The Plants in my Aquarium are Dying
by Brian McGinty
It started simply enough. I was talking idly with a co-worker who knows a lot about aquatic biology, and who is the workplace "go-to guy" on aquarium related issues.
"I put a bunch of plants in there a couple months ago, but they look really thin now. I've got a betta fish, a plecostomus, and a lot of little snails. Are they eating the plants?"
"No, they're dying from lack of light. The plants' cells are actually eating themselves." (How did he know this without even hearing about my setup?)
The aquarium was in a "not especially" dim corner of the living room. There was a lamp with a 40 W incandescent bulb over the aquarium so I thought (insert ill-informed internal monologue concerning the spectrum from a fluorescent tube here) and replaced the bulb with a compact fluorescent over the week end. So, the next Monday:
"Nope, that won't help. Usually people just keep replacing the plants as they die. You'll have to put the tank in direct sunlight."
"Won't that kill the betta?"
"Nothing kills a betta."
OK, by now I am figuring out that I am a little electroweak (chuckle) on the properties of light. I admit this bias, since I'm made of leptons and baryons and therefore photons carry no weight with me (chuckle.) So here's what was wrong with my original idea:
My 40 W bulb put out 400 lux of light, which I replaced with…
A 23 W compact fluorescent that put out 1500 lux of light.
BUT…
Full daylight is 50,000 lux of light.
A lux being a unit of light brightness. For aquatic plants, that dim corner was not quite the ragged edge of survival. The 'perfect aquatic weed', Hydrilla verticillata can get by on 1% of full sunlight - about 500 lux. No wonder my plants weren't dying overnight, but not thriving either. Plants don't see with rhodopsin like we do, or with a p-n junction like a phototransistor, but with chlorophyll.
How could I be off by a factor of around 100 on what I thought was an adequate light intensity? How could I have come across a clearer case of what you see is NOT what you get? It turns out my eye is one of the most versatile optical sensors ever developed, particularly in terms of dynamic range.
The eye can adapt to a light intensity that ranges from 100,000 lux (brightest full daylight) down to 0.00005 lux (starlight.) Moreover, the eye can see even 1 photon (which the brain suppresses as noise) but 2 or 3 photons over a small area for a short time will produce an impression. I'm really not consciously aware of this ability. I know that a candle is 'dimmer' than a 40 W bulb, but I didn't realize it is 400 times dimmer - 400 lux for the light bulb but just 1 lux for the candle.
Don't leave yet, I'm just getting to the analog electronics part - Part Number OP705A to be exact. This is an npn phototransistor from Optek electronics. The chemistry is different, with photons liberating electrons from a silicon junction instead of from a molecule like rhodopsin in your eye, or chlorophyll in a plant. The current output of a phototransistor-based circuit rises slowly until the number of electrons tips the junction over into an avalanche of conduction:
So I could see that this phototransistor would be fully on in my 'not especially dim' corner of the living room. It crosses over into conduction at about 30 candles, and is "maxxed" out at about the normal room light of a fluorescent bulb. I could imagine a cop questioning the three of us as eyewitnesses:
OFFICER: "So, was the corner of the living room well lit?"
PHOTOTRANSISTOR: "It was too bright to look at."
ME: "It was 'not especially' dim."
CHLOROPHYLL: "It was so dark I could barely see."
Both the eye and the phototransistor get their sensitivity from bias. In the case of the silicon junction, enough voltage is applied to the emitter and collector so that the electricity is almost, but not quite able to burst over the barrier and conduct. Enough so that a few photons boosting a few electrons to a higher energy level away from their atoms can make the difference.
A similar concept applies to the eye's light sensing rhodopsin molecule - an incident photon triggers it to untwist, starting a cascade of reactions that lead to a nerve impulse. Then the metabolism of the cell laboriously twists the molecule up again until it is ready to fire. The eye can handle quite a few more orders of magnitude. Let's be generous and say the OP705A can respond over four orders of magnitude (1.5 to 15 to 150 to 1500 lux). The eye can go from 0.00005 lux (starlight) to 50,000 lux (looking at a closer star, the Sun): Nine orders of magnitude.
Chlorophyll looks like the slacker in this case (ballpark it at two orders of magnitude, from 500 to 50,000 lux), but it isn't the trigger for some previously energized reaction. It is doing work, absorbing photons so electrons jump to a higher energy state. It is an elaborate chemical reaction with 10 separate small jumps from the lowest energy state to the highest. (If I recall correctly. I studied the reactions to design an 'artificial chloroplast' for a science fiction story I was writing. After spending a good four pages describing the machine and still not getting close to moving on, I said "The hell with it," and put everything behind a panel that said 'Authorized Personnel Only'.)
What about the aquarium? After a week, the betta is fine. The plants, they look about the same.
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