Centric Diatom Community
If your new tank's water turned faintly cloudy and brown in the first weeks before any film appeared on the glass, you were probably watching centric diatoms bloom. These are the free-floating "spring bloom" specialists — the water-column half of the diatom story, the other half being the brown film of their pennate cousins. Representative taxa are Cyclotella, Asterionella, Fragilaria, and Synedra: small, radially symmetric cells, often solitary or loosely chained, that drift rather than stick. They matter far beyond the aquarium — globally, diatoms run roughly a fifth of all photosynthesis and the single largest flux of biogenic silica on Earth (Nelson et al. 1995), and centric forms dominate that floating fraction.
The spring-bloom specialist
Centric diatoms are textbook r-strategists: they grow fast — close to two doublings a day under good conditions — need a fair amount of light, favour cold water, and live by boom and bust. In temperate lakes they form the dominant phytoplankton bloom of late winter and early spring, when the water is cold, mixing lifts silica and nutrients to the surface, and the grazers that will eventually eat them haven't yet caught up (Reynolds 1984; Sommer et al. 1986). A freshly set-up aquarium reproduces the same conditions in miniature: plenty of dissolved silica from tap water and substrate fuels a rapid water-column bloom that often runs just ahead of — or alongside — the slower brown film that the pennate diatoms lay down on surfaces.
That cool-water preference is part of what makes them a new-tank, not an old-tank, organism. Their photosynthetic optimum sits near 16 °C, well below the warmth most tropical tanks are kept at, so as a tank settles into its running temperature the centrics lose their early edge and warm-adapted planktonic green algae overtake them.
Boom and bust through silica
Every bit of diatom growth demands silicic acid pulled from the water, laid down as a glass shell. Diatoms take up silicon and nitrogen in roughly equal molar step — about two grams of silicon for every gram of nitrogen built into new cells (Brzezinski 1985) — so a vigorous centric bloom can strip the dissolved silica out of a small system within a few weeks. Centrics are the spendthrifts of the two diatom types: they have a weaker grip on dilute silica than pennates do (Sommer 1986; Kilham et al. 1986), exploiting abundant silica quickly but unable to scavenge the dregs. The result is the characteristic silica crash that ends a centric bloom — the same crash, seen from the silicon side, that the Silicon Cycle follows in detail.
When the cells die their glass frustules do not dissolve at once. A small fraction of the silicon — the lightly built parts of the cell and its contents — returns to the water almost immediately, but most enters a pool of intact and broken shell fragments that dissolves only slowly, over roughly ten days (Van Cappellen et al. 2002). That lag is why a post-bloom silica shortage can linger for days to weeks after the diatoms themselves are gone, holding off any regrowth.
A floater, not a sticker
Unlike the mucilage-gluing pennates, centric cells invest almost nothing in attachment. The vast majority of their biomass stays suspended in the water column; the little that settles onto surfaces resuspends easily and tends to spread thinly between surfaces rather than building thick local colonies. This is the behavioural line that separates the centric "water-column specialist" from the pennate "biofilm dweller," even though the model gives both the same underlying floating-plus-attached structure. Because they aren't building an adhesive scaffold, centrics also leak less dissolved organic carbon into the water than pennates do, so they feed the heterotrophic bacterial loop a little less.
Frugal with trace metals
Diatoms are famously thrifty with iron and molybdenum, carrying far less of each per unit of carbon than the average alga (Quigg 2003), and pairing that with unusually high-affinity nitrate-handling machinery. This frugality is exactly why diatoms dominate the iron- and molybdenum-poor open ocean, and why the molybdenum-gated shift between ammonium and nitrate use is subtler for them than for other producers. The Iron Cycle and Micronutrient Cycling pages explain those gates; the exact half-saturations are tabulated in the Parameter Reference.
Food quality and who eats them
Centric diatoms are good food: no toxins, rich in the lipids and protein that growing zooplankton need. But their cell size and glassy shell decide who can actually handle them — copepods take them readily, Daphnia only partially, and the smallest filter feeders such as rotifers and heterotrophic nanoflagellates barely at all, limited by mouth gape and the difficulty of cracking a frustule. Because centric biomass lives in the open water, these are the diatoms the pelagic grazers eat, while the surface-bound pennates feed mostly benthic scrapers — snails, shrimp, and ostracods.
Centric versus pennate at a glance
The two diatom communities share one silicon physiology but live opposite lives:
| Trait | Centric (this page) | Pennate |
|---|---|---|
| Growth pace | Fast — bloom-and-crash | Slow and persistent |
| Temperature | Cool-water, new-tank | Temperate, steadier |
| Light | Moderate requirement | Deeply shade-tolerant |
| Lifestyle | Free-floating in the water | Glued to surfaces as brown film |
| Silica affinity | Weaker — exploits abundance | Stronger — scavenges the dregs |
| Where it shows up | Cloudy brown water, first weeks | Brown dust on glass and leaves |
The exact growth rates, light and silica half-saturations, temperature thresholds, and attachment rates behind this contrast are tabulated in the Parameter Reference.
Further reading
- Pennate Diatom Community — the slow, surface-bound "brown algae" of new tanks
- Silicon Cycle — the silica boom-and-crash that drives the brown-algae succession, week by week
- Producers — how all the algae and plants fit together
- Iron Cycle and Micronutrient Cycling — the trace-metal frugality that lets diatoms dominate poor water
- Planktonic Green Algae — the warm-water competitor that overtakes centrics as a tank settles
- Parameter Reference — every rate, half-saturation, and ratio behind this page, with citations
Key references
- Brzezinski, M.A. (1985). The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables. Journal of Phycology 21, 347–357.
- Kilham, S.S., Kilham, P. & Theriot, E.C. (1986). Speciation in the Stephanodiscus niagarae complex. Hydrobiologia 138, 107–113.
- Nelson, D.M. et al. (1995). Production and dissolution of biogenic silica in the ocean. Global Biogeochemical Cycles 9, 359–372.
- Quigg, A. et al. (2003). The evolutionary inheritance of elemental stoichiometry in marine phytoplankton. Nature 425, 291–294.
- Reynolds, C.S. (1984). The Ecology of Freshwater Phytoplankton. Cambridge University Press.
- Sommer, U. (1986). Phytoplankton competition along a gradient of dilution rates. Oecologia 68, 503–506.
- Sommer, U., Gliwicz, Z.M., Lampert, W. & Duncan, A. (1986). The PEG-model of seasonal succession of planktonic events in fresh waters. Archiv für Hydrobiologie 106, 433–471.
- Van Cappellen, P. et al. (2002). Dissolution kinetics of biogenic silica in marine sediments. Geochimica et Cosmochimica Acta 66, 1149–1158.