Could Purple Non-Sulfur Bacteria Change Reefing?

The future of the reef aquarium industry is microbial. Microbes hold many of the nuanced secrets behind the long-term health and reproduction of corals, reef fish, and invertebrates. All of these creatures are a sum of their associated microbiomes and appreciating how all these tiny worlds orchestrate together is key to achieving success in the reef aquarium.

Microbes present the potential to remediate several of the routine pitfalls of reef aquarium keeping (excess nutrient buildup, proliferation of pest algae, trace metal accumulation, etc.). Microbes can be used as probiotics, enhancing digestion, providing immunostimulation, and suppressing pathogens. Furthermore, live bacteria cells of high nutritional content offer a means to satisfy the diets of commonly kept species as well as numerous marine sponges, NPS corals, and other filter-feeding species currently not available in the hobby.

The key to making all these microbial propositions a reality is the purposeful identification, cultivation, and application of certain bacteria species/strains that will best satisfy the following criteria:

1. Adaptability: They can survive, and propagate to efficacious numbers, under general reef aquarium environmental conditions (salinity, temperature, etc.).  

2. Practicality: They do not produce toxins or grow aesthetically unappealing superstructures such as fuzzies, films, slimes, oozes, etc. 

3. Biofiltration: They provide an element of biofiltration, consuming inevitable aquarium wastes such as organics, phosphates, and ammonia.  

4. Nutrition: They have a beneficial nutritional profile (lipids, bioactive vitamins, pigments, etc.) when consumed by the corals and cleanup crew.

5. Disease Protection: They not only cannot become pathogenic, but actively provide immunostimulation to reef inhabitants and suppress the growth of pathogens.  

6. Lineage: They have been formally studied for decades and are the subject of a host of research papers that explore their abilities/potential applications (also implies that the microbe can be cultivated and distributed as pure cultures).  

It is with these criteria in mind that members of the purple non-sulfur (PNS) bacteria have been identified as highly promising candidates for benefiting the reef aquarium industry. One species in particular, Rhodopseudomonas palustris, has been the subject of substantial aquacultural/agricultural research since the 1960s. R. palustris is naturally prevalent in both freshwater, estuarine, and marine ecosystems worldwide and is capable of colonizing water, soil, and biological microhabitats. 

R. palustris is intricately associated with various wild coral reef ecosystems, being observed colonizing sediments and within rock pores. But perhaps even more interestingly, this species has also been observed to colonize the internal tissues of corals, the root systems of mangroves, and the gastrointestinal tracts of fish. Utilized in captive environments, R. palustris has demonstrated the ability to enhance production and improve water quality in commercial fish/shrimp farms.

Even more exciting, is a growing body of research demonstrating the ability of R. palustris to provide atmospheric immunostimulation to species such as pacific white-legged shrimp (Litopenaeus vannamei); such studies offer a lot of hope that R. palustris can fight back pathogenic Vibrio outbreaks.  

R. palustris (and other purple non-sulfur bacteria) can benefit the reef aquarium industry. With less and less live rock being harvested from wild reefs nowadays, there is a profound discrepancy between the initial microbial profile of reef aquariums today compared to those thirty years ago. Wild reef live rock came with all sorts of sometimes undesirable hitchhikers, but it also came with the precious microbial essence of the wild reef. Albeit this is a microbiome scattered and attenuated by the process of shipping and holding, but still…each chunk of wild rock contained a profound microbial core… which likely contained a plethora of PNS bacteria and a multitude of other bacteria species.

Nowadays, more and more reef aquariums are started with dry rock. This is essentially starting the aquarium with an empty frontier where tough, nasty microbes are likely to thrive–that is unless they are challenged by a population of benign microbes. Purposefully dosing PNS bacteria is a way of preoccupying the various microhabitats of the dry rockwork before more nasty cyanobacteria, diatom, etc. species do.

R. palustris is an ideal ‘pioneer’ microbe for a new aquarium because of its ‘Swiss-army knife’ metabolism. This refers to its ability (not uncommon among the PNS) to alternate between various forms of metabolism. For example, most conventional ammonia and nitrite-consuming biofilter species (Nitrosomonas, Nitrobacter, Nitrospira, etc.) are only capable of living in dark, aerobic environments. This means they cannot successfully operate in the many illuminated parts of an aquarium, nor can they live in anaerobic ‘dead zones.’ PNS bacteria do not have such limitations and thrive in anaerobic environments. Furthermore, they are photosynthetic, allowing them to not only tolerate but utilize light in the aquarium to consume dissolved nitrates and phosphates.

R. palustris does not die when exposed to oxygen but can utilize it to conduct heterotrophy and reduce organic sludge. Due to this metabolic plasticity, PNS bacteria complement conventional biofilter species, while at the same time, allowing aquarists to purposefully fill a wider variety of ecological niches in both new and aging aquariums. 

R. palustris cells consume nitrate through denitrification, phosphate through photosynthesis, or organic carbons through heterotrophy…all convert these aquarium wastes into more R. palustris cells. Each one of these cells is a microscopic jellybean full of microbial proteins, lipids, sugars, and sterols. They also contain more nuanced nutritional elements such as proenzymes and vitamin B12. Because PNS bacteria photosynthesize, they produce pigments for collecting light. These pigments are of profound nutritional value to other organisms, where they function as antioxidizing agents and oxygen modulators. 

When acting as a functional biofilter, PNS bacteria cells exist as biofilms attached to the inner surfaces of rock and other biomedia. Excess cells produced by these biofilm concentrations will eventually become ‘sloughed off’ into the water column. Additionally, the young ‘daughter cells’ are motile and swim among the plankton. Here they provide a source of planktonic forage for sponges, NPS corals, sabellid worms, porcelain crabs, clams, and other filter-feeding organisms. R. palustris cells often form aggregated rosettes of several hundred cells, forming a larger particle size for filter feeders.

R. palustris has many exciting applications as a planktonic live feed for satisfying the nuanced needs of bacterivorous species. This species also has benefits to already established reef species in the hobby, as it is increasingly explored as a stimulant for triggering coral’s feeding response. Even more fascinating is the potential of PNS bacteria to actively colonize the tissues of corals and play a role in stabilizing the holobiont. One potential means R. palustris has to do this is its ability to conduct diazotrophy, or the ability to fix nitrogen gas into ammonia. This is a means by which bacteria can dose ammonia directly to Symbiodinium and other zooxanthellae when there is none in the surrounding water.

Also exciting is the potential of R. palustris to suppress pathogenic Vibrio species through competition for resources and localized secretion of antibiotics (e.g., streptomycin). Exploring the functional relationships between PNS bacteria and various coral, fish, invertebrate, and microbe species will be key to maximizing its profound potential utility.           

Overall Benefits of Rhodopseudomonas palustris to the Reef Aquarium 

Understanding how microbes operate within the context of coral reef ecosystems will be key to resolving existing limitations in the reef aquarium industry. Though not a replacement for mechanical or chemical filtration, the purposeful application of known bacteria species, such as Rhodopseudomonas palustris, offers a plethora of avenues for increasing resource use efficiency in the reef aquarium. Food, water, electricity–these constitute the main cost of any reef aquarium.

Promoting a microbial ecology that recycles food and waste so that it can be re-consumed by filter feeders is key to maximizing water quality/feed efficiency while reducing reliance on external filtration. Coupled with the multitude of ecological benefits associated with PNS bacteria generally, this all suggests an ever-increasing value for the species across the reef aquarium industry.