Updated: Jan 26, 2022
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 understandable 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 wastewater.
Of course, every organism requires a little bit of phosphorus for its survival. This includes all of the various bacteria used for wastewater treatment (and targeted in carbon dosing strategies). But PNSB are special in that they seemingly accomplish the impossible by scrubbing far more phosphate than they could conceivably use for regular metabolism and growth. All of this owes to their ability to accumulate intracellular polyphosphate. In so doing, they maintain a store of polyphosphate granules that can be used later if necessary--an incredibly valuable capability in chronically nutrient-poor environments such as coral reefs.
In a study described in the journal Applied Microbiology Biotechnology (2010), investigators found that R. palustris is (even among its PNSB brethren) exceptionally good at polyphosphate accumulation. Likely due to differences in its pufM gene, this species was shown to build huge stores of internal phosphorus (up to 15% of its dry cell weight!), even after its population density levels off.
Growth rate and biomass, as well as polyphosphate accumulation, was found to be higher under photoheterotrophic (i.e. anaerobic) than under chemoheterotrophic (i.e. aerobic) conditions. As the researchers acknowledged, the results of the study demonstrate that PNSB polyphosphate accumulation is markedly “different from the well-known enhanced biological phosphorus removal model.”
Given the exceptional ability of PNSB to control phosphate concentrations--and their other considerable benefits--they are especially useful in reef and many freshwater planted aquarium applications where maintaining low nutrient levels is critical. But an experienced aquarist might ask, “What if my Redfield ratio is already balanced, or especially if nitrate is already very low? Will that just stunt these bacteria anyway?” The answer is no.
Firstly, PNSB can fix nitrogen, and so cannot “run out” of a biologically available source. Secondly (and perhaps most amazingly), because they can accumulate polyphosphate, they continue to sequester phosphate even when they are not actively growing! They (along with their stored phosphorus) are recycled as they are consumed by protozoa, copepods, corals, bivalve mollusks, sponges, etc. And they (along with their stored phosphorus) are ultimately exported via water changes and protein skimming. Natural, safe, effective, full of additional benefits and indeed tested extensively by literally generations of professional aquaculture technicians!
PNS ProBio™, which contains dense live cultures of R. palustris, has made it relatively easy and comparatively inexpensive to control phosphate pollution biologically. This unique product is photoheterotrophically grown on plant extracts (rather than inorganic carbons such as acetate). It therefore is syntrophically adapted--right out of the bottle--to utilize those stubborn wastes such as dead plant/macroalgae rubbish. Plus, it is loaded with health-promoting and color-enhancing carotenoids!
Bacterioplankton tend to be grossly lacking in most reef aquarium systems. PNS ProBio™ allows you to replenish your plankton to natural densities--without having to take the risks associated with heavy carbon dosing. These bacteria do not require any supplemental carbon dosing. In fact, they possess the enzymes required to degrade poorly digestible organic compounds (polyphenols, cellulose, etc.) that wimpier bacteria leave behind (detritus, yellowing compounds, etc.). After PNS bacteria have consumed all of the available organics, they simply resort to autotrophy, utilizing carbon dioxide similarly to plants and algae. Mopping up phosphate all the while.
Pretty awesome? We think so! That's why PNS ProBio™ was originally developed for use in a large, well-known aquaculture facility--and eventually commercialized for home aquarium use. Because this single-species "bacteria-in-a-bottle" not only helps to maintain good water quality and also serves as a food and probiotic, it arguably is a bit of a game changer! While the benefits of PNSB are many, it's entirely possible that aquarium hobbyists will come to value them especially for their incredible ability to cycle excess phosphate.