Algae: light control

Light control:  algae and light. At the first sign of excessive algae in a tank, you'll immediately think of adjusting the lighting. There are only three variables in the lighting: the spectrum, the intensity and the photoperiod.
 
The photoperiod is easy to control. The planted aquarium should be strongly lit about ten hours a day, and not more.  In controlling true algae, the first thing people have told you to do is to use a timer to cut down on the photoperiod: not to reduce the intensity of the light available, merely to limit the number of "daylight" hours per day to fewer than ten. They were right: get a timer. In the last few years programmable electronic timers have come down in price, and those clunky old mechanical ones with the insertable plastic pins are historic relics.
 
Circadian rhythm of plants. This action will be demystified if you know about the circadian rhythm of plants. Circadian ("sir-KAY-de-an") is derived from Latin circa "about" and dies "a day" and describes the biological daily clock common to cyanobacteria, true algae and other photosynthesizing single-celled organisms and plants. (Animals also have a circadian rhythm.)
 
Circadian rhythm regulates many cellular processes. In plants circadian rhythm regulates not only photosynthesis but also root and stem growth and the timing of flowering. The daily rhythm is set by parts of the genome that are similar in bacteria, protists and all multicellular organisms. They code for two proteins, designate them Period and Timeless, which accumulate slowly in cells. When they reach a certain concentration, they suppress the production of two other proteins — call them Clock and Cycle. But Clock and Cycle are in fact precursors of the first two; without them, the proteins Period and Timeless cannot be made. Thus as Period and Timeless disappear from the cell, Clock and Cycle can be produced again without getting quickly scavenged. The resulting ebb-and-flow rhythm takes place in an approximately twenty-four-hour cycle.
 
The biological clock is constantly re-set by day length. So, after a certain number of hours bathed in light bright enough to trigger photosynthesis, the process shuts down. For a brief but thorough, illustrated explanation of circadian rhythm, from a botanist's perspective, go to the page "Circadian movements"at Botany onLine.
 
Fish as well as plants benefit from a consistent light/dark cycle, by the way. Perhaps strong plant growth is what actually represses algal growth, when the lights are set by a timer. 
 
The "siesta": interrupted light. Some successful keepers of  planted tanks keep algae at bay with a mid-day "siesta", when lights are out for a period of two or three hours. Tom Barr and the plant guru team discuss this technique in several threads at the Barr Report. The German plant firm Dennerle seems to have originated this technique in the 1990s; the current version of their site remarks that "thunderstorms regularly darken the skies in the tropical habitats which are home to most aquarium plants". The authors of the Baensch Atlas advocate the practice (Atlas, I, p.45). Though I'm skeptical of any connection with daily thunderstorms, this interruption in the artificial day has an added value of extending hours of light in the evening, when you're most likely to be home.
 
Diana Walstad provided a more plausible account for the effect, which had developed a certain mystic metaphysic, by suggesting in The Ecology of the Planted Aquarium (1999) that during the dark period fish respiration replenishes the CO₂ that has been diminished by hours of photosynthesis, hopefully of the higher plants. CO₂ is the source of carbon for plant growth. Peter Hiscock's Encyclopedia of Aquarium Plants endorses the practice, reporting the thought that after the dark hours "higher" vascular plants are able to recommence photosynthesis faster than algae.  This has not been demonstrated.
 
The spectrum of light is less easy to control. Algae are more efficient than plants at using light waves in the blue and indigo end of the visible spectrum. "Cool Daylight" bulbs may encourage algae less than "Gro-Lux" bulbs, which are strong in red and blue sections of the spectrum. "Warm Daylight" bulbs put out intense light in the green and yellow  parts of the spectrum to which the human eye is most sensitive; though they may look brighter, they are not brighter in the parts of the spectrum used for photosynthesis.
 
The intensity of the light can be defined as the output (in photons not wattage) times the square of the distance. Concerning the intensity of the light, I find that I'm always being given conflicting advice, viz: "Don't reduce the intensity of your light, measured in  photosynthetically active radiation (PAR), to control algae," says Team A. Algae can get by at lower light levels than "higher" plants. In other words, the photosynthesizing plastids embedded in each algal cell become saturated at a lower intensity than the comparable plastids in plants. Intense light, like sunlight, also has an inhibiting effect, the A Team pros tell me. Algae are photoinhibited at levels that vascular plants can still use. And diatoms ("brown algae") are photoinhibited at the lowest levels of all. But frankly I can't imagine being able to flood my tanks with so much artificial light that algae are genuinely photoinhibited. Instead, I'm noticing that a densely-planted tank I have, which receives a couple of hours of natural sunlight each day, has clearer water than any of my tanks that depend entirely on artificial light.
 
"Do reduce the intensity of your light to control algae," says Team B. Intense light can make iron more available for algae and plants, in a process called "photoreduction." Iron's increased availability under intense lighting is a prime suspect for algal stimulation, rather than the intensity of the light itself. (There's lots more detail about "iron photoreduction" in the aquarium in Diana Walstad's Ecology of the Planted Aquarium, 1999, pp 167-169.) I've noticed that problem algae can slowly become an issue as a fluorescent tube ages; the spectrum of its output may be shifting in a way that encourages algae. The remedy can be as simple as installing a fresh bulb.