Rhodoplanes in Aquaculture: Mechanistic Insights and Emerging Applications

As sustainable aquaculture becomes an increasingly critical component of global food production, the demand for microbial solutions that enhance water quality, promote animal health, and improve system stability is growing. Similarly–albeit somewhat belatedly–a strong interest in sustainability has emerged within the ornamental aquaculture community. Among the purple nonsulfur bacteria (PNSB) candidates under investigation, the genus Rhodoplanes—particularly R. serenus and R. piscinae—shows considerable promise due to its apparent safety, metabolic versatility, and ecological adaptability.

This article explores the mechanistic traits of Rhodoplanes relevant to aquaculture, reviews the emerging body of literature, and considers future applications of these bacteria in aquatic systems.

Taxonomy and Natural Habitat

Members of the genus Rhodoplanes are part of the family Xanthobacteraceae and the class Alphaproteobacteria. They are facultative phototrophs that thrive in aquatic environments, including ponds and aquaculture systems. These bacteria are characterized by their ability to perform anoxygenic photosynthesis, fix nitrogen, and reduce nitrates—all of which are valuable traits in aquaculture settings.

Rhodoplanes sp. (rosette formation) in a Rhodoplanes/Rhodobacter dominated culture. Total Magnification: 1000x, oil immersion. Copyright 2025 Hydrospace LLC

One of the unique features of Rhodoplanes is its ability to grow photoheterotrophically, using organic carbon sources and light energy. This mode of growth is advantageous in aquaculture, where light is abundant and organic substrates are plentiful due to feed input and metabolic waste. Moreover, Rhodoplanes can form biofilms on surfaces such as tank walls and biofilters, where they may contribute to the stabilization of microbial communities and enhancement of nutrient cycle processes. Their ability to persist in both oxic and anoxic zones further supports their utility in complex aquaculture systems.

Nitrogen Cycling: From Waste Reduction to Biofiltration

A central challenge in aquaculture is the accumulation of nitrogenous waste, particularly ammonia, nitrites, and nitrates. Rhodoplanes contributes to nitrogen cycling through two critical anaerobic mechanisms: nitrogen fixation (conversion of nitrogen gas into bioavailable ammonium) and denitrification (reduce nitrates to nitrogen gas).

Rhodoplanes is perhaps best known for its close, beneficial association with plants. It is indeed found living on aquatic plants such as Myriophyllum (Sun et al) and Vallisneria (Zhang et al). Species like R. piscinae have been shown to harbor nitrogenase gene clusters, enabling them to perform nitrogen fixation (Srinivas et al.). This function not only supports microbial biomass growth but also contributes to the nitrogen pool available to primary producers like aquatic plants.

In parallel, R. serenus can perform denitrification, a process that helps to close the nitrogen cycle and prevents the buildup of harmful nitrogen species in closed-loop aquaculture systems (Okamura et al.).

Ecological Presence in Aquaculture Systems

Metagenomic analyses suggest that Rhodoplanes is capable of surviving as a part of the microbial communities in aquaculture systems. Studies such as Xu et al. and Zhang et al. report the detection of Rhodoplanes in sediment and biofilter samples from freshwater ponds and aquaponics systems, often in association with nitrogen-metabolizing consortia. Their presence in these systems, alongside nitrifying and denitrifying bacteria, indicates a role in maintaining nitrogen balance and water quality.

Probiotic Potential: Gut Microbiota and Health Benefits

Although direct studies of Rhodoplanes as an aquaculture probiotic are limited, parallels can be drawn from related PNSB such as Rhodobacter and Rhodopseudomonas, which have been successfully used to improve gut health, immunity, and growth rates in fish and shrimp. These bacteria often function through competitive exclusion of pathogens, production of antimicrobial compounds, and modulation of host immune responses.

Given the taxonomic and metabolic similarities, Rhodoplanes likely exerts similar benefits. The detection of Rhodoplanes in fish pond sediments suggests that they are well-adapted to aquaculture environments and may naturally colonize the gut microbiome of aquatic animals, where they could contribute to nutrient assimilation or pathogen resistance.

Rhodoplanes sp. (biofilm formation). Total Magnification: 1000x, oil immersion. Copyright 2025 Hydrospace LLC

Exploring the Aquacultural Potential of Rhodoplanes 

There is considerable potential to use Rhodoplanes in bioaugmentation strategies, either alone or as part of microbial consortia, to manage water quality and improve animal health. The formulation of stable, cost-effective inoculants will be a key step toward commercialization. Future studies should aim to isolate native strains of Rhodoplanes from aquaculture systems and evaluate their nutritional, probiotic, and water conditioning functions in controlled experiments. Genomics-driven functional characterization could help identify pathways for antimicrobial production, stress resistance, and host interaction.

With regard to health and growth performance, it will be necessary to determine factors that influence probiotic activity of Rhodoplanes, particularly within the fish and invertebrate gut. To date, Rhodoplanes has already been evaluated for such purposes in several studies:

• Composition and abundance of gut bacterial communities in key aquaculture fish groups, most notably how gut bacterial diversity of farm-raised fish diverges from that of wild-caught fish due to ecological and management differences (Kanika et al).

• Differences in the microbiome of the gut, gill, and skin of grass carp in response to stocking density (Yang et al).

• Changes of gut microbial activity in rice field eels following certain dietary restrictions (Hu, Cai, and Chu).

• Changes of gut microbial diversity in response to plant-based protein sources (low fish meal) and subsequent long-term health impact on olive flounder (Niu et al).

• Comparison of responses of the gut microbiota to fasting/starvation across several vertebrate hosts including Nile tilapia (Kohl et al).

• Impact of environmental contaminants on the gut microbiome of livebearer killifish (Nolorbe-Payahua et al).

• Identification of bacterial genera potentially hosting antibiotic resistance genes, which infers how the fish gut microbiome could have implications for managing antibiotic resistance in aquaculture and other ecosystems (Jiang).

Additionally, it may be advantageous to distinguish Rhodoplanes from those related PNSB that are better known to aquarists. For example, while it is quite similar to Rhodopseudomonas (the genus Rhodoplanes was in fact assembled from several former Rhodopseudomonas species), it does differ in some important ways concerning aquarium care. Notably, unlike Rhodopseudomonas, most Rhodoplanes strains do not flourish in saltwater; on the other hand, Rhodoplanes is a more capable denitrifier (Hiraishi and Imhoff).

Rhodoplanes dominated culture exhibiting an orange-red suspension and red-purple biofilms. Copyright 2025 Hydrospace LLC

Conclusion 

As with so many PNSB, the prospects of Rhodoplanes for enhancing sustainability in aquaculture are strong. With demonstrated capabilities in nitrogen metabolism, gut colonization, and ecological fitness in aquatic systems, Rhodoplanes represents a novel and grossly underexplored candidate for aquacultural biotechnology. Unlocking its full potential will require a combination of intensive laboratory validation, field trials, and microbial formulation expertise!

Works Cited 

Okamura, Keiko, et al. "Rhodoplanes serenus sp. nov., a phototrophic alphaproteobacterium isolated from pond water." International Journal of Systematic and Evolutionary Microbiology, vol. 59, no. 2, 2009, pp. 298–304. https://doi.org/10.1099/ijs.0.000562-0.

Srinivas, T. N. R., et al. "Rhodoplanes piscinae sp. nov., a phototrophic alphaproteobacterium isolated from a freshwater fish pond." International Journal of Systematic and Evolutionary Microbiology, vol. 62, no. 7, 2012, pp. 1668–1673. https://doi.org/10.1099/ijs.0.032284-0.

Xu, Zhen, et al. "Functional microbial communities and nitrogen fixation genes in paddy fields revealed by DNA-SIP." Science of the Total Environment, vol. 807, part 1, 2022, Article 150702. https://doi.org/10.1016/j.scitotenv.2021.150702.

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