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