Updated: Jan 18
Phosphorus pollution is a common frustration of both freshwater and marine aquarists. Phosphorus is easily imported via organic sources such as fish foods, but is much less easily exported. The most reliable method of export, water exchange, is costly and laborious. Chemical filtrants such as aluminum oxid media are shown to irritate sensitive animals (including corals). While preferable in that it is a natural process, algal sequestration (whether using seaweeds or phytoplankton) can have limited results where biologically available (i.e. “fixed”) nitrogen is limiting.
Increasingly, aquarists are finding success with bacterial sequestration. Though it is also quite natural, it too is generally nitrogen-limited. Moreover, traditional methodologies (e.g. carbon dosing) that target aerobic heterotrophs carries a high risk of oxygen depletion.
Well, don’t write off the possibility of controlling your phosphates with bacteria quite yet. The aquarium community has only just begun to earnestly examine the use purple non-sulfur bacteria (PNSB). Indeed, the hobby’s relatively new interest in these primordial photosynthetic microbes is somewhat belated, as commercial scale aquaculturists and aquaponic technicians (especially fish and shrimp farmers) have been relying on them (sometimes heavily) since the 1970s. The longstanding and widespread use of PNSB in aquaculture is understanding considering their many positive attributes:
They are completely natural, fulfilling important nutrient cycling roles in virtually all freshwater and marine ecosystems.
Because they are primarily anaerobic, they pose no risk of oxygen depletion.
They are entirely nonpathogenic and safe with all types of aquarium livestock; indeed, they are proven to confer certain healthful probiotic qualities.
They provide a nutritious and carotenoid-rich source of nutrition for filter-feeders and zooplankton such as copepods.
They utilize recalcitrant organic wastes (e.g. detritus) as a carbon source whilst sequestering nitrate and phosphate.
Let’s focus on that last point for a moment. When you’re talking about PNSB in general, and Rhodopseudomonas palustris in particular, you’re talking about one of the most metabolically versatile organisms known to science. Because R. palustris has been a subject of great curiosity for biologists, its many capabilities are fairly well studied and characterized; it was, in fact, among the first microbes to have its entire genome mapped. Even so, we’ve only begun to understand why it’s so incredibly good at removing phosphate from w