Cheating Malthus even more: some amateur musings

[neutrino_cannon]

I was speaking to a friend of mine about photovoltaic cell chemistry, and to be honest the majority of it went way over my head, but the issue of efficiency came up. As I was able to comprehend it, the issue is that once the electrons get bumped around by the incoming photons, the resultant charge tends to want to dissipate via pathways that do not end up creating electricity. It is my further understanding that this exact same problem applies to biological photosynthesis as well, and if wikipedia is to be believed: http://en.wikipedia.org/wiki/Photosynthetic_efficiency#Upper_light_limits_to_efficiency it would appear that the best man-made photovoltaics are better at turning sunlight into electricity than plants are at turning sunlight into sugar.

Not an apples to apples comparison exactly, but it's rather rare for an artificial technology to beat its natural analogue on any sort of efficiency comparison. So, the issue as I see it, is that we can't turn electricity into sugar.

We are pretty good at breeding plants that spend more of their energy making edible bits. We are pretty good at figuring out the ideal soil chemistry and other conditions to make the plants produce at optimal yield. With a little will, the efficiency of fertilizer use, crop transport, storage and use could surely all be optimized. Once all that's been perfected though, we're still limited by the amount of megawatts per acre that can be used to convert inedible compounds into edible compounds, and the energetic efficiency of that process is frankly abysmal.

So, if you had to come up with a process of making a process that's more efficient than plant agriculture to generate edible material, how would you go about doing it? I had a vague notion that you might be able to breed sulfur-eating bacteria to store lots and lots of energy as proteins, and then taking their metabolic waste products and reducing those back into compounds the bacteria can use again, and then I realized that I know absolutely nothing about the metabolism of anaerobic bacteria or electrolysis. And then I realized that people here probably do.

Thoughts?

 

[quarky]

Single cell organisms beat larger foods, due to less structural requirements. Amplifying sunlight could up production by reflectors that steal it from other areas. C-4 plants are more efficient photo synthesizers.

 

[soylent]

Blue-green algae and cyanobacteria can be pretty efficient; 5-10% sunshine-to-food calories efficiency under just the right conditions. Admittedly, they're not terribly tasty; spirulina can cause stomach irritation when eaten in > 100 g amounts.

Those problems may have solutions. Raw wheat berries taste pretty terrible to. Wheat only became delicious when we invented beer, booze, pasta, bread, porridge and combined products like lasagna, ravioli, pizza, pies, pancakes, wheat-gluten fake vegetarian meat, béchamel sauce(the 'mother sauce' from which many non-cream sauces and thickened gravies are made) etc.

Near the equator you get ~200-300 W/m^2 average sunshine. At 5-10% efficiency you need 3 to 10 m^2 of algae/cyanobacteria to supply a 2 kcal diet for 1 person. You'll probably need a few more square meters of PV to run pumps, stirers, dryers and so on.

 

[soylent]

Single cell organisms beat larger foods, due to less structural requirements. Amplifying sunlight could up production by reflectors that steal it from other areas. C-4 plants are more efficient photo synthesizers.

There are many ways in which macroscopic plants are inefficient. They contain too much clorophyll; allowing a leaf to absorb more sunshine than it can put to good use, for the sake of shading its competitors(but there is also a lot of self-shading and shading of adjacent food crops). Excess energy captured by clorophyll must be disposed off to prevent it from damaging the plant. Plants are green; what that means is, they don't capture green light, they reject it; what they want is red and blue light. There is precedent for photosynthetic organisms that capture green light, purple bacteria. Ideally plants could be made to utilize all frequencies of light, with some mechanism(e.g. white fuzzy hairs) to prevent the albedo of the Earth from being drastically altered.

Rubisco, which is involved in capturing and integrating CO2 into organic molecules using energy from photosynthesis, is a pretty terrible enzyme and it may be possible to engineer a much, much better enzyme(though a very hard problem). Rubisco captures and catalyzes reactions that fixes CO2 from air into organic molecules at a miserable rate of a few times per second; that's really bad, and plants have to produce a lot of rubisco to compensate. Sometimes rubisco captures an oxygen molecule, which wastes a CO2-molecule and some energy to fix.

Perennial crops don't need to rebuild their root system from scratch every year, which means they can have a much deeper root system built over multiple years at no additional energy cost; capturing more water and nutrients from the soil.

 

[quarky]

Good post above.

So many factors in this equation. I think its hard to do better than a climax forest, as per sun light and production, yet such an approach doesn't yield well to mono-culture prejudices. There are tropical trees with edible (to us) leaves. There is wood. Iguanas. Nuts. Drugs. Nature preserves/touristry; flood control; climate control, etc; etc; all worthy of being part of any bold, final analysis.

 

[soylent]

Near the equator you get ~200-300 W/m^2 average sunshine. At 5-10% efficiency you need 3 to 10 m^2 of algae/cyanobacteria to supply a 2 kcal diet for 1 person.

Errata: that would be a 2000 kcal diet.

 

[macdoc]

Quarky - that's way too vague - you are looking at a broad brush utilization that stretches as far as the carbon cycle and the water cycle in addition to sunlight capture....

The OP is dealing with efficiency in a very narrow zone of turning sunlight to food source - this is effectively bio-engineering for a narrow goal.

Plants can be inefficient as there are lots of them .....and they grow themselves.

We want to mix our engineering understanding with biological profundity to get our goal cheaply.

 

[neutrino_cannon]

Unfortunately I had to leave on a trip just after I posted this thread, but thanks for the responses. A few thoughts:

-If you put your algae hydroponics installations in space on one of the earth/moon Lagrange points you gain about 30% more sunlight wattage for a given area and you get rid of that pesky day/night cycle. For such a scheme to be practical you'd need cheap launch technology and an efficient means of transporting large amounts of bulky organic matter back and forth. Maybe an orbit like a spy satellite uses could keep an orbital greenhouse in the sun most of the time and not be so far away?

-Most of the responses seem focused on using biological photosynthesis, green or otherwise. Are any of the purple-pigment or other exotic photosynthetic cycles any more efficient than the green-plant ones? Just talking the cycle itself, not losses due to weird macroscopic adaptations.

-Supposing one does manage to genetically engineer a more efficient plant metabolism (tough but theoretically possible), what happens if it gets loose in the wild? Does the more efficient lab-designed organism displace the natural ones? Does it immediately die because there aren't particularly strong selective pressures towards high photosynthetic efficiency in the first place?

 

[soylent]

Are any of the purple-pigment or other exotic photosynthetic cycles any more efficient than the green-plant ones?

I suppose not; but there is little overlap between the frequencies they capture. It's a proof of concept that you could engineer grayish plants that capture most frequencies of light.

 

[neutrino_cannon]

So you're advocating a dual cycle photosynthesis? Interesting. I wonder if such an organism could survive outside a lab. My recollection is that some of the more unusual photosynthetic pigments needed manganese and suchlike. Doubling or tripling up on different ones would mean some pretty specific nutrient needs for the plant.