources, such as high-pressure sodium lamps (HPS) and typical overhead lighting (OHL) as used for room illumination. Tomato would be a good choice for Martian greenhouse food, since it will grow onto vertical wires, making efficient use of space.

This study showed that the electrical conversion efficiency of LED illumination into fruit biomass was 75% higher than HPS and OHL light sources.[1] There are quite a few factors that would encourage such a growth method here on Earth, as well as on Mars. The principal factor is the high cost and large carbon footprint of the traditional horticultural system of growing such fruits in warmer climates and then transporting them thousands of miles to where they're eaten.[2] The United States imports about a third of its tomatoes, which are picked green to ripen during shipment, a process that reduces fruit quality.[2]

The LEDs run cool, so they can be placed in close proximity to the plants. They can illuminate not just the tops of leaves, but the bottoms, also. Says study coauthor Gómez,
"The leaves are photosynthesizing on the lower parts of the plants, and that may be helping with the plant's energy... We're getting the high intensity of the LEDs close to the plants because they're not hot like a high-pressure sodium lamp. If you put one of those close to the plants, you'd scorch it."[2]

Purdue University greenhouse with LED illumination.

Purdue University agricultural scientists, Cary Mitchell (left), and Celina Gómez, tend to tomatoes grown using red and blue light-emitting diodes.

(Purdue Agricultural Communication photo by Tom Campbell.)


Quite a few others are developing this ultimate "locally-grown" technology for horticulture, including designs that integrate living spaces with their own food production environments.[3] Although the Purdue experiments used LEDs to supplement natural light, some others are working on garden factories that are completely enclosed and out of reach of insects and extreme weather.[3]

Caliber Biotherapeutics has a 150,000 square-foot "pink house" facility in Texas in which more than two million plants are grown in vertical stacks up 50 feet high.[3] Instead of growing tomatoes, this facility grows a high-value-added tobacco-like plant for drug and vaccine production.[3] One advantage of the enclosed system is that water use is minimal, which is something that may become an important future factor.

Let's switch topics from future photosynthesis to photosynthesis that occurred more than two billion years ago. The origin of oxygenic photosynthesis, the splitting of water molecules into oxygen upon which animal life on Earth depends, is unknown. As in other evolutionary processes, it's unlikely that the process we see today was the first process that nature tried.

A team of geobiologists from the California Institute of Technology (Pasadena, California), the Stanford Synchrotron Radiation Lightsource (Stanford University, Menlo Park, California), the Massachusetts Institute of Technology (Cambridge, Massachusetts), and the Tokyo Institute of Technology (Tokyo, Japan) have just published evidence in the Proceedings of the National Academy of Sciences that photosynthesis evolved from a transitional photosystem involving single-electron oxidation reactions of manganese.[4-5]

As materials scientists and inorganic chemists know, manganese is a slippery character, having a myriad of oxidation states, including +2, +3, +4, +6 and +7. As such, it's easy for it to be a part of photoreactions. As summarized by study coauthor Woodward Fischer, assistant professor of geobiology at Caltech,
"Water-oxidizing or water-splitting photosynthesis was invented by cyanobacteria approximately 2.4 billion years ago and then borrowed by other groups of organisms thereafter... Algae borrowed this photosynthetic system from cyanobacteria, and plants are just a group of algae that took photosynthesis on land, so we think with this finding we're looking at the inception of the molecular machinery that would give rise to oxygen."[5]
The research team based its hypothesis on the analysis of manganese deposits in drill core specimens of 2.415 billion-year-old South African marine sedimentary rocks.[4-5] The analysis showed that the manganese was deposited as the oxide, and not as manganese that was subsequently oxidized as oxygen on the Earth increased.[4-5]

Manganese in South African rock

Caltech graduate student Jena Johnson, a coauthor of the PNAS paper, examines a South African rock formation where evidence of a 2.415 billion year old manganese-oxidizing photosystem was found.

(Caltech photo.)[5)]


The research team also plans to analyze manganese-bearing rocks from western Australia similar in age to the South African samples.[5] However, the most interesting part of the research program is the plan to mutate cyanobacteria to perform manganese-oxidizing photosynthesis. Investigation of such a system, says Fischer, "could help target technologies for energy production from artificial photosynthesis."[5]

References:

  1. Celina Gómez, Robert C. Morrow, C. Michael Bourget, Gioia D. Massa and Cary A. Mitchell, "Comparison of Intracanopy Light-emitting Diode Towers and Overhead High-pressure Sodium Lamps for Supplemental Lighting of Greenhouse-grown Tomatoes," HortTechnology, vol. 23, no. 1 (February, 2013 ), pp. 93-98.
  2. Brian Wallheimer, "LEDs reduce costs for greenhouse tomato growers, study shows," Purdue University Press Release, April 29, 2013.
  3. Michaeleen Doucleff, "Vertical 'Pinkhouses:' The Future Of Urban Farming?" NPR, May 21, 2013.
  4. Jena E. Johnson, Samuel M. Webb, Katherine Thomas, Shuhei Ono, Joseph L. Kirschvink and Woodward W. Fischer, "Manganese-oxidizing photosynthesis before the rise of cyanobacteria," Proc. Natl. Acad. Sci., Online before Print, June 24, 2013, doi: 10.1073/pnas.1305530110.
  5. Katie Neith, "A Stepping-Stone for Oxygen on Earth," Caltech Press Release, June 26, 2013.

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