Iron Cycle

For a high-level tour of how every element moves through the tank, start with Nutrient Cycling. Iron is the trace nutrient planted-tank keepers worry about most — the one behind pale new leaves, the rust-coloured tints in soft water, and a surprising amount of what happens to phosphorus as a tank ages.

Iron is the element that flips with the oxygen

Almost everything iron does in an aquarium is dictated by whether it is sitting in oxygenated water or in oxygen-free sediment. In the well-lit, oxygen-rich water column, dissolved iron oxidizes within hours into insoluble rust-like particles (ferrihydrite) that settle out — iron is effectively banished from open water. Down in the anoxic sediment, microbes run the reaction in reverse, using organic carbon to reduce those rust particles back into soluble iron and recharging a pore-water reservoir far richer than the water above. Every meaningful iron flux in the model is a consequence of this split: oxidized and locked-away above, reduced and mobile below.

This is not only an iron story. Phosphate sticks tightly to the surface of fresh rust particles, so every time iron oxidizes near the sediment surface it drags phosphate down with it, and every time iron is reduced in anoxic sediment it releases that phosphate back. This coupling — internal P-loading — is one of the most important controls on long-term phosphorus availability in real lakes, ponds, and aquariums, and it is the reason an aging planted tank can suddenly find itself with an algae problem it didn't have when it was young.

On top of all that, iron is structurally essential to most of the enzymes that drive the nitrogen cycle and to photosynthesis itself: nitrogenase for nitrogen fixation, ammonia monooxygenase for nitrification, the nitrate-reducing enzymes of denitrification, and the chlorophyll, cytochromes, and ferredoxins every photosynthesizer needs. No other trace nutrient in the model reaches into so many processes at once.

Free iron versus chelated iron

Not all dissolved iron behaves the same way, and the difference is something aquarists exploit every time they dose a fertilizer. Free iron — the bare aquo ion — oxidizes fast, with a half-life of minutes to a couple of hours in oxygenated water (Stumm & Morgan 1996). Iron bound to organic molecules — the humic and fulvic acids of natural water, or the synthetic chelators in plant fertilizers — oxidizes far more slowly, surviving for days (Emmenegger et al. 2001; Rose & Waite 2003). In any tank with meaningful dissolved organic matter, most of the dissolved iron is in this protected, chelated form.

The model keeps the two separate and lets them interconvert: dissolved organic matter captures free iron into the protected pool, and the protected pool slowly releases it back. In a tea-stained, planted tank rich in organics, fresh iron is grabbed and protected before it can rust away, which is why such tanks hold onto bioavailable iron so well. In a sterile RO tank with no organics, iron stays free and oxidizes quickly — the correct, and frustrating, low-organic behaviour. Crucially, plants and algae can use either form: their root and surface reductases pry iron out of the chelate, so growth responds to the total dissolved iron, not just the free fraction.

This is also why chelated iron supplements exist. A dose of plain iron sulfate is mostly oxidized and gone within hours. A dose of EDTA-, DTPA-, or gluconate-chelated iron lingers for days, staying available to plants — and the model reproduces this, holding dosed chelated iron in a protective form that releases gradually at a rate set by the specific chelator and the water's pH. The exact release timescales for each form are tabulated in the Parameter Reference.

How much iron your water and substrate carry

Iron in the water column starts from your source water, and the range across common water types spans nearly four orders of magnitude:

Water source	Iron level	Why
RO / remineralized	very low	RO membranes strip iron almost completely
Hard limestone tap	low	iron precipitates in alkaline, oxygenated karst water
Typical municipal tap	low–moderate	treatment removes most iron
Volcanic aquifer	moderate	volcanic groundwater carries dissolved iron
Soft, peaty water	elevated	humic acids hold iron in protected complexes
Reducing well water	elevated	anoxic groundwater is iron-rich
Surface ocean	almost none	the classic iron-limited marine regime

The larger iron reservoir in a planted tank, though, is the substrate. Organic and mineral soils are seeded with a stock of rust particles that the sediment microbes can later reduce and recycle. The richness varies enormously by soil: iron-rich volcanic clays (the basis of many commercial aquasoils) carry a heavy complement, ordinary aged garden soil somewhat less, and acidic peat very little. A soil's iron stock is one of the things that determines how long it can keep feeding plants — and, as the next sections show, how long it can buffer a sulfide problem. Exact starting values for each water and soil preset live in the Parameter Reference.

The redox cycle in the sediment

In oxygenated water, dissolved iron oxidizes and settles as rust, consuming a little oxygen and releasing acid as it goes. The free form does this much faster than the chelated form, and faster still as the water gets more alkaline and warmer — so a hard, warm tank scrubs iron out of its water column far more aggressively than a soft, cool one.

In the anoxic sediment, microbes reverse the reaction, using the buried rust as a substitute for oxygen while they burn organic carbon — releasing soluble iron, consuming acid (which nudges alkalinity back up), and mineralizing the nitrogen and phosphorus in that organic matter into the pore water alongside it. This reduction only runs where the sediment is genuinely oxygen-free; the model switches it on sharply as oxygen disappears and keeps it firmly off in oxygenated conditions, matching the real behaviour in which sediment iron turns over only below the oxic skin.

Up in the lit water column, any rust that hasn't settled gets a second chance at life: sunlight, working through dissolved organics, photochemically reduces suspended rust particles back to soluble iron during the day (Barbeau 2006; Voelker & Sulzberger 1996). In a bare-bottom planted tank with no soil reservoir, this daytime photoreduction is the main thing keeping a trickle of usable iron in circulation.

Why dissolved iron stays near zero in a healthy tank

There is a subtlety that explains a common observation — that even a tank with active sediment iron cycling shows almost no iron in a water test. In real sediments, the soluble iron produced by reduction only diffuses upward a millimetre or two before it meets oxygen at the sediment surface and is immediately re-oxidized, never reaching the open water. The model captures this "cryptic" cycle: most of the iron produced by reduction is trapped and re-deposited right back onto the sediment grains. The result is that iron turns over busily inside the sediment while the water column stays clean — exactly what is measured in healthy oxic tanks. Only when the water column itself goes hypoxic does the trap relax and let reduced iron escape into the open water, mirroring the iron release seen in seasonally stagnant lakes.

Internal P-loading: where iron meets phosphorus

The most consequential thing the iron cycle does to a tank is shuttle phosphorus around. Freshly forming rust scavenges dissolved phosphate onto its surface as it precipitates (Gunnars & Blomqvist 1997), and that phosphate stays locked away as long as the iron stays oxidized. But when the sediment goes anoxic and the iron is reduced, the phosphate is released back into the pore water — even though no new phosphorus has entered the tank.

This is the engine behind the phosphate pulses that aging planted tanks are famous for. A young tank with a fresh, oxygenated substrate keeps phosphorus locked onto iron; as the substrate ages, accumulates organic matter, and develops anoxic zones, the iron starts releasing its phosphorus load, and a tank that was phosphate-limited can tip into a phosphate-replete, algae-friendly state. Rooted plants drive a parallel version of the same chemistry right at their root surfaces — the rice-paddy pattern — releasing iron-bound phosphorus into the pore water where their roots can immediately use it. The Phosphorus Cycle follows this from the phosphorus side.

Iron limitation of growth

Most organisms in the model need iron, and growth slows when dissolved iron runs short. Because the various iron-dependent enzymes differ in how tightly they bind iron, scarcity bites them in a characteristic order. Nitrogen fixation fails first, because nitrogen-fixing cyanobacteria carry roughly ten times the iron demand of ordinary phytoplankton (Kustka et al. 2003); nitrification and denitrification follow; and general producer growth, which can lean on whatever iron remains, holds out longest. Diatoms are the frugal exception — they are adapted to thrive on the sub-nanomolar iron of the open ocean (Sunda & Huntsman 1995) and draw it down only slowly. The specific half-saturation values for each enzyme and species are listed in the Parameter Reference.

Iron limitation combines with nitrogen, phosphorus, and (for diatoms) silica through Liebig's law of the minimum: growth is set by whichever single resource is scarcest, and stops entirely when iron runs out.

How rooted plants get their iron

Rooted plants like Cryptocoryne and Vallisneria mostly bypass the iron-poor water column and mine the sediment instead. Their roots release organic compounds and create oxygen gradients that activate iron-reducing bacteria right at the root surface, liberating soluble iron straight into the pore water where the roots can take it up. As long as the substrate still holds a reserve of rust, these plants are effectively iron-replete even in a tank whose water column would starve a floating plant. The same plants run the reverse reaction too: where their roots leak oxygen, some of that pore-water iron re-precipitates as the visible rust-coloured iron plaque on the root surface, scavenging phosphate as it forms. The net result is a continuous iron-and-phosphorus shuttle around the root zone, with no equivalent in a bare-bottom tank. This is the rice-paddy strategy: rooted plants keep access to both iron and phosphorus in a sediment where the open water offers neither.

Iron and the sulfide buffer

Iron has one more decisive role: it is the substrate's defence against toxic sulfide. When sulfide (produced by sulfate-reducing bacteria in deep anoxic sediment) meets dissolved iron in the pore water, the two precipitate together almost instantly as black iron sulfide. So as long as the substrate still has iron left to reduce, sulfide is captured the moment it forms and never reaches the water column. The iron stock in the soil acts as a sulfide buffer that can absorb months — sometimes years — of sulfate reduction before it is used up.

When that buffer finally saturates, the substrate's character changes. With no iron left to bind it, sulfide begins to diffuse upward faster than the oxygenated water can neutralize it, and free hydrogen sulfide — acutely toxic at tiny concentrations — starts to accumulate and escape into the air. This is the model's representation of the dreaded "old tank syndrome": the rotten-egg smell and livestock losses of a substrate that has exhausted its iron buffer. Disturbing or oxygenating the substrate (a deep gravel vacuum, or oxygen leaking from plant roots) can slowly recharge the buffer by re-oxidizing the iron sulfide. The full sulfide story — its toxicity, its pH dependence, and its venting to the headspace — is told in the Sulfur Cycle; iron sulfide is where the two cycles meet.

Putting it together: the iron map

All of the above is really one connected loop. Iron is seeded into the water from your source water and into the substrate as buried rust; from there it shuttles between dissolved and solid forms depending on whether it finds itself in oxygen-rich water or oxygen-poor sediment, and it is borrowed by living things along the way:

      WATER COLUMN  (oxygen-rich)                 SEDIMENT  (oxygen-poor)
      ──────────────────────────                  ──────────────────────

  source water                                    organic soil
       │                                               │
       ▼            capture (fast)                      ▼
   Free Fe²⁺ ─────────────────────►  Chelated Fe    buried rust (Fe³⁺)
       │     ◄─────────────────────  (DOM-bound)       │         ▲
       │            release (slow)        │            │         │
  fast │ oxidation                   slow │ oxidation  │ anoxic  │ cryptic
       ▼                                  ▼            │ microbial   re-oxidation
       └──────────►  suspended rust (Fe³⁺)  ◄──────────┤ reduction   traps most
                          │      ▲                      │         │   Fe²⁺ back as
            settling      │      │  sunlight frees      ▼         │   a grain coating
             down ────────┤      │  it (daytime)    dissolved Fe²⁺┘
                          ▼      │                      │
                       buried rust                      │  escapes to open water
                                                        │  only when the water
                                                        ▼  itself goes anoxic
                                                  (re-enters Free Fe²⁺)

  LIVING SYSTEM
       Free + Chelated Fe ──uptake──► living tissue ──death/grazing──► detritus & DOM
              ▲                                                              │
              └──────────────────── decomposition releases Fe ◄──────────────┘

  PHOSPHORUS rides along:   rust forms     → scavenges phosphate  (locked away)
                            rust dissolves → releases phosphate   (internal P-loading)

The two halves of the picture barely talk to each other in a healthy tank — the cryptic cycle keeps sediment iron turning over without leaking it upward — which is why the water stays clear of iron while the substrate quietly recharges its reserves. It is when that separation breaks down (the water column goes hypoxic, or the sediment's sulfide buffer saturates) that iron, and the phosphorus riding on it, spill into the open water.

How iron interacts with the other cycles

Phosphorus — the headline coupling. Iron scavenges phosphate when it oxidizes and releases it when it reduces, driving the internal P-loading pulses of an aging tank, and rooted-plant root chemistry feeds phosphorus directly into the root zone. See Phosphorus Cycle.
Nitrogen — every major nitrogen-cycle enzyme carries an iron cofactor, so iron scarcity throttles fixation, then nitrification and denitrification, in that order. See Nitrogen Cycle.
Carbon — sediment iron reduction is one of the anaerobic ways organic carbon gets oxidized, competing with denitrification and sulfate reduction for the same buried detritus.
Oxygen — oxidizing iron consumes oxygen, and the cryptic cycle at the sediment surface keeps drawing a little oxygen continuously even when no iron reaches the open water. In a strongly reducing substrate under moderately oxygenated water, this interface demand is a real oxygen sink.

Conservation

Iron is strictly conserved in the model — there is no gaseous escape the way nitrogen has. Every atom is accounted for across the dissolved forms, the suspended and buried rust pools, the iron sulfide buffer, dosing reserves, and the iron held in living tissue and detritus. The only things that legitimately change the total are water changes (which dilute the water-column forms while leaving the buried sediment alone) and a small, expected bookkeeping artifact when diatoms — which carry unusually little iron — die. The mass-balance diagnostics account for both.

Key references

Emmenegger, L., King, D.W., Sigg, L. & Sulzberger, B. (2001). Oxidation kinetics of Fe(II) in a eutrophic Swiss lake. Environ. Sci. Technol. 35, 3214–3219.
Gunnars, A. & Blomqvist, S. (1997). Phosphate exchange across the sediment–water interface when shifting from anoxic to oxic conditions. Biogeochemistry 37, 203–226.
Kustka, A. et al. (2003). A revised estimate of the iron use efficiency of nitrogen fixation. Limnol. Oceanogr. 48, 1869–1884.
Lovley, D.R. & Phillips, E.J.P. (1988). Organic carbon oxidation coupled to dissimilatory reduction of iron. Appl. Environ. Microbiol. 54, 1472–1480.
Stumm, W. & Morgan, J.J. (1996). Aquatic Chemistry, 3rd ed. Wiley-Interscience.
Sunda, W.G. & Huntsman, S.A. (1995). Iron uptake and growth limitation in oceanic and coastal phytoplankton. Mar. Chem. 50, 189–206.
Voelker, B.M. & Sulzberger, B. (1996). Effects of fulvic acid on Fe(II) oxidation by hydrogen peroxide. Environ. Sci. Technol. 30, 1106–1114.