Planktonic Cyanobacteria Community
These are the bloom-formers — the free-floating cyanobacteria that turn a warm, fertile lake pea-soup green in high summer and lay a paint-like scum across the surface. Representative taxa are Microcystis (colonial and toxin-producing), Dolichospermum (formerly Anabaena, heterocystous and filamentous), Aphanizomenon (rafting colonies), and Cylindrospermopsis (an invasive tropical filament). Their phycocyanin pigment gives the blue-green sheen, and between them they make the cyanotoxins — microcystin, anatoxin, cylindrospermopsin — behind most of the world's freshwater harmful algal blooms (Paerl & Huisman 2008). For the aquarist they are mostly a curiosity: a home tank rarely gets warm, alkaline, and nutrient-loaded enough to support a Microcystis bloom, and the BGA hobbyists actually battle is the benthic Phormidium/Oscillatoria mat. In this simulator they are tracked as cyanobacteria — kept separate from the green algae in the food web and the accounting — because of their nitrogen-fixing physiology.
Fast bloomers of warm, bright water
Planktonic cyanobacteria are r-strategists: fast-growing, warm-adapted, with a high light requirement and boom-and-bust dynamics. They flourish in the warm, stratified surface layer of a summer lake, where buoyancy regulation by gas vesicles lets them float to the best light and grab nutrients before sinking algae can reach them (Reynolds 1987). Their high light demand is the surface-scum habit made physiological — cells tuned to saturating midday sun, casting shade over the competitors below.
Nitrogen fixation — the defining trait
The heterocystous forms — Dolichospermum, Aphanizomenon, Cylindrospermopsis — switch on nitrogenase when combined nitrogen runs short and fix atmospheric N₂ straight into ammonium, inside specialised heterocyst cells. A heterocyst keeps a permanently oxygen-free interior — a thick glycolipid envelope, no oxygen-producing photosystem, and elevated respiration together strip oxygen out — which decouples the enzyme from the oxygen in the surrounding water (Fay 1992; Wolk et al. 1994). That is why Anabaena, Aphanizomenon, and Dolichospermum routinely bloom and fix nitrogen in fully air-saturated summer lakes, conditions that would shut down a fixer without heterocysts. This oxygen protection is the single biggest physiological divide between the planktonic bloom-formers and the benthic mats, which have no heterocysts and must hide their nitrogenase inside microaerobic mat interiors instead.
Fixation is held in check by several conditions at once:
| Brake | What it means |
|---|---|
| Light | The enzyme runs on ATP from photophosphorylation, so fixation only operates in the light |
| Dissolved nitrogen | Plenty of ammonium or nitrate represses the nifH gene; fixation switches on only once the water has been drawn down to roughly a tenth of a milligram of nitrogen per litre |
| Oxygen | A brake on most fixers, but relaxed here because the heterocyst interior stays anoxic whatever the bulk water does |
| Phosphorus | Building nitrogenase demands phosphorus, so fixation co-limits on it |
| Iron and molybdenum | The enzyme's iron-molybdenum cofactor makes diazotrophs about ten times more iron-hungry than ordinary phytoplankton, and each active site needs a single molybdenum atom; in iron- or molybdenum-poor water fixation can fail before any other brake bites (see Micronutrient Cycling) |
The net effect is that fixation adds genuinely new nitrogen to the system — the one process in the model that can raise total nitrogen, checkable as a net gain in a sealed, nitrogen-limited mass balance. Because the nitrogen brake only releases after the bloom has used up the starting combined-nitrogen pool, fixation then tops growth up further, producing the familiar collapse in the nitrogen-to-phosphorus ratio that marks a maturing cyanobacterial bloom (Smith 1983).
The cost of fixing nitrogen
Pulling nitrogen out of N₂ gas is expensive — the reaction burns a great deal of ATP, equivalent to roughly a quarter again as much carbon respired for every unit of nitrogen fixed. The model charges this surcharge directly: a fraction of the carbon is burned back to DIC and balanced by oxygen use. Fixation stays a net energetic cost even when nitrogen is scarce, which is why it is a fallback strategy rather than a default one.
Thriving where the water is alkaline
Cyanobacteria carry a potent carboxysome-based carbon concentrating mechanism, giving them a strong grip on bicarbonate at the high pH where free CO₂ has all but vanished. This lets the planktonic bloom-formers dominate alkaline, eutrophic lakes where daytime photosynthesis routinely pushes pH past 9 — conditions that suppress most competing green algae. The planktonic forms tolerate the highest pH of any producer in the model, matching their affinity for extreme-alkaline bloom water.
A floater, not a sticker
Planktonic cyanobacteria put almost nothing into surface adhesion. The great majority of their biomass stays in the water column; the little that settles resuspends readily and spreads thinly between surfaces rather than knitting into a cohesive mat — the opposite of their benthic relatives.
Poor food for grazers
Planktonic cyanobacteria are notoriously bad food for filter-feeding zooplankton. Colonies can be huge — Microcystis aggregates reach millimetre scale — and mucilage sheaths clog the feeding baskets of rotifers and daphnids, on top of the possibility of toxins (Gliwicz 1990). The model reflects this with broadly reduced grazer preferences across the board: benthic scrapers manage them better than the open-water filter feeders, but none find them good eating.
Planktonic versus benthic cyanobacteria at a glance
The two cyanobacterial communities share nitrogen-fixing physiology but live opposite lives:
| Trait | Planktonic (this page) | Benthic |
|---|---|---|
| Growth pace | Fast — bloom-and-crash | Slow, persistent mats |
| Lifestyle | Free-floating scum | Glued to substrate as a mat |
| Light | High-light, surface scum | Deeply shade-adapted |
| Temperature | Warm, summer-bloom | Temperate, cold-hardy |
| Nitrogen fixation | Heterocysts — oxygen-protected, runs in fully aerated water | No heterocysts — microaerobic mat interior, oxygen-sensitive |
| High-pH tolerance | Extreme-alkaline specialist | Broad but moderate |
| What the hobbyist sees | Pea-soup or surface scum (rare in tanks) | Black slimy mat, "BGA" |
The exact growth rates, light and pH thresholds, fixation rates, and regulation half-saturations behind this contrast are tabulated in the Parameter Reference.
Further reading
- Benthic Cyanobacteria — the mat-forming "BGA" that hobbyists actually fight
- Producers — how cyanobacteria, algae, and plants fit together, including iron limitation on nitrogen fixation
- Nitrogen Cycle — fixation as the one pathway that adds nitrogen to a sealed system
- Iron Cycle and Micronutrient Cycling — the trace-metal demands of nitrogen fixation
- Parameter Reference — every rate, half-saturation, and threshold behind this page, with citations
Key references
- Fay, P. (1992). Oxygen relations of nitrogen fixation in cyanobacteria. Microbiological Reviews 56, 340–373.
- Wolk, C.P., Ernst, A. & Elhai, J. (1994). Heterocyst metabolism and development. In The Molecular Biology of Cyanobacteria (Bryant, ed.), Kluwer, pp. 769–823.
- Gliwicz, Z.M. (1990). Why do cladocerans fail to control algal blooms? Journal of Plankton Research 12, 240–254.
- Howarth, R.W. et al. (1988). Nitrogen fixation in freshwater, estuarine, and marine ecosystems. Limnology and Oceanography 33, 669–687.
- Paerl, H.W. & Huisman, J. (2008). Blooms like it hot. Science 320, 57–58.
- Reynolds, C.S. (1987). Cyanobacterial water-blooms. Advances in Botanical Research 13, 67–143.
- Smith, V.H. (1983). Low nitrogen to phosphorus ratios favor dominance by blue-green algae in lake phytoplankton. Science 221, 669–671.