Benthic Cyanobacteria Community ("BGA")
This is the BGA — blue-green algae — that planted-tank keepers dread. A dark green-to-black, slimy film creeps across the substrate and up plant leaves, smells faintly of mould or swamp, peels off in sheets, and grows right back within days. Despite the hobbyist name it isn't algae at all: it is a mat of cyanobacteria, taxa like Oscillatoria, Phormidium, Lyngbya, and Nostoc — filamentous bacteria that knit themselves into thick, cohesive, foul-smelling microbial mats. A Phormidium outbreak can smother crypts and stem plants under a blanket that shrugs off manual removal and is notoriously hard to clear without a blackout, hydrogen peroxide, or an antibiotic. In the wild these same organisms are the dominant primary producers of nutrient-poor streams, mineral and hot springs, and polar ponds, and they build most of the world's microbial-mat ecosystems. In this simulator they are tracked as cyanobacteria — kept separate from the green algae and diatoms in the food web and the accounting — because their nitrogen-fixing physiology sets them apart.
Slow to arrive, stubborn to leave
Benthic cyanobacteria are K-strategists: slow-growing — less than one doubling a day, under half the pace of their planktonic bloom-forming relatives — deeply shade-tolerant (as shade-adapted as pennate diatoms), temperate, and the most persistent producer in the model, dying off more slowly than anything else. They are not explosive bloomers. Instead they colonise the substrate quietly over weeks and then dig in. This slow-but-durable habit is exactly why a Phormidium outbreak builds silently before it is ever noticed and then takes weeks to months of determined effort to remove.
The slime scaffold
The defining physical feature of a benthic mat is its heavy investment in extracellular polymeric substances — the EPS slime. Benthic cyanobacteria pour a large share of their fixed carbon into this matrix, far more than planktonic forms, and it does several jobs at once. It binds filaments tightly to the substrate so the mat stays exactly where it forms and resists physical disturbance; the leaked carbon also feeds a dedicated community of heterotrophic bacteria living beneath the mat, which adds to the labile DOM pool. And the scaffold is a defence: it mechanically excludes many grazer mouthparts, and field mats often carry antibacterial and antiprotistan compounds, plus a UV-screening pigment, as chemical deterrents. The model represents that defence implicitly, through low grazer preferences and the mat's physical toughness, rather than as explicit allelopathy.
Fixing nitrogen without heterocysts
Most benthic mat-formers lack heterocysts — the specialised, oxygen-free cells that planktonic cyanobacteria use to protect their nitrogenase. Instead they exploit oxygen gradients within the mat itself. By day, photosynthesis at the mat surface makes oxygen that diffuses away; deeper in the EPS, heterotrophic bacteria consume oxygen fast, carving out anoxic microzones where nitrogenase can run even while the bulk water above is well oxygenated (Paerl & Bebout 1988). That protection is only partial — it needs a thick, active mat, and bulk-water oxygen still penetrates the top millimetre — so benthic mat-formers are markedly more oxygen-sensitive than the heterocyst-shielded planktonic blooms. Fixation is therefore strongest at night, when respiration draws the surrounding oxygen down, or deep inside a mature, active mat. The maximum fixation rate is also a touch lower than in the heterocystous forms, reflecting the lack of dedicated cellular machinery. The same four-way regulation — by light, by dissolved nitrogen, by oxygen, and by phosphorus — applies as for the planktonic cyanobacteria, and both pay the same energetic surcharge for fixing nitrogen.
Light, pH, and cold
The same carboxysome-based carbon concentrating mechanism that planktonic forms use lets benthic mats keep photosynthesizing in carbon-depleted, alkaline water. But they tolerate high pH less than their planktonic relatives — they thrive across a broad but more moderate pH range, the springs and temperate streams rather than the extreme-alkaline eutrophic blooms. They are, however, notably cold-tolerant: in cold polar and alpine waters, benthic cyanobacterial mats are often the dominant primary producers (Hawes et al. 1999), and Phormidium mats persist through winter in unheated aquaria and outdoor ponds.
The boundary-layer advantage
Like pennate diatoms and benthic green algae, surface-bound cyanobacteria gain from the still diffusion boundary layer against the substrate, where nitrogen runs richer than in the open water. The same three sources feed it — settled-detritus mineralization, pore-water proximity, and local nitrification — and only the attached mat benefits, not the negligible floating fraction (see biofilm enrichment).
Why almost nothing eats it
Benthic cyanobacterial mats are among the most grazer-resistant producers in the model, and hobbyists read this correctly when they say "nothing eats BGA." Between the EPS scaffold, the toughness of the mat, and the chemical deterrents, baseline grazer preferences are low. Ostracods and bladder snails — the most capable biofilm scrapers — can damage a mat mechanically, and nerite snails are occasionally seen grazing Phormidium in real tanks, but no grazer in the model can crash a mature mat on its own. This is why keepers facing BGA reach for environmental fixes — a blackout, a peroxide spot-treatment, an antibiotic course — rather than trusting a cleanup crew.
Benthic versus planktonic cyanobacteria at a glance
The two cyanobacterial communities share nitrogen-fixing physiology but live opposite lives:
| Trait | Benthic (this page) | Planktonic |
|---|---|---|
| Growth pace | Slow, persistent mats | Fast — bloom-and-crash |
| Lifestyle | Glued to substrate as a mat | Free-floating scum |
| Light | Deeply shade-adapted | High-light, surface scum |
| Temperature | Temperate, cold-hardy | Warm, summer-bloom |
| Nitrogen fixation | No heterocysts — microaerobic mat interior, oxygen-sensitive | Heterocysts — oxygen-protected, runs in fully aerated water |
| High-pH tolerance | Broad but moderate | Extreme-alkaline specialist |
| What the hobbyist sees | Black slimy mat, "BGA" | Pea-soup or surface scum (rare in tanks) |
The exact growth rates, light and pH thresholds, fixation rates, and oxygen sensitivities behind this contrast are tabulated in the Parameter Reference.
Further reading
- Planktonic Cyanobacteria — the heterocystous, free-floating bloom-formers of warm eutrophic lakes
- 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
- Biofilm Nutrient Enrichment — the boundary-layer enrichment that sustains surface mats
- 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
- Hawes, I., Howard-Williams, C. & Vincent, W.F. (1999). Desiccation and recovery of Antarctic cyanobacterial mats. Polar Biology 21, 242–249.
- Paerl, H.W. & Bebout, B.M. (1988). Direct measurement of O₂-depleted microzones in marine Oscillatoria: relation to N₂ fixation. Science 241, 442–445.
- Stal, L.J. (1995). Physiological ecology of cyanobacteria in microbial mats and other communities. New Phytologist 131, 1–32.
- Wood, S.A. et al. (2012). Benthic cyanobacterial mats as a possible cause of dog deaths: Phormidium and anatoxin-a. Toxicon 60, 1248–1256.
- Fay, P. (1992). Oxygen relations of nitrogen fixation in cyanobacteria. Microbiological Reviews 56, 340–373.