Here are some of my thoughts on algae's appearance in our tanks and its possible relation to nutrient deficiencies.
There are several interesting scenarios that we are all familiar with:
1) The healthy tank, uses chemical fertilizers according to EI or PPS-Pro with very little algae in the tank.
2) Established soil tanks, very little algae.
3) Tanks with plants that don't grow well and lots of algae.
Conventional thinking used to be that it was an excess of nutrients that caused algae to appear. Lots of effort was put into eliminating excess nutrients. Frequent water changes, using carbon in our filters, phosphate scrubbers, reduced lighting, etc all reduced these "excesses" and eventually the algae disappeared as well - likely eventually succumbing to resource deficiencies. This particular method of keeping an aquarium works well with fish-only tanks, but does not work well with planted tanks. Plants develop deficiencies and die as well as the algae.
Later on the EI method allowed people to grow lots of plants quickly and over time people realized that an excess of nutrients was not the sole cause of algae in our tanks. This has been shown countless times where people often have 20+ ppm of nitrate, and 2+ ppm of phosphate along with excesses of all the other nutrients in the water column and there are no algae blooms. Under these conditions algae does not flourish so long as there are enough plants in the tank.
Diana Walstad's book - Ecology of the Planted Aquarium mentioned how many species of aquatic plants produce chemical compounds that can inhibit other species of plants. This concept made me wonder if the same could be true of plants vs. algae. After reading through obscure plant and algae journals I found out that this is in fact true as well. Plants do make allopathic chemicals that significantly reduce algae's growth. Furthermore, algae does not seem to be able to adapt to these compounds and so they remain effective even though many generations of algae are made over time.
From my botany class days where we grew mono cultures of various algae species for experiments the standard procedure was to grow algae in small glass containers which contained all the mineral nutrients that we dose in our tanks. The algae we grew developed quickly and grew healthily in these conditions. The major difference between our tanks and those test tubes is - there were no plants in the test tubes. This implies that the presences of plants influences algae's ability to grow and further supports the journal articles mentioned above.
Miquel Lürling, Gerben van Geest, Marten Scheffer. Importance of Nutrient Competition and Allelopathic Effects in Suppression of the Green Alga Scenedesmus obliquus by the Macrophytes Chara, Elodea and Myriophyllum: Hydrobiologia Journal, February 2006, Volume 556, Issue 1, pp 209-220
My own research into nutrient deficiencies has uncovered many interesting relationships between nutrients and plants, along with some connections between nutrients and algae. For example, it is possible to give algae deficiencies when a tank is deprived of a nutrient.
During a discussion on what causes BBA to grow in our tanks (a lack of CO2 or an abundance of dissolved organic material in our tanks) I found several interesting articles which described how many species of algae can actually directly consume dissolved organic molecules in place of nutrients like nitrates and phosphates. In other words, algae can feed off organic molecules that plants release and dissolved partially broken down organic molecules.
Putting all of these seemingly unrelated facts together I think many, if not all of the algae outbreaks we observe in our tanks might be explained.
Plants suppress algae growth in a number of ways. They compete with algae by shading them out, by releasing allopathic chemicals into the water column that inhibit or kill algal species and also by growing quickly and not leaking organic molecules which algae can feed on.
Brij Gopal, Usha Goel. Competition and allelopathy in aquatic plant communities. The Botanical Review Journal: July–September 1993, Volume 59, Issue 3, pp 155-210
This is likely the reason why EI and soil tanks work so well to suppress algae. The EI method prevents plants from ever running out of any nutrient. This encourages fast growth which allows plants to shade out algae, to have plenty of excess resources to make the costly allopathic chemicals which stunt algae and also allows plants to maintain healthy intact cells which do not leak out their organic contents into the water column. These processes all combine and inhibit most algae species in our tanks from growing and taking over. Algae has to struggle to make a living and is generally confined to tiny patches near the substrate which receive enough light and which do not receive a lot of water flow (and the allopathic chemicals that are constantly being supplied in the current).
Soil tanks work in a slightly different way. After soil tanks have finished their break-in period where nutrients leach out of the soil and accumulate in the water column they become mature tanks which are extremely stable over the long term and generally have very few algae problems for many years (until the soil nutrients become depleted). Soil tanks feed the plants (via the roots) and ensure they never become deficient which in turn allows plants to shade out algae, produce expensive allopathic chemicals and maintain healthy cell walls that don't leak. However, soil tanks also tend to maintain a water column that is constantly devoid of nutrients. This nutrient deficient water column adds a fourth safeguard against algae by forcing algae to try make a living in nutrient deficient conditions on top of the other three inhibitions. Since algae does not have the ability to grow root systems it cannot move nutrients out of the soil layer through the substrate cap and into the water column area where the light that drives photosynthesis is located. These four factors combine to create an incredibly stable long term tank system that has very few algal issues. This method also has the added benefit that fertilizer delivery is automatic and does not depend on the aquarist correctly measuring out enough chemicals or remembering to fertilize their tank. This is also likely the same reason that Amano's ADA Aquasoil works so well, producing beautiful planted tanks with no algae for many years.
This idea explains why algae seems to pop up when tanks run out of nutrients. For example, when the CO2 runs out in a moderately planted tank and you see algae develop. This is not because high CO2 levels directly harm algae (algae uses CO2 as a nutrient just like plants do), but rather because the plants cannot spend as many resources on producing allopathic chemicals which normally inhibit the algae. On the other hand, if you have an extremely densely planted tank and you lose CO2 you might not see any algae - because there are enough plants to maintain a fairly high level of allopathic chemicals even though each plant is making less than it did before when CO2 was plentiful.
When a mineral nutrient runs out in a tank, like nitrogen (KNO3) for example, plants and algae begin to develop deficiencies at a fairly similar rate. Plant's develop the normal symptoms of nitrogen deficiency (decaying older leaves and smaller new growth which eventually stops growing). However, as the plant's old leaves die they release all of their proteins (high in nitrogen) and other contents into the water column where algae can directly consume it and use it for growth. This relieves the nutrient deficiency condition for the algae only. Unfortunately plants are specialists and cannot consume organic matter directly. They require nutrients to be present in mineral form (KNO3, etc) and so they remain nutrient deficient and continue to release more and more organics into the water where algae uses it. Interestingly this also seems to correspond to the location where certain species of algae will grow. Most notably BBA. BBA seems to grow on older parts of plants, at the nodes between stems where cellular connections are weaker, on filter return pipes, and on slow growing species that maintain their leaves for long periods of time. These locations are all locations where the plant cells are old, weakened and most likely leaking organics into the water column. The filter return line is loaded with organic material from the filter where bacteria have partially digested fish food and dead plant matter. In addition to the organic nutrient source plants can no longer spend resources on producing allopathic chemicals or continue to grow and shade out algae. The collapse of normal algae inhibition enables algae species that used to be suppressed by multiple processes to suddenly take off, flourish and ruin our tanks.
This idea might also explain why algae blooms are notoriously difficult to predict. After all, it is difficult to monitor our nutrient levels accurately and continually. So one line of reason suggests that the cause of "random" algae outbreaks might in fact be caused by temporary nutrient deficiencies. If plants in our tank run out of nutrients in the water column temporarily, they switch over to using up their reserve nutrient stores and start cutting corners when growing, reducing their growth rate, reducing allopathic chemical production (a luxury at this point) and preparing for starvation. This reduction of allopathic chemicals might temporarily allow opportunistic and fast growing algae species to bloom. This may also explain why we generally tend to see only a handful of algae species in our tanks compared to the thousands of species of freshwater algae that exist. The algae species we commonly get are the best species at taking advantage of sudden lapses of inhibition, blooming quickly and then going dormant until favorable conditions return.
Ultimately the take home message here is that if we have enough plants in our tanks, keep our plants well supplied with nutrients and never let them run out then algal issues will be minimized or completely eliminated.
Here is an image of what I just said above for all you visual people: