Aquarium plants play a vital role in maintaining the health and stability of aquatic environments. The uptake of nitrogenous compounds by aquarium plants is the reason why heavily planted tanks rarely have problems with fish health. Nitrogenous compounds, particularly ammonia and nitrite, are extremely toxic to fish, even at low levels. In order to combat this problem, hobbyists rely heavily on the bacterial process of nitrification or ‘biological filtration’ to convert these toxic compounds into non-toxic nitrates. However, the role of plants in preventing the accumulation of these toxic compounds is often overlooked by hobbyists. Hobbyist’s often incorrectly assume that aquarium plants mainly take up nitrates, however, that is not the case.
Aquatic Plants Behave Differently to Terrestrial Plants
Many terrestrial plants grow better in the presence of nitrates than in the presence of ammonium. Thus, it was assumed that aquatic plants would behave in much the same way. However, experimental studies have suggested otherwise.
Experimental studies have shown that a majority of aquarium plants prefer ammonium over nitrates, as their preferred source of nitrogen. In one study investigating the preference of ammonium over nitrate, duckweed (Lemna gibba), removed 50% of the ammonium in a nutrient solution within 5 hours, even though the solution contained over a hundred times more nitrates than ammonium. Elodea nuttallii, placed in a mixture of ammonium and nitrates, removed 75% of the ammonium within 16 hours while leaving the nitrates virtually untouched. Only when the ammonium was gone, did the plant begin to take up nitrates.
The ammonium removal witnessed during these studies cannot be attributed to nitrification, as the plants were grown under sterile conditions. The uptake of ammonium was also shown to be in relation to plant growth, the plants grew rapidly during the study confirming that the uptake of ammonium was accompanied by an increase in plant biomass, demonstrating that the uptake of ammonium was a result of metabolic requirements.
Plant Uptake of Nitrite
Although ammonium has been shown to be the preferred source of nitrogen by aquatic plants when presented with both ammonium and nitrate, the plants preference for nitrite remains in question. Although plants can use nitrite as a nitrogen source, the question remains; do aquatic plants remove the toxic nitrite before the non-toxic nitrate? Scientific literature on this subject is lacking and there is no clear evidence to support any current scientific theory. However, the chemical reduction of nitrites to ammonium requires less energy than the chemical reduction of nitrates to ammonium. (A plant must convert both nitrites and nitrates to ammonium before it can use them to make its proteins.) Thus, it is my personal opinion that aquatic plants do in fact take up nitrites, whether or not this is in preference to nitrate, is debatable.
Aquatic Plants versus Biological Filtration
All photosynthesizing organisms (plants, algae, bacteria etc.) use the nitrogen from ammonium to produce their proteins. If the plant takes up nitrate, it must first be converted to ammonium in an energy-requiring process called ‘nitrate reduction’. Nitrate reduction in plants is the exact opposite of the bacterial process of nitrification. Nitrifying bacteria gain the energy they need by oxidizing ammonium to nitrates. Thus, if nitrifying bacteria in biological filters convert all available ammonium to nitrates, plants will be forced, at a significant energy cost, to convert all the nitrates back to ammonium. This may explain why aquatic plants seem to grow better with ammonium or an ammonium/nitrate mixture than when they are forced to grow with pure nitrates. The nitrogen cycle is often presented incorrectly to hobbyists as nitrifying bacteria converting ammonium to nitrates and then plants taking up nitrates. However, plants will only take up nitrates in the absence of ammonium. Thus, nitrates may accumulate even in planted ponds and aquariums. It can then be concluded that plants and bacteria compete for ammonium in the aquarium.
Nitrification is essential for protecting fish from toxic ammonia in aquariums without plants. However, in planted aquaria the plants perform the task of removing toxic ammonia. Thus, it is possible to remove filters from heavily planted tanks because biological filtration is no longer needed. Moreover, the nitrifying bacteria could actually limit plant growth in heavily planted tanks, as the bacteria compete with plants for ammonium.
Filtration in Heavily Planted Aquariums
The filters on heavily planted aquariums can often be removed once the aquarium has been established for a few months. This is due to the fact that the plants are able to remove all the toxic compounds produced by fish waste. Once a heavily planted aquarium has become established the role of the filter is reduced to water circulation (there has to be a degree of circulation in planted aquariums to ensure that nutrients in the water column are evenly distributed). The well-known hobbyist and author, Diana Walstad, is a firm believer in the efficiency of plants in maintaining water quality and has herself removed filters from heavily planted tanks without any ill effect. (For more on Diana Walstad: Ecology of the Planted Aquarium and Mycobacteriosis – the Stealth Disease)
In summary, there is considerable experimental evidence in the scientific literature demonstrating that aquatic plants vastly prefer ammonium over nitrates as their nitrogen source. Even in the presence of abundant nitrates, aquatic plants will take up ammonium before taking up nitrates. I hope this explains why (in terms of fish health) it is worth the trouble keeping plants in aquariums.