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R. palustris Reliably Consumes Plant/Macroalgal Rubbish

Updated: Sep 23, 2020

Pondkeepers as well as freshwater and marine aquarists (especially in commercial operations) are increasingly relying on purple non-sulfur bacteria (PNSB) to remediate wastewater. Though these microbes are generally used to target excess nitrogenous compounds (ammonia, nitrate, etc.), they are also useful for removing organic wastes. This includes both dissolved and solid organic matter. Sure, there are a number of heterotrophic microbes (i.e. sludge digesters) used to remove solid organics; however, among these, only a tiny handful (including certain PSNB) have the ability to remove cellulose- and lignin-based plant/algae detritus. The implication of this is huge for anyone maintaining a water garden or aquarium/refugium.

Before we get into the meat of this, we'll impress upon you how special this ability really is.

Lignocellulosic compounds (i.e. substances that contain cellulose and lignin) are extremely stable chemically and are therefore difficult to degrade. This is exactly why so few organisms ever evolved to synthesize them in the first place. Cellulose is quite ancient, having first emerged among a few types of microbes (especially in their biofilms) such as cyanobacteria. The ability to produce cellulose was passed on to algae, which incorporate the compound in their cell walls. Similarly, chlorophytes (green algae) conserved this ability as they evolved into plants.

Lignins and tannins are chemically similar to cellulose, but are even more resilient. These phenolic substances serve primarily as a structural/protective component of leaves, bark and seeds (lignins are somewhat more resistant than tannins). As plants decompose, tannic and lignic compounds are among the last to degrade (imparting a yellow-brown color to many water bodies). Compared to cellulose, they are (in terms of evolutionary history) quite new.

Indeed, the first occurrence of lignin on Earth defines the Carboniferous Period (a mere 300-350 million years ago) as plants started incorporating the stuff into bark and other woody tissues. Decomposing microbes and fungi had not yet evolved the enzymes capable of digesting the resistant lignic polyphenols. Even to this day, not many organisms can degrade it; while some can consume cellulose, few effectively digest lignin (which is why it tends to accumulate into humic soils and waters). No wonder that, after cellulose, it is now the most abundant polymer in nature! The same sort of build-ups occur in our aquaria.

This is where the purple non-sulfur bacterium Rhodopseudomonas palustris comes in. Some time during or immediately following the Carboniferous Period, this adaptive photoheterotrophic microbe evolved the enzymes to completely consume lignin. And it does so quite well. It is now a naturally abundant occupant of lignin-rich habitats.

Under anoxic and moderately-brightly illuminated conditions, R. palustris breaks lignin down into simpler compounds (i.e. p-coumarate and benzoate) and then into even simpler compounds (acetate and carbon dioxide). Not only can R. palustris derive carbon and energy from degrading lignin, but also from degrading each of the resulting byproducts. And, it does so in an amazingly efficient manner.

Generally, decomposition processes are carried out by a multi-species consortium of fairly specialized microorganisms (a phenomenon called syntrophy). This teamwork reduces the effort required to maintain chemical stability at each syntrophic level. Here, however, the unusually versatile R. palustris forms its own single-species syntrophic team. Genetically identical "subpopulations" of these consortia specialize to manage particular metabolic modes. In other words, instead of trying to consume lignin and all of its catabolic byproducts, each cell instead focuses on a single compound. As certain byproducts accumulate in the environment, subsequent generations of this bacterium respond by forming new, specialized subpopulations. While decomposing lignin, three or four subpopulations of R. palustris can eventually develop in this manner.

When used in an aquacultural system to remove excess nitrogenous compounds, R. palustris can utilize almost anything as a carbon source. It may use rotting organic matter (e.g. uneaten fish food) or even inorganic carbon (e.g. carbon dioxide). But it loves lignocellulosic and tannic compounds, which are rich in energy and (since most other microbes cannot consume them) are free for the taking. For aquarists/water gardeners who are frustrated by build-ups of algal/plant detritus, these bacteria are a real blessing. Perhaps even better yet, for enthusiasts of blackwater/botanical style aquaria, the activity of these bacteria is quite fascinating to observe.

PNS ProBio is uniquely suited for these purposes. This product contains highly concentrated, fully developed cultures of live R. palustris. Its growth medium is comprised of extracts of Eurasian watermilfoil (Myriophyllum spicatum), an aquatic plant that is famously rich in tannins--specifically the ellelopathic ellagitannin Tellamagrandin II. What this means is that several syntrophic subpopulations, adapted to fully degrading rot-resistant organic matter, are present. What that means is that the bacteria in this product immediately get to work digesting pockets of leaf litter, dead seaweed, etc. right out of the bottle. It even consumes "yellowing compounds" secreted into the water by living plants and algae. Best of all, as meiofauna such as amphipods consume it, they pass on its abundant B vitamins and carotenoids up the food chain (e.g. to fish). Really, PNSB don't just remediate your system water--they truly complete your captive ecosystem!

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