Phosphate cycle
Phosphate cycle. Other cycles of nutrients are less vividly represented in the aquarium than the familiar nitrogen cycle. Phosphorus, for example, is an essential nutrient whose "cycling" in the aquarium isn't often described.
Though phosphorus is a vital macronutrient that all living cells require, elemental phosphorus is so strongly reactive— even explosive— that it doesn't normally appear in its unoxidized form in nature, but always as phosphate, in which the P atom is surrounded and tamed by four oxygen atoms (PO4); the phosphate unit is passed along unbroken in the various transformations phosphorus undergoes, whether inorganic, that is, mineralized, or organic, that is, bound up in a giant organic molecule.
Organic phosphates are essential: they form part of DNA and RNA; phosphate is used in photosynthesis; in blood, a phosphate buffer keeps pH stable; and phospholipids are the major components of cell membranes. Phosphate also participates in every living cell as the P in ATP (adenosine triphosphate), the basic "energy currency" molecule that all life uses to transfer energy. In short, organically-bound phosphates are everywhere. Manmade organophosphates are particularly lethal because they are so quickly taken up by living organisms.
Phosphate phases. Phosphates cycle naturally both in these organic forms, where PO4 groups form parts of organic molecules, and in inorganic, mineralized forms, called orthophosphates. These can string together in chains called polyphosphates, which tend to break apart. And phosphate groups of either kind may either be dissolved in the water or they may exist as particulates, whether precipitated out or adsorbed to a substrate. So there are many phases of PO4 in the aquarium.
Only phosphates that are already built into organic molecules can be used by animals, from blackworms and daphnia to fish or yourself. On the other hand, only mineralized orthophosphates in their soluble phase are available to algae and plants: the biologists who study freshwater systems term them "soluble reactive phosphates" (SRP), because the insoluble orthophosphates are unavailable to organisms in that phase, thus they're "unreactive."
Testing for phosphate. Your phosphate test kit is measuring only this soluble inorganic phosphate (SRP). To measure the organic phosphates you'd have to boil your sample in acid, topping up repeatedly with distilled water, to break down the organic PO4 to SRP. Not a project for the aquarist's home kitchen. And bacterial action is constantly tapping the aquarium's reservoirs of organic phosphate, mineralizing it to SRP, while algae and plants rebind it to organic molecules. So your test result might be different tomorrow. This is why I rarely test for PO4 and I'm skeptical of the results I get, when I do.
Some phosphate in food is metabolized and deposited with calcium in fish bones, teeth and scales, where it is mineralized as calcium phosphate, or apatite. You could think of these organic forms of PO4 as insoluble too— locked up and insoluble, like the phosphates more temporarily locked up in living plankton. Acids would slowly dissolve it, but animals aren't able to take up inorganic phosphates, even if they are dissolved. Instead, soluble reactive phosphate (SRP) is scavenged by bacteria and algae and also quickly taken up and used— or stored— by plants. The "higher"vascular plants have this ability to store PO4, so in an aquatic system where plants are in control, living plants form a major reservoir of the system's phosphate.
Once phosphates are stored in plant tissue they are no longer available to bacteria and algae. Instead, grazers or detritivores assimilate the organic phosphates, which get passed up the food web. What the zooplankton or the fish can't use, they excrete in their feces, where gut bacteria have already begun reworking it. So, when plants or animals die, bacterial decomposition remineralizes their phosphates.
Waters rich and poor in phosphates. All this cycling moves fast, so fast that unpolluted natural waters are normally extremely low in phosphates, which may measure in parts per billion. But algae can get by on a PO4 diet of 1 to 10 ppb: they're not easily phosphate-starved. In deep lakes, however, the productivity of freshwater algae is limited by the availability of scarce phosphate. British experiments in Lake Windemere confirmed this. When phosphates were stripped from the wastewater and run-off entering the lake after 1992, the concentration of blanketweed, a cladophora alga, was reduced. What happens in deep lakes that don't experience upwelling of enriched bottom waters is that both phytoplankton and the planktonic grazers that have scavenged the available phosphate, eventually sink to the bottom far below, taking the phosphate out of circulation.
In aquaria, by contrast, phosphates are generally over-plentiful, and they are credited with algal blooms. Though perhaps phosphate and algae are too rashly linked, many of us are still interested in lowering phosphate levels. There are four main approaches. Two phosphate sinks are natural: they can be assimilated by plants (and algae) or sequestered by co-precipitation in undisturbed sediment (see "Phosphate burial" below). Two other artificial techniques are part of your chemical filtration arsenal: phosphates can be precipitated with alum or ferric chloride (FeCl3) or adsorbed by ion-exchange granules.
Sources of phosphate. Where does the aquarium's phosphate come from? Unlike nitrate, which is constantly generated in the aquarium ecosystem from ammonia, phosphate can't be generated through biological processes. Also, there's no store of PO4 in the atmosphere to draw on, no gas phase for phosphorus, as there is for nitrogen.
Animal feces contain significant amounts of phosphate. Think of seabird or bat guano. The accumulated droppings of fish-eating seabirds are so rich in the unmetabolized phosphates that have passed through their systems, that guano forms the main mineral wealth of some oceanic islands. Similarly, the major source of phosphate in the aquarium comes from the fish meal content of flake feed, where the ground-up fish of "fish meal" includes the calcium phosphate (apatite, CaPO4) of bones and scales. Though it provides healthy roughage, calcium phosphate is indigestible in this form and passes out in fish feces: "fish guano."
Recently some "lowered-phosphate" flake feeds have come onto the market to answer fishkeepers' concerns about phosphate levels. In the aquarium, organic phosphates remain in plant detritus as well as fish feces. Bacterial degradation will swiftly reduce them to algae-available orthophosphate. Though the vascular plants tend to take up PO4 before algae can use it, this is a major incentive for you to siphon out the detritus that collects on the surface of the substrate, to prune out softening leaves and to rinse your filter!
Unnecessary phosphate can enter the aquarium's closed system through other paths. Phosphate may enter the system in many water conditioners, and some common pH buffers still contain phosphate. It may be released from some forms of granular activated carbon; several brands of carbon were tested by Steven Pro and reported in his article "Activated carbon and phosphates". Some houseplant fertilizers that have been re-labelled (but not reformulated) for aquarium use will contain phosphate— it's the "P" of familiar N-K-P, after all. There's phosphate, too, in the rock-wool that wraps the roots of your new hydroponically-grown plants, if you haven't snipped open their plastic baskets and picked it all away. It's easy enough to eliminate these sources of phosphate in your tanks.
Tapwater polyphosphates. It's not so easy to eliminate the phosphates that enter your system in your tapwater. Long-chain polyphosphates are often added to the public water supply, both for protecting water mains from corrosion and carbonate scale and also for some softening effect, when the PO4 renders calcium or magnesium ions inactive by sequestering them. These forms of PO4 are orthophosphates, which are available for plant and algal uptake. Protecting these useful added phosphates is the reason it's illegal to dump ferrric chloride FeCl (see below) in your drain.
Phosphate and water cloudiness. Phosphate carries strong triple negative charges, represented as PO4-3— and so do colloidal clay and humus particles. But the clay and humus attract positively-charged cations, which cluster round, and the negatively-charged phosphates bind to those. Heavy PO4 loading in the water can contribute to persistent initial cloudiness caused by colloidal clay, while the not-yet-neutralized negative charges repel one another and keep the charged colloidal silt from settling out. In aquaria this can be an issue in a newly established aquarium where laterite is part of the substrate mix and phosphate-based buffers are forcing down the pH. In a similar fashion, in wastewater treatment plants high levels of phosphate can interfere with coagulation in settling tanks. The phosphates adhering to the colloids can give the colloids net negative charges that encourage them to repel each other and stay dispersed in the water.
Phosphate burial. But natural reactions are constantly pulling phosphate from the water. For example, iron is quickly oxidized in oxygen-rich waters, and the resulting positively-charged iron hydroxides scavenge phosphate from the water, bind it, and carry it as a component of ferric precipitates, which form on the substrate. In a moment I'll note how this reaction can be harnessed to reduce phosphate in the water. In highly buffered alkaline waters, however, phosphates tend to bind directly to calcium instead. So, with one kind of partner or another, phosphates are continually being extracted from the water and buried in the substrate— as long as the substrate contains some colloidal clay and flocs of humus, or some suitable iron or calcium compounds. Below the surace layers of the substrate, as the oxygen gradient falls away, the ferric phosphate bons are loosed. This releases soluble reactive phosphate (SRP) in the substrate, where plant roots can get at it, but where it's not available to algae. "Indeed," says Diana Walstad, who's explained these things in The Ecology of the Planted Aquarium, "if a soil sample is shaken with a concentrated phosphate solution, it will remove the phosphate." In a natural environment, where phosphates are bound in sediments, many plants prefer to take them up by their ordinary root respiration. In a tank with no rooted plants, you'll quickly see how phosphate could accumulate in the sediment, safely taken out of circulation, until you stir it up with a gravel vacuum, releasing it into the water column.
Part of what makes a substrate "age" is the accumulation of phosphate; sometimes long-established aquaria slowly develop intractable algae problems, and the phosphates in the substrate are part of the problem. Thoroughly washed conventional "aquarium" gravel has few appropriate sites to hold phosphates, for clay and humus are not elements found in ordinary "aquarium" gravel. Yet any orthophosphates that remain in the water column encourage the growth of single-cell algae. If you sense, as I do, that there's a connection between plain gravel substrates and problems with algae and green water, lack of a natural phosphate sink might be the invisible link. Some desirable colloidal silt, such as unbaked laterites offer, mixed into the original substrate, may be contributing to low phosphate levels in the aquarium water.
Precipitating phosphate with iron. So, following these trends of thought, to reduce PO4 levels in the water, some advanced aquarists have been adding iron, in the form of ferric chloride (FeCl3), an ingredient, it would appear, in Tetra's EasyBalance water conditioner. The iron reacts with phosphate to form insoluble iron phosphate, which precipitates out. These avant-garde types were inspired by Peter Peterson's translation, in Aquatic Gardener, vol. 7, no 1, of a German article from AquaPlanta. The technique is to slowly reduce PO4 by adding a 0.3% solution of FeCl3 over the course of several days. Paul Sears and Neil Frank discussed this in the Aquatic-Plants Digest, 4 Apr 1996 etc. If you do decide to use FeCl3, make sure it all goes eventually into the aquarium, for it's against federal law to dispose of it down the drain.
Luca, posting in the Aquatic-Plants Digest, 3 Feb 1999, gave the operative chemical reactions (it's my translation):
FeCl3 + 3.H20 -> Fe(OH)3 + 3.HCl
"Ferric chloride reacts with 3 molecules of water to give ferric hydroxide and 3 molecules of hydrochloric acid."
FeCl3 + Na3PO4 -> FePO4 + 3.NaCl
"And ferric chloride also reacts with sodium phosphate to give iron phosphate and 3 molecules of common salt."
2.HCl + Ca(HCO3)2 -> CaCl2 + H2O +2.CO2
"Then two molecules of the hydrochloric acid react with two of calcium carbonate [from the buffer] to give one molecule of calcium chloride, a molecule of water and two of carbon dioxide."
Precipitating phosphates with alum. Alum (aluminum sulfate, AlSO4) is a flocculant used as a water clarifier in many public utilities. It does not affect the alkalinity. Dissolved in water, it forms a floc (aluminum hydroxide, the main ingredient of Maalox) that may temporarily cloud the water while it reacts strongly with dissolved phosphate, precipitating it out as insoluble aluminum phosphate. Aquarists and pondowners who are bent on controlling algae by reducing PO4 may want to use AlSO4 to flocculate PO4 and collect phytoplankton as it settles out. Sweetwater Technology Corp. of Aitkin MN outlines the water chemistry involving alum in an article "What is alum and how does it control algae?" in ponds.
Phosphate-reducing granules. Resin granules that adsorb phosphate can be included in your filtration. Tim Hovanec downplays the effectivenesss of chemical PO4-reducing granules or pads as too slow-acting to be effective and also apt to be quickly colonized by the bacteria that normally re-solubilize the adsorbed phosphate. Bacteria on the surfaces of this filter substrate could re-release orthophosphate into the water. One answer is to remove the phosphate adsorbing granules or pads after 24 hours and let them dry thoroughly. In my experience, healthy growing plants and good sanitation practices go farther than chemical filtration media to keep phosphate levels low.
Wastewater management engineers find that the bacteria in activated sewage sludge also have a high tendency to absorb phosphates from wastewater. So when you rinse the filter media, you remove organic-bound phosphates before bacterial action can mineralize it to orthophosphate.
Re-releasing adsorbed phosphate. In the aquarium, phosphate adsorbed to colloidal silt and floc becomes soluble again in anoxic underlayers of your substrate, where it is made available to plant roots. If you vigorously stir up your substrate at intervals, this soluble PO4 is released into the water column temporarily. Before it precipitates out once more, it is available to algae. This is a cause of frustration for aquarists whose gravel-cleaning and general sanitation practices are meticulous and yet are plagued by cloudiness and green water.
A shortcut in phosphate cycling. You've probably detected from what I've been saying that in these transformations of phosphate in the aquarium there's no complete "cycle". Phosphate simply undergoes transmutations between mineralized and organic forms, and as solutes or precipitates. Phosphates are only recycled on the planetary scale, in vast slow biogeochemical processes that take tens of millions of years. In the shorter run, all the earth's phosphates are on a slow ecological conveyor belt that finally deposits them on the oceans' abyssal plains,. There tectonic seafloor spreading will make them available again to a future age. In a closed ecosystem, such as an aquarium, phosphate must be continually reintroduced, and then it is transmuted back and forth between its organophosphate and orthophosphate forms, until it's eventually lost through water changes, filter rinsing or plant prunings — or the removal of a dead fish.
The enzymes called phosphatase. Within aquatic ecosystems, continual regeneration of orthophosphate from organic phosphates is performed by a range of enzymes collectively called phosphatase. These enzymes are the keys to access otherwise unavailable resources of organic phosphate. The process is rapid: turnover rates for scavenged phosphate are measured in matters of minutes and hours. Though many aquatic organisms, ranging from bacteria and cyanobacteria to algae and even higher plants, can synthesize these enzymes, the two major sources of phosphatases are algae and aerobic bacteria.
Anoxic conditions inhibit the production and activity of phosphatase: thus the anoxic layers of an unplanted substrate can become a phosphate sump. Whenever local levels of orthophosphate drop low, bacterial communities in the biofilm draw on phosphate bound in the sediment: in response to low availability of free inorganic PO4, synthesis of phosphatase will switch on within 24 hours or less.
Conversely, sufficient supplies of orthophosphate tend to repress the production of phosphatase, as organisms generally take the metabolically cheaper route. Accumulating organic matter allows an increase in biofilm bacterial populations. More bacteria exert more demand for phosphorus. The less orthophosphate is available, the more necessary local recycling becomes, and the more freely phosphatase is produced. So a kind of stabilizing feedback system is established, which is characteristic of aquatic systems as a whole — and perhaps characteristic of the greater aquarium that is Gaia. You knew that vacuuming out the mulm and decaying plants would help control PO4 levels: now you know why. And now you also see that the aquarium will never run out of phosphate.
Cyanobacteria and phosphate? In marine ecosystems, cyanobacteria have a much higher rate of phosphatase production than algae. "This enhanced enzyme gives cyanobacteria an advantage over algae in P-limited conditions in the presence of labile organic matter [in Florida Bay]" one observer noted. Perhaps there is a clue here to the blooms of cyanobacteria we sometimes encounter in freshly set-up freshwater aquaria also. The sheets of cyanobacteria seem to subside as algae and the biofilm become established, and as PO4 begins to accumulate in sediments. In our freshwater aquaria, PO4 is rarely limited, except in newly-set up systems. Could cyanobacteria have a similar phosphate-scavenging advantage in freshwater aquaria?
Phosphate links. Wilkes University has a phosphate website where these questions are explored. Sheila Murphy offers a "General introduction to phosphate" at the Boulder, Colorado, municipal water site. At FishChannel an archived article on phosphate by Randy Holmes-Farley, from Aquarium Frontiers, "Phosphate...what is it and why you should care," escaped me for a long time, because it was "about" marine aquaria and didn't seem to apply to me. I found there's a lot here for the freshwater fishkeeper too. Holmes-Farley discusses inorganic and organic phosphates, sources of phosphate in fish food and tapwater and phosphate sinks. At the high pH of seawater, PO4 precipitates onto calcium and magnesium carbonates, minerals that are in short supply in my soft water, and in your water too perhaps, unless you're maintaining an aquarium for Lake Tanganyika's alkali-loving cichlids.
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