Purple Non-Sulfur Bacteria as a Probiotic Aquarium Food Additive

Updated: Feb 26

In recent years, we’ve seen a heightened interest in probiotic products throughout the aquarium industry. This development closely follows an increased use of probiotic microbes in various agricultural sectors, including aquaculture, over the last decade. Such is fairly understandable, as certain probiotic additives present a natural, organic, innocuous and generally effective alternative to antimicrobial chemicals and disinfectants.

It is the relatively gentle, nontoxic nature of these products that make them especially attractive as food supplements. However, some species of bacteria are far better suited for this use than others. This is due to fact that animal digestive tracts are, by design, extremely inhospitable to bacteria (and other invading organisms), rendering many such additives useless. Worse yet, some microbes potentially may turn on their host, suddenly becoming pathogenic.


Purple non-sulfur bacteria (PNSB) have become quite popular by professional aquaculturists. This is for a great many reasons; among these, PNSB significantly promote growth/health in diverse species (from copepods to corals to fish). Indeed, they are proven to increase feed conversion, contain essential dietary components such carotenoids, B vitamins and fatty acids as well as synthesize powerful antibiotic substances. They are demonstrated to remain viable as they pass through an animal’s gut. They are highly digestible and present absolutely no threat of pathogenesis. Moreover, they do no harm to, and are indeed known to work synergistically with, other probiotic genera such as Lactobacillus.

The potential advantages of using purple non-sulfur bacteria in this manner are many. Of course, PNSB are quite nutritious and can, owing to their rich carotenoid (e.g. astaxanthin) content, increase animal coloration. They are known to benefit hermatypic corals in particular as diazotrophs. But, as a food supplement, they are most beneficial for their probiotic abilities. By increasing host food conversion, they increase growth rates and reduce waste output. And, by synthesizing antibiotics in situ, they provide continuous and powerful disease prevention.


It is certainly worthwhile here to further elaborate upon their capacity to produce antibiotic substances. PNSB such as Rhodopseudomonas palustris can make copious amounts of antibiotics. These include antibiotics that target highly pathogenic gram-negative microbes such Vibrio, which has been implicated in many common and deadly aquarium diseases ranging from wasting disease in seahorses to rapid tissue necrosis in corals. Principle among these antibiotic substances are streptomycin, kanamycin and neomycin. Most of these are absorbed (unlike many other antibiotic compounds) from the gut into the bloodstream where they may prevent/treat disease internally.

For these reasons, we could easily, and strongly, suggest adding live PNSB products (such as PNS ProBio™) to any type of aquarium/aquaculture diet. There are several ways such could be done. Perhaps the ideal practice (especially for finicky fish) is to gutload PNSB into live feed just prior to use; live food animals such as brine shrimp have been shown to readily consume PNSB. One might also soak dry fish or coral foods in PNSB medium before use. It may similarly be mixed into frozen foods just after thawing. Finally, in many cases, fine filter-feeders (e.g. sponges, tunicates, many corals) will take up PNSB directly from the water column when “dosed” on a regular basis.